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[Objective] The sedimentary paleoenvironmental characteristics of the Lower Cambrian Niutitang Formation in the Southern Guizhou Depression were studied, and the mechanism of organic matter enrichment was analyzed to support shale gas exploration and development in the area. [Methods] Based on the core and testing analysis data of Well Guidudi 1, the mineral composition, lithofacies characteristics, and geochemical characteristics of the Niutitang Formation were analyzed to study the sedimentary processes and characteristics of ancient marine environments, such as redox conditions, paleoproductivity, input of terrestrial debris, and degree of water retention. The sedimentary evolution process and organic matter enrichment mechanism of the Niutitang Formation were explored. [Results] The results indicate that the first section of the Niutitang Formation developed organic rich siliceous shale, organic rich silty siliceous shale, and organic mixed shale, mainly involves suspended sedimentation of mud and biological sedimentation of siliceous matter. The second section of the Niutitang Formation was mainly composed of poor organic clayey shale, with the main sedimentary model being mud coagulation and sedimentation. The third section of the Niutitang Formation developed organic and calcareous mixed shale, with the main sedimentary model being calcium chemical deposition and mud flocculation deposition. [Conclusions] The ancient marine environment and organic matter enrichment during the sedimentation period of the Niutitang Formation in the study area showed obvious stages. During the first sedimentary period of the Niutitang Formation, the ancient marine environment was controlled by the high productivity level and strong reducing conditions formed by rapid marine invasion, resulting in a significant enrichment of organic matter. During the second sedimentary period of the Niutitang Formation, the organic carbon content was low due to the combined effects of oxidative environment and dilution from high terrestrial debris supply. During the third sedimentary period of the Niutitang Formation, it is speculated that the strong retention environment promoted the deposition of carbonate minerals and the enrichment of organic matter due to the influence of underwater ancient uplift. The research results can provide a reference for the evaluation of shale gas resource potential in the Niutitang Formation of Southern Guizhou Depression.
[Objective] The sedimentary paleoenvironmental characteristics of the Lower Cambrian Niutitang Formation in the Southern Guizhou Depression were studied, and the mechanism of organic matter enrichment was analyzed to support shale gas exploration and development in the area. [Methods] Based on the core and testing analysis data of Well Guidudi 1, the mineral composition, lithofacies characteristics, and geochemical characteristics of the Niutitang Formation were analyzed to study the sedimentary processes and characteristics of ancient marine environments, such as redox conditions, paleoproductivity, input of terrestrial debris, and degree of water retention. The sedimentary evolution process and organic matter enrichment mechanism of the Niutitang Formation were explored. [Results] The results indicate that the first section of the Niutitang Formation developed organic rich siliceous shale, organic rich silty siliceous shale, and organic mixed shale, mainly involves suspended sedimentation of mud and biological sedimentation of siliceous matter. The second section of the Niutitang Formation was mainly composed of poor organic clayey shale, with the main sedimentary model being mud coagulation and sedimentation. The third section of the Niutitang Formation developed organic and calcareous mixed shale, with the main sedimentary model being calcium chemical deposition and mud flocculation deposition. [Conclusions] The ancient marine environment and organic matter enrichment during the sedimentation period of the Niutitang Formation in the study area showed obvious stages. During the first sedimentary period of the Niutitang Formation, the ancient marine environment was controlled by the high productivity level and strong reducing conditions formed by rapid marine invasion, resulting in a significant enrichment of organic matter. During the second sedimentary period of the Niutitang Formation, the organic carbon content was low due to the combined effects of oxidative environment and dilution from high terrestrial debris supply. During the third sedimentary period of the Niutitang Formation, it is speculated that the strong retention environment promoted the deposition of carbonate minerals and the enrichment of organic matter due to the influence of underwater ancient uplift. The research results can provide a reference for the evaluation of shale gas resource potential in the Niutitang Formation of Southern Guizhou Depression.
Abstract:
Accurate identification of mineral composition and microstructural features in sedimentary rock thin sections is an important basis for provenance analysis, reconstruction of depositional environments, and reservoir evaluation. Although traditional manual identification based on polarized light microscopy is highly reliable, it is limited by low efficiency, strong subjectivity, and insufficient standardization. Therefore, it is difficult to meet the growing demand for high-throughput, quantitative, and reproducible analysis in data-driven sedimentological research. Focusing on the development history and technical evolution of intelligent mineral identification in sedimentary rocks, recent progress in traditional image processing, machine learning, and deep learning methods for rock thin-section image analysis is systematically summarized. The applications of convolutional neural networks and Vision Transformers in image classification, object detection, and image segmentation are reviewed, with particular emphasis on their adaptability to the recognition of sedimentological features. The roles of weakly supervised, semi-supervised, and unsupervised learning in reducing dependence on manual annotation are further discussed. The potential of generative models in data augmentation, class balancing, and rare-sample supplementation is also analyzed. In addition, the development trends of vision foundation models, large language models, and multimodal frameworks are examined in terms of geological knowledge integration, model generalization, and interpretive consistency. Intelligent mineral identification is shifting from the application of single models toward an integrated analytical paradigm that combines low-label learning, generative models, and multimodal large models. Future research should strengthen the construction of standardized rock thin-section datasets, the incorporation of geological prior knowledge, cross-domain model adaptation, and interpretability evaluation. A sedimentary rock intelligent analysis system with high generalization ability and geological interpretive consistency is expected to provide efficient and reliable technical support for sedimentological research.
Accurate identification of mineral composition and microstructural features in sedimentary rock thin sections is an important basis for provenance analysis, reconstruction of depositional environments, and reservoir evaluation. Although traditional manual identification based on polarized light microscopy is highly reliable, it is limited by low efficiency, strong subjectivity, and insufficient standardization. Therefore, it is difficult to meet the growing demand for high-throughput, quantitative, and reproducible analysis in data-driven sedimentological research. Focusing on the development history and technical evolution of intelligent mineral identification in sedimentary rocks, recent progress in traditional image processing, machine learning, and deep learning methods for rock thin-section image analysis is systematically summarized. The applications of convolutional neural networks and Vision Transformers in image classification, object detection, and image segmentation are reviewed, with particular emphasis on their adaptability to the recognition of sedimentological features. The roles of weakly supervised, semi-supervised, and unsupervised learning in reducing dependence on manual annotation are further discussed. The potential of generative models in data augmentation, class balancing, and rare-sample supplementation is also analyzed. In addition, the development trends of vision foundation models, large language models, and multimodal frameworks are examined in terms of geological knowledge integration, model generalization, and interpretive consistency. Intelligent mineral identification is shifting from the application of single models toward an integrated analytical paradigm that combines low-label learning, generative models, and multimodal large models. Future research should strengthen the construction of standardized rock thin-section datasets, the incorporation of geological prior knowledge, cross-domain model adaptation, and interpretability evaluation. A sedimentary rock intelligent analysis system with high generalization ability and geological interpretive consistency is expected to provide efficient and reliable technical support for sedimentological research.
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[Objective] Forearc basins along active continental margins can effectively preserve important sedimentary records of the tectonic, magmatic, and metamorphic evolution of adjacent magmatic arcs. The Hengdan Group exposed in the Bikou area at the northwestern margin of the Yangtze Block, serves as a well-preserved Neoproterozoic forearc-basin succession. However, its response to arc-related tectono-magmatic processes in adjacent areas remains poorly constrained. [Methods] In situ U–Pb dating and trace-element analyses were conducted on detrital apatite from sedimentary rocks of the lower Baiyang Formation and middle-upper Yangtianba Formation of the Hengdan Group. These data were combined with genetic-type discrimination of apatite to reveal changes in provenance composition and their response to the Neoproterozoic subduction–accretion evolution along the northwestern margin of the Yangtze Block. [Results] Detrital apatite ages from the Hengdan Group are dominated by Neoproterozoic populations, with major age peaks at approximately 810 Ma and 770 Ma for the Baiyang and Yangtianba formations, respectively. The Baiyang Formation contains relatively more Paleoproterozoic to Mesoproterozoic detrital components, indicating a greater contribution from ancient basement materials during its deposition. Based on apatite trace-element characteristics and SVM classification results, detrital apatite from the Hengdan Group is dominated by I-type granitoids and mafic igneous rocks (IM), with ages mainly concentrated between ca. 740 and 860 Ma, corresponding to regional Neoproterozoic arc magmatism. High-grade metamorphic apatite (HM) shows age peaks concentrated at 800–900 Ma, which may record the exhumation and input of regional high-grade metamorphic basement during arc–continent convergence. Low- to medium-grade metamorphic and metasomatic apatite (LM) displays two age groups of 800–1000 Ma and 600–700 Ma. The former may be related to low- to medium-grade metamorphism under a prolonged early Neoproterozoic subduction setting, whereas the latter may record the influence of late Neoproterozoic intraplate magmatic thermal events. The age peak of IM-type detrital apatite youngs from ca. 810 Ma in the Baiyang Formation to ca. 770 Ma in the Yangtianba Formation. Together with the age-related variations in Sr/Mn and Sr/Y ratios, this trend indicates that the provenance of the basin shifted from relatively diverse sources to a dominance of young arc-derived magmatic materials during deposition of the Hengdan Group, and that the magmatic arc itself evolved from a juvenile arc system into a more mature continental arc system. [Conclusion] Integrated detrital apatite U-Pb geochronology and trace-element characteristics suggest that the main deposition of the Hengdan Group most likely occurred during the late stage of active continental-margin forearc-basin development under a long-lived Neoproterozoic subduction–accretionary setting along the northwestern margin of the Yangtze Block, or during the early stage of transition to post-orogenic extension environment. The maximum depositional age of the Hengdan Group should be close to 720 Ma, and the succession was subsequently overprinted by late Neoproterozoic regional thermal events.
[Objective] Forearc basins along active continental margins can effectively preserve important sedimentary records of the tectonic, magmatic, and metamorphic evolution of adjacent magmatic arcs. The Hengdan Group exposed in the Bikou area at the northwestern margin of the Yangtze Block, serves as a well-preserved Neoproterozoic forearc-basin succession. However, its response to arc-related tectono-magmatic processes in adjacent areas remains poorly constrained. [Methods] In situ U–Pb dating and trace-element analyses were conducted on detrital apatite from sedimentary rocks of the lower Baiyang Formation and middle-upper Yangtianba Formation of the Hengdan Group. These data were combined with genetic-type discrimination of apatite to reveal changes in provenance composition and their response to the Neoproterozoic subduction–accretion evolution along the northwestern margin of the Yangtze Block. [Results] Detrital apatite ages from the Hengdan Group are dominated by Neoproterozoic populations, with major age peaks at approximately 810 Ma and 770 Ma for the Baiyang and Yangtianba formations, respectively. The Baiyang Formation contains relatively more Paleoproterozoic to Mesoproterozoic detrital components, indicating a greater contribution from ancient basement materials during its deposition. Based on apatite trace-element characteristics and SVM classification results, detrital apatite from the Hengdan Group is dominated by I-type granitoids and mafic igneous rocks (IM), with ages mainly concentrated between ca. 740 and 860 Ma, corresponding to regional Neoproterozoic arc magmatism. High-grade metamorphic apatite (HM) shows age peaks concentrated at 800–900 Ma, which may record the exhumation and input of regional high-grade metamorphic basement during arc–continent convergence. Low- to medium-grade metamorphic and metasomatic apatite (LM) displays two age groups of 800–1000 Ma and 600–700 Ma. The former may be related to low- to medium-grade metamorphism under a prolonged early Neoproterozoic subduction setting, whereas the latter may record the influence of late Neoproterozoic intraplate magmatic thermal events. The age peak of IM-type detrital apatite youngs from ca. 810 Ma in the Baiyang Formation to ca. 770 Ma in the Yangtianba Formation. Together with the age-related variations in Sr/Mn and Sr/Y ratios, this trend indicates that the provenance of the basin shifted from relatively diverse sources to a dominance of young arc-derived magmatic materials during deposition of the Hengdan Group, and that the magmatic arc itself evolved from a juvenile arc system into a more mature continental arc system. [Conclusion] Integrated detrital apatite U-Pb geochronology and trace-element characteristics suggest that the main deposition of the Hengdan Group most likely occurred during the late stage of active continental-margin forearc-basin development under a long-lived Neoproterozoic subduction–accretionary setting along the northwestern margin of the Yangtze Block, or during the early stage of transition to post-orogenic extension environment. The maximum depositional age of the Hengdan Group should be close to 720 Ma, and the succession was subsequently overprinted by late Neoproterozoic regional thermal events.
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Abstract：[Objective] The Upper Permian Changhsing Formation in the Mianyang–Guang'an Trough, Sichuan Basin, is characterized by deeply buried limestone reservoirs. Clarifying the depositional and diagenetic evolution processes can provide new geological insights for deep carbonate exploration in the Sichuan Basin. [Methods] Based on core and drilling data, petrological and geochemical investigations were conducted on the reef- and shoal-bearing limestone reservoirs within Member 2 of the Changhsing Formation in the Mianyang–Guang'an Trough. [Results] Reservoirs in the study area are developed in the 2nd Member of the Changhsing Formation. The lithofacies, including sponge framestone, sponge dolomitic framesone, sponge doloframestone, foraminifer/crinoid bioclastic grainstone, packstone and wackestone, indicate sedimentary environments of reef, bioclast shoal, and inter-shoal settings. Fabric-selective dissolution along fusulinid foraminifer shells is the dominant mechanism for porosity formation, which occurred under meteoric leaching conditions associated with short-term subaerial exposure. In terms of δ13C compositions, calcites from various lithofacies show values consistent with coeval seawater, reflecting formation in an early seawater-dominated diagenetic environment. In contrast, the relationships between δ18O values in calcite and dolomite contents, and between stratigraphic distribution of dolomite and sea-level variations, suggest penecontemporaneous dolomitization, which occurred under intensified evaporative conditions during sea-level lowstands. These limestone reservoirs evolved into relatively closed diagenetic systems during the Induan. Based on the integration of sedimentary and diagenetic processes, three sedimentary-diagenetic facies associations (FA1, FA2, and FA3) can be identified, with FA2 being the best limestone reservoir type. [Conclusions] The formation of effective limestone reservoirs with a deeply burial depth can be attributed to the interplay between deposition and penecontemporaneous diagenesis, which are controlled by sea-level fluctuations. Therefore, “sedimentary-diagenetic facies associations” are of importance with aspects of finding deeply buried limestone reservoirs.
Abstract：[Objective] The Upper Permian Changhsing Formation in the Mianyang–Guang'an Trough, Sichuan Basin, is characterized by deeply buried limestone reservoirs. Clarifying the depositional and diagenetic evolution processes can provide new geological insights for deep carbonate exploration in the Sichuan Basin. [Methods] Based on core and drilling data, petrological and geochemical investigations were conducted on the reef- and shoal-bearing limestone reservoirs within Member 2 of the Changhsing Formation in the Mianyang–Guang'an Trough. [Results] Reservoirs in the study area are developed in the 2nd Member of the Changhsing Formation. The lithofacies, including sponge framestone, sponge dolomitic framesone, sponge doloframestone, foraminifer/crinoid bioclastic grainstone, packstone and wackestone, indicate sedimentary environments of reef, bioclast shoal, and inter-shoal settings. Fabric-selective dissolution along fusulinid foraminifer shells is the dominant mechanism for porosity formation, which occurred under meteoric leaching conditions associated with short-term subaerial exposure. In terms of δ13C compositions, calcites from various lithofacies show values consistent with coeval seawater, reflecting formation in an early seawater-dominated diagenetic environment. In contrast, the relationships between δ18O values in calcite and dolomite contents, and between stratigraphic distribution of dolomite and sea-level variations, suggest penecontemporaneous dolomitization, which occurred under intensified evaporative conditions during sea-level lowstands. These limestone reservoirs evolved into relatively closed diagenetic systems during the Induan. Based on the integration of sedimentary and diagenetic processes, three sedimentary-diagenetic facies associations (FA1, FA2, and FA3) can be identified, with FA2 being the best limestone reservoir type. [Conclusions] The formation of effective limestone reservoirs with a deeply burial depth can be attributed to the interplay between deposition and penecontemporaneous diagenesis, which are controlled by sea-level fluctuations. Therefore, “sedimentary-diagenetic facies associations” are of importance with aspects of finding deeply buried limestone reservoirs.
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[Objective] Girvanella, a cyanobacterial fossil commonly found in Cambrian carbonates, plays a crucial role in the formation of many microbialites. Despite considerable advancements in elucidating calcifying Girvanella and its biomineralization mechanisms, the understanding of iron-mineralized Girvanella and its iron-mineralization mechanism is still relatively vague. [Methods]This paper explores the structural characteristics of Girvanella and its iron mineralization in ferruginous microbialites from the Mantou Formation Member Ⅲ (Cambrian Miaolingian Series) of Weihui area, northern Henan through an integrated analysis of sedimentology, mineralogy and geochemistry. [Results]The results indicate that the lamina or clots of ferruginous microbialites in study area are dominated by densely distributed Girvanella. The tube core of Girvanella is primarily composed of crystalline calcite crystals, whereas the tube wall of Girvanella is composed of a large number of sheet-like hematite crystals with diameter of 0.1-0.5 μm and random distribution. [Conclusions]Through CO2-concentrating mechanism, Girvanella generates a large number of anions in its surroundings，which adsorb Fe3? cations along with Al、Ca 、Mg and Si, facilitating nucleation on Girvanella EPS, leading to the formation of cryptocrystalline hematites. This kind of microbially induced hematite is significantly much complex in mineral morphology and elemental composition than chemically precipitated hematite. The co-occurrence of cryptocrystalline hematite minerals formed by the iron mineralization of Girvanella and idiomorphic chamosite crystals formed by chemical precipitation within ferruginous microbialites indicate that there are fluctuating changes in the redox gradient and iron mineral phase, as well as involving the coexistence of ferric and ferrous minerals within the cyanobacterial microbial mat（film）.
[Objective] Girvanella, a cyanobacterial fossil commonly found in Cambrian carbonates, plays a crucial role in the formation of many microbialites. Despite considerable advancements in elucidating calcifying Girvanella and its biomineralization mechanisms, the understanding of iron-mineralized Girvanella and its iron-mineralization mechanism is still relatively vague. [Methods]This paper explores the structural characteristics of Girvanella and its iron mineralization in ferruginous microbialites from the Mantou Formation Member Ⅲ (Cambrian Miaolingian Series) of Weihui area, northern Henan through an integrated analysis of sedimentology, mineralogy and geochemistry. [Results]The results indicate that the lamina or clots of ferruginous microbialites in study area are dominated by densely distributed Girvanella. The tube core of Girvanella is primarily composed of crystalline calcite crystals, whereas the tube wall of Girvanella is composed of a large number of sheet-like hematite crystals with diameter of 0.1-0.5 μm and random distribution. [Conclusions]Through CO2-concentrating mechanism, Girvanella generates a large number of anions in its surroundings，which adsorb Fe3? cations along with Al、Ca 、Mg and Si, facilitating nucleation on Girvanella EPS, leading to the formation of cryptocrystalline hematites. This kind of microbially induced hematite is significantly much complex in mineral morphology and elemental composition than chemically precipitated hematite. The co-occurrence of cryptocrystalline hematite minerals formed by the iron mineralization of Girvanella and idiomorphic chamosite crystals formed by chemical precipitation within ferruginous microbialites indicate that there are fluctuating changes in the redox gradient and iron mineral phase, as well as involving the coexistence of ferric and ferrous minerals within the cyanobacterial microbial mat（film）.
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[Objective]The Hechuan-Tongnan block in the central Sichuan Basin has drilled multiple high-yield wells in the second member of the Permian Maokou Formation dolostones, revealing significant exploration prospects. However, the characteristics and genesis of dolomite reservoirs remain unclear, hindering the prediction of favorable reservoir distribution. [Methods]Based on observations of cores and thin sections, combined with geochemical methods such as carbon-oxygen isotopes, rare earth elements, and carbonate U-Pb dating, a systematic study was conducted on the genesis and controlling factors of the second member of the Maokou dolomite reservoir. [Results]bioclastic-bearing micrite limestone, micrite bioclastic limestone, dolomitic limestone, and grain-bearing dolomite. Among these, grain-bearing dolomite is the primary reservoir rock type. There are three types of reservoir spaces: intergranular dissolution pores, dissolution pores, and dissolution fractures, with dissolution pores being the most developed. The formation of dolomite occurred during the quasi-syngenetic shallow burial period, with early matrix dolomite and late saddle dolomite formed at 262 ± 16 Ma and 259.1 ± 4.7 Ma, when high Mg/Ca ratio seawater infiltrated and flowed back into the porous beach sediments in the confined environment between the shoals, leading to dolomitization.[Conclusions]The reservoir is controlled by sedimentary facies, early dolomitization, and burial hydrothermal dissolution related to structural fractures. The high-energy particle beach facies in the elevated areas of paloe-morphology provide the basis for reservoir development. Dolomitization during the quasi-syngenetic shallow burial period created conditions for the preservation of early pores and their transformation during the mid-deep burial period. The hydrothermal dissolution process related to the Emei taphrogenesis leads to the heterogeneous development of dissolution pores, which is key to reservoir development.
[Objective]The Hechuan-Tongnan block in the central Sichuan Basin has drilled multiple high-yield wells in the second member of the Permian Maokou Formation dolostones, revealing significant exploration prospects. However, the characteristics and genesis of dolomite reservoirs remain unclear, hindering the prediction of favorable reservoir distribution. [Methods]Based on observations of cores and thin sections, combined with geochemical methods such as carbon-oxygen isotopes, rare earth elements, and carbonate U-Pb dating, a systematic study was conducted on the genesis and controlling factors of the second member of the Maokou dolomite reservoir. [Results]bioclastic-bearing micrite limestone, micrite bioclastic limestone, dolomitic limestone, and grain-bearing dolomite. Among these, grain-bearing dolomite is the primary reservoir rock type. There are three types of reservoir spaces: intergranular dissolution pores, dissolution pores, and dissolution fractures, with dissolution pores being the most developed. The formation of dolomite occurred during the quasi-syngenetic shallow burial period, with early matrix dolomite and late saddle dolomite formed at 262 ± 16 Ma and 259.1 ± 4.7 Ma, when high Mg/Ca ratio seawater infiltrated and flowed back into the porous beach sediments in the confined environment between the shoals, leading to dolomitization.[Conclusions]The reservoir is controlled by sedimentary facies, early dolomitization, and burial hydrothermal dissolution related to structural fractures. The high-energy particle beach facies in the elevated areas of paloe-morphology provide the basis for reservoir development. Dolomitization during the quasi-syngenetic shallow burial period created conditions for the preservation of early pores and their transformation during the mid-deep burial period. The hydrothermal dissolution process related to the Emei taphrogenesis leads to the heterogeneous development of dissolution pores, which is key to reservoir development.
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[Objective] The sedimentary environment evolution and organic matter enrichment mechanisms of the Datangpo Formation in northeastern Guizhou lack systematic investigation. This study aims to reconstruct the paleoenvironmental evolution sequence and clarify the main controlling factors of organic matter enrichment through high-resolution geochemical analysis. [Methods] A total of 43 core samples from the Datangpo Formation in Well KZ8 in northeastern Guizhou were systematically analyzed for total organic carbon (TOC) and major and trace element geochemistry. The samples cover Member I (manganese carbonate rocks and manganese-bearing shales), Member II (black shales), and Member III (grey shales). [Results] The study reveals that Member I of the Datangpo Formation was deposited under a background of limited post-glacial climatic warming, within a moderately to strongly restricted basin. This interval is characterized by high-frequency alternations between ferruginous and sulfidic reducing conditions, accompanied by significant hydrothermal activity and intermittent marine incursions. The relatively high TOC content in the manganese carbonate sub-member was primarily controlled by high productivity driven by hydrothermal and seawater-derived nutrient inputs. In contrast, productivity in the manganese-bearing shales sub-member declined due to limited nutrient supply, yet its TOC content did not decrease significantly owing to effective preservation under sulfidic conditions. Additionally, some organic matter in Member I was consumed during manganese mineralization. Member II was deposited in a strongly restricted environment under warm climatic conditions, featuring persistent sulfidic reduction and high terrigenous input. The efficient coupling between productivity sustained by enhanced terrigenous nutrients and superior preservation conditions within the strongly restricted sulfidic water column led to peak TOC content. Member III, however, exhibits very low TOC content due to collapsed productivity and completely oxic water conditions. [Conclusion] This study clarifies the pattern of organic matter enrichment mechanisms co-controlled by “productivity-preservation conditions” and their evolution alongside environmental changes in the study area, providing direct insights for regional shale gas exploration. These findings are based on data from a single well; thus, their regional representativeness requires further verification through additional drilling. Moreover, the quantitative contributions of various environmental factors to TOC have not been precisely determined. Future work should focus on establishing a more universal enrichment model.
[Objective] The sedimentary environment evolution and organic matter enrichment mechanisms of the Datangpo Formation in northeastern Guizhou lack systematic investigation. This study aims to reconstruct the paleoenvironmental evolution sequence and clarify the main controlling factors of organic matter enrichment through high-resolution geochemical analysis. [Methods] A total of 43 core samples from the Datangpo Formation in Well KZ8 in northeastern Guizhou were systematically analyzed for total organic carbon (TOC) and major and trace element geochemistry. The samples cover Member I (manganese carbonate rocks and manganese-bearing shales), Member II (black shales), and Member III (grey shales). [Results] The study reveals that Member I of the Datangpo Formation was deposited under a background of limited post-glacial climatic warming, within a moderately to strongly restricted basin. This interval is characterized by high-frequency alternations between ferruginous and sulfidic reducing conditions, accompanied by significant hydrothermal activity and intermittent marine incursions. The relatively high TOC content in the manganese carbonate sub-member was primarily controlled by high productivity driven by hydrothermal and seawater-derived nutrient inputs. In contrast, productivity in the manganese-bearing shales sub-member declined due to limited nutrient supply, yet its TOC content did not decrease significantly owing to effective preservation under sulfidic conditions. Additionally, some organic matter in Member I was consumed during manganese mineralization. Member II was deposited in a strongly restricted environment under warm climatic conditions, featuring persistent sulfidic reduction and high terrigenous input. The efficient coupling between productivity sustained by enhanced terrigenous nutrients and superior preservation conditions within the strongly restricted sulfidic water column led to peak TOC content. Member III, however, exhibits very low TOC content due to collapsed productivity and completely oxic water conditions. [Conclusion] This study clarifies the pattern of organic matter enrichment mechanisms co-controlled by “productivity-preservation conditions” and their evolution alongside environmental changes in the study area, providing direct insights for regional shale gas exploration. These findings are based on data from a single well; thus, their regional representativeness requires further verification through additional drilling. Moreover, the quantitative contributions of various environmental factors to TOC have not been precisely determined. Future work should focus on establishing a more universal enrichment model.
Abstract:
Fine-grained sedimentation differs fundamentally from conventional coarse-grained sedimentation in terms of its dynamic mechanisms. It is therefore necessary to move beyond qualitative descriptions and establish quantitative dynamic models specifically for fine-grained materials. Based on the authors’ simulation results, this paper systematically reviews process modelling of fine-grained sediment across three hierarchical levels: (1) The grain scale represented by the floc mean-size model. The Winterwerp flocculation dynamics equation and the influence of fractal dimension are elucidated, revealing the evolution of floc size under the equilibrium of collision–flocculation–breakup processes; (2) The depositional-body scale, represented by Delft3D. Cohesive and noncohesive sediments are differentiated explicitly. It employs the Partheniades–Krone erosion–deposition formulation and salinity-controlled settling velocity, and effectively reproduces the impact of flocculation on deltaic depositional patterns; and (3) The basin scale, represented by Dionisos (Diffusion), Badlands (Stream Power), and Lecode (Marker-in-Cell). They are evaluated in terms of their applicability to fine-grained sediment simulation, parameterization strategies distinguishing fine- and coarse-grained sediments, and their ability to reproduce spatial patterns of various fine-grained depositional processes such as suspended settling, turbidity currents, debris flows, hillslope processes, and fluvial transport. This study deepens the understanding of the intrinsic dynamics of fine-grained sedimentation and provides perspectives for improving the adaptability of numerical models in fine-grained sediment research.
Fine-grained sedimentation differs fundamentally from conventional coarse-grained sedimentation in terms of its dynamic mechanisms. It is therefore necessary to move beyond qualitative descriptions and establish quantitative dynamic models specifically for fine-grained materials. Based on the authors’ simulation results, this paper systematically reviews process modelling of fine-grained sediment across three hierarchical levels: (1) The grain scale represented by the floc mean-size model. The Winterwerp flocculation dynamics equation and the influence of fractal dimension are elucidated, revealing the evolution of floc size under the equilibrium of collision–flocculation–breakup processes; (2) The depositional-body scale, represented by Delft3D. Cohesive and noncohesive sediments are differentiated explicitly. It employs the Partheniades–Krone erosion–deposition formulation and salinity-controlled settling velocity, and effectively reproduces the impact of flocculation on deltaic depositional patterns; and (3) The basin scale, represented by Dionisos (Diffusion), Badlands (Stream Power), and Lecode (Marker-in-Cell). They are evaluated in terms of their applicability to fine-grained sediment simulation, parameterization strategies distinguishing fine- and coarse-grained sediments, and their ability to reproduce spatial patterns of various fine-grained depositional processes such as suspended settling, turbidity currents, debris flows, hillslope processes, and fluvial transport. This study deepens the understanding of the intrinsic dynamics of fine-grained sedimentation and provides perspectives for improving the adaptability of numerical models in fine-grained sediment research.
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Abstract: [Objective] Glycerol dialkyl glycerol tetraethers (GDGTs) are chemically stable and can be preserved for long periods under natural conditions. Their composition and distribution show systematic responses to environmental conditions and have therefore been widely applied in recent years to paleoenviron-mental and paleoelevation reconstructions. The southeastern margin of the Tibetan Plateau is characterized by complex topography and highly variable climate, and the applicability of temperature and pH calibration equations developed from a single basin remains highly uncertain at the regional scale. Consequently, it is necessary to investigate the distribution patterns of branched GDGTs (brGDGTs) and their responses to temperature and pH across multiple basins in this region, to establish robust regional calibration equations that provide modern-process constraints for accurate quantitative reconstructions of regional paleotemperature and paleoelevation using sedimentary brGDGTs.[Methods] A total of 56 surface soil samples were collected from nine basins (Maqu, Gannan, Zoigê, Songpan, Nangqên, Leiwuqi, Gongjue, Mangkang, and Daocheng) on the southeastern margin of the Tibetan Plateau. brGDGTs were systematically analyzed and combined with instrumentally measured mean annual air temperature (MAAT) and mean annual precipitation (MAP) from meteorological stations, as well as measured soil pH data. Pearson correlation analysis was used to compare and evaluate published proxy indices, and stepwise regression was applied to develop regional calibration equations for temperature and pH.[Results] brGDGTs in surface soils of the study area are dominated by Ia, IIa, and IIIa, with 5-methyl brGDGTs accounting for the highest proportion. The regional MAAT calibration model established via stepwise regression shows high reconstruction accuracy within the low-temperature range (approximately ?4°C to 4°C). Among indices based on Tibetan Plateau surface soils, CBT'' exhibits the strongest correlation with measured pH.[Conclusions] The composition and distribution of surface-soil brGDGTs on the southeastern margin of the Tibetan Plateau are jointly controlled by temperature and pH, and global- or regional-scale calibration equations cannot be directly applied to this environment. The newly developed regional temperature and pH calibration equations show better applicability in high-altitude, cold environments. These results provide critical scientific constraints for developing and applying sedimentary brGDGTs to quantitatively reconstruct the evolutionary history of paleotemperature and paleoelevation on the Tibetan Plateau.
Abstract: [Objective] Glycerol dialkyl glycerol tetraethers (GDGTs) are chemically stable and can be preserved for long periods under natural conditions. Their composition and distribution show systematic responses to environmental conditions and have therefore been widely applied in recent years to paleoenviron-mental and paleoelevation reconstructions. The southeastern margin of the Tibetan Plateau is characterized by complex topography and highly variable climate, and the applicability of temperature and pH calibration equations developed from a single basin remains highly uncertain at the regional scale. Consequently, it is necessary to investigate the distribution patterns of branched GDGTs (brGDGTs) and their responses to temperature and pH across multiple basins in this region, to establish robust regional calibration equations that provide modern-process constraints for accurate quantitative reconstructions of regional paleotemperature and paleoelevation using sedimentary brGDGTs.[Methods] A total of 56 surface soil samples were collected from nine basins (Maqu, Gannan, Zoigê, Songpan, Nangqên, Leiwuqi, Gongjue, Mangkang, and Daocheng) on the southeastern margin of the Tibetan Plateau. brGDGTs were systematically analyzed and combined with instrumentally measured mean annual air temperature (MAAT) and mean annual precipitation (MAP) from meteorological stations, as well as measured soil pH data. Pearson correlation analysis was used to compare and evaluate published proxy indices, and stepwise regression was applied to develop regional calibration equations for temperature and pH.[Results] brGDGTs in surface soils of the study area are dominated by Ia, IIa, and IIIa, with 5-methyl brGDGTs accounting for the highest proportion. The regional MAAT calibration model established via stepwise regression shows high reconstruction accuracy within the low-temperature range (approximately ?4°C to 4°C). Among indices based on Tibetan Plateau surface soils, CBT'' exhibits the strongest correlation with measured pH.[Conclusions] The composition and distribution of surface-soil brGDGTs on the southeastern margin of the Tibetan Plateau are jointly controlled by temperature and pH, and global- or regional-scale calibration equations cannot be directly applied to this environment. The newly developed regional temperature and pH calibration equations show better applicability in high-altitude, cold environments. These results provide critical scientific constraints for developing and applying sedimentary brGDGTs to quantitatively reconstruct the evolutionary history of paleotemperature and paleoelevation on the Tibetan Plateau.
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In order to deeply study the development process and sedimentary model of the distal shallow-water braided river delta, based on the modern sedimentary hydrodynamic conditions, the initial model and boundary conditions of the hydrodynamic depositional numerical simulation were designed. The sedimentary process of the distal shallow-water braided river delta was numerically simulated. The characteristics of the sedimentary thickness and flow velocity were analyzed. The results of the hydrodynamic depositional numerical simulation and the modern sedimentary examples were compared and analyzed. The controlling effect of distributary channels of different levels on the proliferation of shallow-water braided river delta. The sedimentary model based on the classification of distributary channel were summarized. The results show that: 1) in a complete flood-interflood
Abstract:
Abstract:[Objective]The northern foothills of the Qinling Mountains lie in the transitional zone between China's northern and southern geomorphological units, serving as a critical link between the Loess Plateau and the Qinling orogenic belt. Conducting research on magnetic characteristics of river sediments in this region holds significant scientific value and theoretical implications for understanding regional environmental evolution processes, sedimentary response mechanisms, and exploring basin-wide environmental change patterns. [Methods]This study focused on the floodplain sediment profile of the lower Ba Wang River in the northern Qinling foothills. Through systematic environmental magnetism analysis combined with principal component analysis (PCA), we investigated magnetic characteristics and dominant mechanisms across different sedimentary stages. [Results]The sedimentary profile exhibited distinct differentiation at the 124cm depth marker, with generally weak magnetic properties. In the lower section of Stage I (308-124 cm), the sediments exhibit relatively weak magnetism, with magnetic minerals primarily consisting of hematite and siderite, along with minor magnetite, with magnetic particle sizes gradually coarsening into multi-domain (MD) particles. Stage Ⅱ (124-24cm) sediments showed significantly enhanced magnetism, primarily consisting of small sized pseudo-single domain (PSD) magnetite and magnetite, alongside minor hematite content. PCA results further confirmed stratification features: the cumulative contribution rate of two principal components ferromagnetic mineral content and magnetic particle size reached 89%. Variations in magnetite content within ferromagnetic minerals were the dominant factor explaining magnetic differences across sedimentary stages. [Conclusion]Stage I sediments remained submerged in a reducing environment for prolonged periods, leading to dissolution of magnetite under early diagenetic processes. Stage Ⅱ sediments, exposed to an oxidizing environment, exhibited magnetic characteristics shaped by hydrodynamic sorting and weak soil-forming processes. This study enhances our understanding of factors influencing sediment diagenesis, providing crucial reference materials for analyzing the characteristics of sedimentary environment changes and climatic environmental variations in the Wei River Basin.
Abstract:[Objective]The northern foothills of the Qinling Mountains lie in the transitional zone between China's northern and southern geomorphological units, serving as a critical link between the Loess Plateau and the Qinling orogenic belt. Conducting research on magnetic characteristics of river sediments in this region holds significant scientific value and theoretical implications for understanding regional environmental evolution processes, sedimentary response mechanisms, and exploring basin-wide environmental change patterns. [Methods]This study focused on the floodplain sediment profile of the lower Ba Wang River in the northern Qinling foothills. Through systematic environmental magnetism analysis combined with principal component analysis (PCA), we investigated magnetic characteristics and dominant mechanisms across different sedimentary stages. [Results]The sedimentary profile exhibited distinct differentiation at the 124cm depth marker, with generally weak magnetic properties. In the lower section of Stage I (308-124 cm), the sediments exhibit relatively weak magnetism, with magnetic minerals primarily consisting of hematite and siderite, along with minor magnetite, with magnetic particle sizes gradually coarsening into multi-domain (MD) particles. Stage Ⅱ (124-24cm) sediments showed significantly enhanced magnetism, primarily consisting of small sized pseudo-single domain (PSD) magnetite and magnetite, alongside minor hematite content. PCA results further confirmed stratification features: the cumulative contribution rate of two principal components ferromagnetic mineral content and magnetic particle size reached 89%. Variations in magnetite content within ferromagnetic minerals were the dominant factor explaining magnetic differences across sedimentary stages. [Conclusion]Stage I sediments remained submerged in a reducing environment for prolonged periods, leading to dissolution of magnetite under early diagenetic processes. Stage Ⅱ sediments, exposed to an oxidizing environment, exhibited magnetic characteristics shaped by hydrodynamic sorting and weak soil-forming processes. This study enhances our understanding of factors influencing sediment diagenesis, providing crucial reference materials for analyzing the characteristics of sedimentary environment changes and climatic environmental variations in the Wei River Basin.
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[Objective] Braided river deltas are one of the most important types of hydrocarbon reservoirs. Extensive studies have been conducted on their depositional processes, evolutionary characteristics, and controlling factors. However, how key boundary conditions quantitatively control the growth, evolution, and genetic sandbody characteristics of lacustrine braided river deltas still requires further investigation.[Methods] UThe Delft3D sedimentary numerical simulation software was employed to design a series of single-factor comparative experiments involving five key boundary conditions, including lake-inflow discharge, basin slope, total sediment concentration, sand–mud ratio, and lake-level variation. By quantitatively extracting relevant indicators, the differences in depositional characteristics under different parameter settings were systematically analyzed. [Results](1) Lake-inflow discharge shows a significant positive correlation with delta area and sandbody scale. As discharge increases, distributary bifurcation becomes more active, the number of distributary mouth bars increases, and chute channels on bar tops become more developed.(2) Basin slope mainly controls the lateral and vertical distribution of sediments. With increasing slope, the delta changes from a broad and gentle morphology to a narrow and thick one, lateral bifurcation is suppressed, and distributary mouth bars become fewer but thicker.(3) An increase in total sediment concentration accelerates the rapid progradation of the delta front during the middle to late stages of simulated evolution. However, under higher concentration conditions, accelerated frontal deposition tends to promote channel siltation and abandonment, thereby reducing both the frequency and number of channel bifurcations.(4) A lower sand–mud ratio promotes the distal transport of fine-grained sediments, resulting in greater elongation in the source-to-sink direction and a more divergent channel pattern.(5) Lake-level fall triggers rapid progradation, forming depositional bodies with larger areal extent, deeper channel incision, and better-developed chute channels, whereas lake-level rise leads to enhanced vertical aggradation and planar shrinkage. [Conclusions] Strong hydrodynamic conditions and lake-level fall are favorable for the development of dominant sandbodies interconnected by chute channels. Steeper slopes and higher sediment-concentration settings tend to form thicker sandbodies with better extension in the sourceward direction. These experimental results provide a theoretical basis for reservoir prediction of braided river deltas in continental basins.
[Objective] Braided river deltas are one of the most important types of hydrocarbon reservoirs. Extensive studies have been conducted on their depositional processes, evolutionary characteristics, and controlling factors. However, how key boundary conditions quantitatively control the growth, evolution, and genetic sandbody characteristics of lacustrine braided river deltas still requires further investigation.[Methods] UThe Delft3D sedimentary numerical simulation software was employed to design a series of single-factor comparative experiments involving five key boundary conditions, including lake-inflow discharge, basin slope, total sediment concentration, sand–mud ratio, and lake-level variation. By quantitatively extracting relevant indicators, the differences in depositional characteristics under different parameter settings were systematically analyzed. [Results](1) Lake-inflow discharge shows a significant positive correlation with delta area and sandbody scale. As discharge increases, distributary bifurcation becomes more active, the number of distributary mouth bars increases, and chute channels on bar tops become more developed.(2) Basin slope mainly controls the lateral and vertical distribution of sediments. With increasing slope, the delta changes from a broad and gentle morphology to a narrow and thick one, lateral bifurcation is suppressed, and distributary mouth bars become fewer but thicker.(3) An increase in total sediment concentration accelerates the rapid progradation of the delta front during the middle to late stages of simulated evolution. However, under higher concentration conditions, accelerated frontal deposition tends to promote channel siltation and abandonment, thereby reducing both the frequency and number of channel bifurcations.(4) A lower sand–mud ratio promotes the distal transport of fine-grained sediments, resulting in greater elongation in the source-to-sink direction and a more divergent channel pattern.(5) Lake-level fall triggers rapid progradation, forming depositional bodies with larger areal extent, deeper channel incision, and better-developed chute channels, whereas lake-level rise leads to enhanced vertical aggradation and planar shrinkage. [Conclusions] Strong hydrodynamic conditions and lake-level fall are favorable for the development of dominant sandbodies interconnected by chute channels. Steeper slopes and higher sediment-concentration settings tend to form thicker sandbodies with better extension in the sourceward direction. These experimental results provide a theoretical basis for reservoir prediction of braided river deltas in continental basins.
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Abstract: [Objective] Tight gas sandstone within the Shanxi Formation is a significant gas-producing zone in the northern Ordos Basin. However, the variations in the vertical stacking patterns and lateral distribution of sandbodies across different intervals of the Shanxi Formation pose challenges for development well placement and gas production area selection. [Methods] This study establishes a three-dimensional digital outcrop model of the Shanxi Formation through measured stratigraphic sections and drone oblique photography modeling. Based on lithofacies classification and hierarchical bounding surface identification, sandbody architecture analysis was conducted. [Results]The results demonstrate that:①The Shanxi Formation represents a delta plain depositional environment, where sandbodies primarily correspond to the braided distributary channel microfacies. Under warm and humid climatic conditions, well-developed vegetation restricted crevasse formation, with overbank flooding (splay deposition) being the dominant process instead.②Four main types of sandbody architecture are identified: longitudinal bar, transverse bar, incised channel, and laminated sandstone.③The vertical succession of sandbody architectures within the Shanxi Formation, from base to top, is characterized by: composite longitudinal bars and laminated sandstones in the lower part, transverse bars in the middle part, and incised channels in the upper part. [Conclusions]The systematic variation in fluvial channel sandbody architecture within the study area records the gradual closure of the northern Solon Ocean and coeval tectonic uplift in the Ordos Basin's north. This uplift drove the evolution of distributary channels from an initial wide and shallow form, through a wide and deep intermediate stage, to a final narrow and deep, incised morphology. Our study elucidates the response mechanism linking channel evolution, architectural differentiation, and reservoir heterogeneity under coupled tectonic-climatic controls, offering a novel perspective for research on the Shanxi Formation reservoirs in the northeastern Ordos Basin.
Abstract: [Objective] Tight gas sandstone within the Shanxi Formation is a significant gas-producing zone in the northern Ordos Basin. However, the variations in the vertical stacking patterns and lateral distribution of sandbodies across different intervals of the Shanxi Formation pose challenges for development well placement and gas production area selection. [Methods] This study establishes a three-dimensional digital outcrop model of the Shanxi Formation through measured stratigraphic sections and drone oblique photography modeling. Based on lithofacies classification and hierarchical bounding surface identification, sandbody architecture analysis was conducted. [Results]The results demonstrate that:①The Shanxi Formation represents a delta plain depositional environment, where sandbodies primarily correspond to the braided distributary channel microfacies. Under warm and humid climatic conditions, well-developed vegetation restricted crevasse formation, with overbank flooding (splay deposition) being the dominant process instead.②Four main types of sandbody architecture are identified: longitudinal bar, transverse bar, incised channel, and laminated sandstone.③The vertical succession of sandbody architectures within the Shanxi Formation, from base to top, is characterized by: composite longitudinal bars and laminated sandstones in the lower part, transverse bars in the middle part, and incised channels in the upper part. [Conclusions]The systematic variation in fluvial channel sandbody architecture within the study area records the gradual closure of the northern Solon Ocean and coeval tectonic uplift in the Ordos Basin's north. This uplift drove the evolution of distributary channels from an initial wide and shallow form, through a wide and deep intermediate stage, to a final narrow and deep, incised morphology. Our study elucidates the response mechanism linking channel evolution, architectural differentiation, and reservoir heterogeneity under coupled tectonic-climatic controls, offering a novel perspective for research on the Shanxi Formation reservoirs in the northeastern Ordos Basin.
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[Significance] Foamy dolostone is widely distributed in China’s two major oil and gas basins and is closely associated with microbialites. It represents one of the key reservoir rock types and holds significant importance for sedimentological research and carbonate hydrocarbon exploration. [Progress] Previous studies on foamy dolostone have mainly focused on its petrographic and mineralogical characteristics. However, its formation mechanisms and controlling factors remain insufficiently understood and lack systematic investigation. Based on an extensive review of domestic and international literature, combined with the research team’s previous findings, this study systematically summarizes the developmental characteristics and lithological assemblages of foamy dolostone. Comparative analyses of geochemical and reservoir properties from different stratigraphic units in two regions were conducted to elucidate their genetic mechanisms. [Prospects] On this basis, the study identifies existing scientific challenges and outlines future research directions for foamy dolostone. These insights provide new perspectives for advancing the understanding of ancient marine environments and offer theoretical guidance for hydrocarbon exploration in carbonate reservoirs.
[Significance] Foamy dolostone is widely distributed in China’s two major oil and gas basins and is closely associated with microbialites. It represents one of the key reservoir rock types and holds significant importance for sedimentological research and carbonate hydrocarbon exploration. [Progress] Previous studies on foamy dolostone have mainly focused on its petrographic and mineralogical characteristics. However, its formation mechanisms and controlling factors remain insufficiently understood and lack systematic investigation. Based on an extensive review of domestic and international literature, combined with the research team’s previous findings, this study systematically summarizes the developmental characteristics and lithological assemblages of foamy dolostone. Comparative analyses of geochemical and reservoir properties from different stratigraphic units in two regions were conducted to elucidate their genetic mechanisms. [Prospects] On this basis, the study identifies existing scientific challenges and outlines future research directions for foamy dolostone. These insights provide new perspectives for advancing the understanding of ancient marine environments and offer theoretical guidance for hydrocarbon exploration in carbonate reservoirs.
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[Objectives] The End-Triassic Mass Extinction (~201.5 Ma) is one of the “Big Five” mass extinction events of the Phanerozoic and is characterized by a two-pulsed extinction pattern in marine ecosystems. While widespread volcanism associated with the Central Atlantic Magmatic Province (CAMP) and subsequent massive carbon emissions are widely regarded as the principal driving mechanism, the precise spatiotemporal relationship between specific CAMP magmatic phases and the two discrete extinction pulses remains a subject of intense debate. This study aims to clarify whether these pulses were driven by immediate volcanic toxicity or delayed environmental feedbacks triggered by sustained igneous activity. [Methods] To address this, we present high-resolution sedimentary mercury (Hg) records from Dove’s Nest drilling cores, located in the Cleveland Basin (UK). Mercury is utilized here as a sensitive proxy for large-scale volcanic outgassing. To move beyond qualitative descriptions, we employed a coupled mercury?carbon emission model to quantitatively evaluate the degassing processes and the scale of CAMP activity. Furthermore, we utilized potassium/aluminum (K/Al) ratios as a robust proxy for continental weathering intensity. By integrating these geochemical datasets with previous high-resolution biostratigraphy and paleontological records, we investigated the dynamic coupling between volcanic pulses, continental weathering rates, and the staged collapses of marine biodiversity. [Results] Our analysis identifies two distinct phases of Hg enrichment within the Dove’s Nest drilling core. The first enrichment phase occurs at the base of the Cotham Member in the Rhaetian; however, its occurrence is restricted to specific local profiles, suggesting it represents a regional signal rather than a global volcanic event. In contrast, the second Hg enrichment phase spans from the top of the Cotham Member to the Langport Member. This latter phase is characterized by a significant, sustained increase in Hg concentrations that coincides precisely with the first marine extinction pulse and slightly predates the onset of the second pulse. Crucially, this second Hg anomaly is globally correlative, appearing in both marine and terrestrial sections across different paleolatitudes, which confirms its origin from large-scale CAMP activity. [Conclusions] The widespread distribution of sedimentary Hg enrichment across global marine and terrestrial profiles during the late Rhaetian provides strong evidence for massive, episodic CAMP activity. We propose a staggered casual mechanism for the ETME: the initial phase of volcanism triggered the first extinction pulse through direct, short-term mechanisms. These likely included rapid sea-surface temperature spikes and the fallout of toxic substances (e.g., Hg and SO?), leading to an immediate decline in regional and global biodiversity. Subsequently, the sustained and cumulative release of CO? drove prolonged global warming and a significant intensification of the hydrological cycle, as evidenced by elevated K/Al ratios. This intensified continental weathering accelerated the flux of terrestrial nutrients (such as phosphorus) into the oceans, fueling primary productivity and causing the expansion and intensification of marine anoxia, indirectly triggering the second extinction pulse. This study highlights the staged nature of environmental collapse and biological extinction driven by large igneous provinces in deep time, providing an important deep-time analogue for understanding the complex responses of marine ecosystems to ongoing global warming.
[Objectives] The End-Triassic Mass Extinction (~201.5 Ma) is one of the “Big Five” mass extinction events of the Phanerozoic and is characterized by a two-pulsed extinction pattern in marine ecosystems. While widespread volcanism associated with the Central Atlantic Magmatic Province (CAMP) and subsequent massive carbon emissions are widely regarded as the principal driving mechanism, the precise spatiotemporal relationship between specific CAMP magmatic phases and the two discrete extinction pulses remains a subject of intense debate. This study aims to clarify whether these pulses were driven by immediate volcanic toxicity or delayed environmental feedbacks triggered by sustained igneous activity. [Methods] To address this, we present high-resolution sedimentary mercury (Hg) records from Dove’s Nest drilling cores, located in the Cleveland Basin (UK). Mercury is utilized here as a sensitive proxy for large-scale volcanic outgassing. To move beyond qualitative descriptions, we employed a coupled mercury?carbon emission model to quantitatively evaluate the degassing processes and the scale of CAMP activity. Furthermore, we utilized potassium/aluminum (K/Al) ratios as a robust proxy for continental weathering intensity. By integrating these geochemical datasets with previous high-resolution biostratigraphy and paleontological records, we investigated the dynamic coupling between volcanic pulses, continental weathering rates, and the staged collapses of marine biodiversity. [Results] Our analysis identifies two distinct phases of Hg enrichment within the Dove’s Nest drilling core. The first enrichment phase occurs at the base of the Cotham Member in the Rhaetian; however, its occurrence is restricted to specific local profiles, suggesting it represents a regional signal rather than a global volcanic event. In contrast, the second Hg enrichment phase spans from the top of the Cotham Member to the Langport Member. This latter phase is characterized by a significant, sustained increase in Hg concentrations that coincides precisely with the first marine extinction pulse and slightly predates the onset of the second pulse. Crucially, this second Hg anomaly is globally correlative, appearing in both marine and terrestrial sections across different paleolatitudes, which confirms its origin from large-scale CAMP activity. [Conclusions] The widespread distribution of sedimentary Hg enrichment across global marine and terrestrial profiles during the late Rhaetian provides strong evidence for massive, episodic CAMP activity. We propose a staggered casual mechanism for the ETME: the initial phase of volcanism triggered the first extinction pulse through direct, short-term mechanisms. These likely included rapid sea-surface temperature spikes and the fallout of toxic substances (e.g., Hg and SO?), leading to an immediate decline in regional and global biodiversity. Subsequently, the sustained and cumulative release of CO? drove prolonged global warming and a significant intensification of the hydrological cycle, as evidenced by elevated K/Al ratios. This intensified continental weathering accelerated the flux of terrestrial nutrients (such as phosphorus) into the oceans, fueling primary productivity and causing the expansion and intensification of marine anoxia, indirectly triggering the second extinction pulse. This study highlights the staged nature of environmental collapse and biological extinction driven by large igneous provinces in deep time, providing an important deep-time analogue for understanding the complex responses of marine ecosystems to ongoing global warming.
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Abstract：[Objective] The primary controls on high-quality deep clastic reservoirs in the Huangliu Formation (Ledong area, Yinggehai Basin) remain poorly constrained, limiting deep gas exploration. [Methods] We integrate core description, blue-epoxy petrography, SEM, cathodoluminescence, fluid-inclusion homogenization temperatures, PVTx- and equation-based paleo-pressure reconstructions, and compaction simulation to characterize lithology, diagenesis, pore systems, and formation pressure, and to establish a genetic model. [Results and Discussions] Reservoirs are dominated by feldspathic quartz sandstones. Diagenesis is marked by moderate–strong compaction, weak dissolution (local leaching of feldspar grains and bioclasts), and predominantly calcite cementation with subordinate silica. Pore systems are chiefly primary intergranular pores with minor feldspar-dissolution pores. Lithofacies set the baseline porosity and compaction resistance, whereas diagenesis drives petrophysical differentiation: compaction and cementation govern tightness, while dissolution mainly modulates pore type and connectivity with limited net porosity gain. Since ~6 Ma, medium–strong overpressure—generated by undercompaction and hydrocarbon generation and transmitted upward from ~2300 m—suppressed compaction and cementation, preserving early primary porosity. Porosity declines with burial but the rate diminishes below ~3300 m; even at >4000 m, sandstones remain primary-pore dominated, indicating substantial deep-reservoir potential. [Conclusions] We propose a “facies foundation–diagenetic differentiation–overpressure preservation” model, identifying overpressure conditions, medium- to coarse-grained lithofacies, and moderate compaction/cementation as the principal controls. These findings provide a basis for predicting high-quality deep gas reservoirs and support efficient exploration in the deep-water South China Sea. Keywords: Yinggehai Basin; Huangliu Formation; deep reservoirs; reservoir quality; lithofacies; diagenesis; overpressure
Abstract：[Objective] The primary controls on high-quality deep clastic reservoirs in the Huangliu Formation (Ledong area, Yinggehai Basin) remain poorly constrained, limiting deep gas exploration. [Methods] We integrate core description, blue-epoxy petrography, SEM, cathodoluminescence, fluid-inclusion homogenization temperatures, PVTx- and equation-based paleo-pressure reconstructions, and compaction simulation to characterize lithology, diagenesis, pore systems, and formation pressure, and to establish a genetic model. [Results and Discussions] Reservoirs are dominated by feldspathic quartz sandstones. Diagenesis is marked by moderate–strong compaction, weak dissolution (local leaching of feldspar grains and bioclasts), and predominantly calcite cementation with subordinate silica. Pore systems are chiefly primary intergranular pores with minor feldspar-dissolution pores. Lithofacies set the baseline porosity and compaction resistance, whereas diagenesis drives petrophysical differentiation: compaction and cementation govern tightness, while dissolution mainly modulates pore type and connectivity with limited net porosity gain. Since ~6 Ma, medium–strong overpressure—generated by undercompaction and hydrocarbon generation and transmitted upward from ~2300 m—suppressed compaction and cementation, preserving early primary porosity. Porosity declines with burial but the rate diminishes below ~3300 m; even at >4000 m, sandstones remain primary-pore dominated, indicating substantial deep-reservoir potential. [Conclusions] We propose a “facies foundation–diagenetic differentiation–overpressure preservation” model, identifying overpressure conditions, medium- to coarse-grained lithofacies, and moderate compaction/cementation as the principal controls. These findings provide a basis for predicting high-quality deep gas reservoirs and support efficient exploration in the deep-water South China Sea. Keywords: Yinggehai Basin; Huangliu Formation; deep reservoirs; reservoir quality; lithofacies; diagenesis; overpressure
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Abstract: [Significance] The genetic mechanism of river-dominated subaqueous distributary channels in lacustrine deltas has long been controversial, as traditional fluvial sediment dynamics fail to adequately explain their formation and distribution patterns. Based on recent advances in global research, this study systematically investigates the formation mechanism of subaqueous distributary channels in lacustrine deltas and explores new insights from hyperpycnal flow depositional theory regarding deltaic depositional processes and sandbody distribution patterns. [Progress] Hyperpycnal flows originate from the delta-front river mouth zone, transporting terrigenous sediments through the prodelta and shallow lake into deep-water depositional environments. While significant breakthroughs have been made in deep-water hyperpycnal flow deposition, descriptions and interpretations of hyperpycnal flow deposits in delta-shallow lake systems remain limited. The new concept of hyperpycnal deltas integrates river mouth deposition with gravity-driven processes, revealing that hyperpycnal channels serve as critical links between deltas and deep-water deposits. Modern and ancient hyperpycnal flow deposits demonstrate the development of hyperpycnal channels in delta-front to shallow lake or marine environments, suggesting that hyperpycnal flows could be the key dynamic force behind subaqueous distributary channel formation. The erosional mechanisms of these channels are closely associated with supercritical hyperpycnal flows (density Froude exceed the critical value). [Conclusions and Prospects] Subaqueous distributary channels are likely formed by episodic hyperpycnal flows rather than being mere underwater extensions of delta plain distributaries. The finding challenges classical delta theories and highlights flood-generated high-density hyperpycnal flows as a dominant geological force controlling deltaic depositional process. Revising existing models based on hyperpycnal flow dynamics provides a novel theoretical framework for studying delta-front depositional patterns. Future research should integrate quantitative sediment physical simulations with analyses of ancient delta-front deposits to advance hyperpycnal flow dynamics in deltaic systems, offering new theoretical foundations for lacustrine sedimentology.
Abstract: [Significance] The genetic mechanism of river-dominated subaqueous distributary channels in lacustrine deltas has long been controversial, as traditional fluvial sediment dynamics fail to adequately explain their formation and distribution patterns. Based on recent advances in global research, this study systematically investigates the formation mechanism of subaqueous distributary channels in lacustrine deltas and explores new insights from hyperpycnal flow depositional theory regarding deltaic depositional processes and sandbody distribution patterns. [Progress] Hyperpycnal flows originate from the delta-front river mouth zone, transporting terrigenous sediments through the prodelta and shallow lake into deep-water depositional environments. While significant breakthroughs have been made in deep-water hyperpycnal flow deposition, descriptions and interpretations of hyperpycnal flow deposits in delta-shallow lake systems remain limited. The new concept of hyperpycnal deltas integrates river mouth deposition with gravity-driven processes, revealing that hyperpycnal channels serve as critical links between deltas and deep-water deposits. Modern and ancient hyperpycnal flow deposits demonstrate the development of hyperpycnal channels in delta-front to shallow lake or marine environments, suggesting that hyperpycnal flows could be the key dynamic force behind subaqueous distributary channel formation. The erosional mechanisms of these channels are closely associated with supercritical hyperpycnal flows (density Froude exceed the critical value). [Conclusions and Prospects] Subaqueous distributary channels are likely formed by episodic hyperpycnal flows rather than being mere underwater extensions of delta plain distributaries. The finding challenges classical delta theories and highlights flood-generated high-density hyperpycnal flows as a dominant geological force controlling deltaic depositional process. Revising existing models based on hyperpycnal flow dynamics provides a novel theoretical framework for studying delta-front depositional patterns. Future research should integrate quantitative sediment physical simulations with analyses of ancient delta-front deposits to advance hyperpycnal flow dynamics in deltaic systems, offering new theoretical foundations for lacustrine sedimentology.
Abstract:
[Objective] Europium (Eu), a redox-sensitive rare earth element (REE), serves as a proxy for paleo-ocean reconstruction. The Middle Permian Maokou Formation, Member 1 (hereafter Mao-1 Member) in the Sichuan Basin exhibits a characteristic Eu negative anomaly, yet its genesis remains unclear. [Methods] Utilizing integrated approaches including field section measurement, core observation, thin-section petrography, major element, trace element, in-situ micro-scale carbon-oxygen isotope, and rare earth element analyses, the rock types within the Mao-1 Member were classified. The characteristics of the Eu negative anomaly across different lithologies were summarized. Combined with the mechanisms governing REE occurrence and fractionation, a genetic model for the Eu negative anomaly in the sepiolite-containing successions of the Mao-1 Member was established. [Results] The Mao-1 Member primarily consists of rhythmic alternations of mudstone, argillaceous limestone, and limestone. Laminated and lenticular sepiolite predominantly occurs in mudstones and argillaceous limestones, while sporadic sepiolite is common in limestones. All three rock types exhibit REE distribution patterns with Eu negative anomalies, with the anomaly intensity decreasing in the order: mudstone > argillaceous limestone > limestone. Within the cool-water realm, the Eu negative anomaly intensifies with increasing temperature. Vertically, trends in paleoenvironmental proxies (Sr/Cu, ??Sr/??Sr, δ13C, δ1?O) show good synchronicity with δEu. Based on the modes of occurrence and fractionation characteristics, a genetic model for the Eu negative anomaly is proposed. The sepiolite-containing successions of the Mao-1 Member were deposited in a cool-water environment, with paleo-seawater temperatures ranging from 5 to 15°C. In seawater, Eu primarily existed as the carbonate complex [Eu(CO?)]- 2 releasing free Eu3? ions capable of substituting for Ca2? and being adsorbed by both sepiolite and organic matter. [Conclusions] During the limestone depositional stage, with paleo-seawater temperatures ranging from 7.47 to 11.08°C, the low abundance of Eu3? and its low partition coefficient hindered its incorporation into the aragonite lattice, resulting in a Eu negative anomaly. During the argillaceous limestone depositional stage, with paleo-seawater temperatures ranging from 8.37 to 12.62°C, sepiolite precipitated extensively, selectively adsorbing Sm3?/Eu3? to form unstable outer-sphere complexes, while Tb3? was adsorbed to form stable inner-sphere complexes. Furthermore, the quantity of REEs adsorbed by sepiolite far exceeded that incorporated into the carbonate lattice, significantly exacerbating the relative depletion of Eu. During the mudstone depositional stage, with paleo-seawater temperatures ranging from 8.15 to 14.25°C, sepiolite synergistically adsorbed Tb3? with organic matter, further intensifying the Eu negative anomaly. During the diagenetic stage, the stability of the complexes increased, preserving the Eu negative anomaly. Therefore, the Eu negative anomaly in the Mao-1 Member was formed by the differential complexation of REEs by sepiolite within a cool-water environment and was preserved from the depositional stage through the diagenetic stage.
[Objective] Europium (Eu), a redox-sensitive rare earth element (REE), serves as a proxy for paleo-ocean reconstruction. The Middle Permian Maokou Formation, Member 1 (hereafter Mao-1 Member) in the Sichuan Basin exhibits a characteristic Eu negative anomaly, yet its genesis remains unclear. [Methods] Utilizing integrated approaches including field section measurement, core observation, thin-section petrography, major element, trace element, in-situ micro-scale carbon-oxygen isotope, and rare earth element analyses, the rock types within the Mao-1 Member were classified. The characteristics of the Eu negative anomaly across different lithologies were summarized. Combined with the mechanisms governing REE occurrence and fractionation, a genetic model for the Eu negative anomaly in the sepiolite-containing successions of the Mao-1 Member was established. [Results] The Mao-1 Member primarily consists of rhythmic alternations of mudstone, argillaceous limestone, and limestone. Laminated and lenticular sepiolite predominantly occurs in mudstones and argillaceous limestones, while sporadic sepiolite is common in limestones. All three rock types exhibit REE distribution patterns with Eu negative anomalies, with the anomaly intensity decreasing in the order: mudstone > argillaceous limestone > limestone. Within the cool-water realm, the Eu negative anomaly intensifies with increasing temperature. Vertically, trends in paleoenvironmental proxies (Sr/Cu, ??Sr/??Sr, δ13C, δ1?O) show good synchronicity with δEu. Based on the modes of occurrence and fractionation characteristics, a genetic model for the Eu negative anomaly is proposed. The sepiolite-containing successions of the Mao-1 Member were deposited in a cool-water environment, with paleo-seawater temperatures ranging from 5 to 15°C. In seawater, Eu primarily existed as the carbonate complex [Eu(CO?)]- 2 releasing free Eu3? ions capable of substituting for Ca2? and being adsorbed by both sepiolite and organic matter. [Conclusions] During the limestone depositional stage, with paleo-seawater temperatures ranging from 7.47 to 11.08°C, the low abundance of Eu3? and its low partition coefficient hindered its incorporation into the aragonite lattice, resulting in a Eu negative anomaly. During the argillaceous limestone depositional stage, with paleo-seawater temperatures ranging from 8.37 to 12.62°C, sepiolite precipitated extensively, selectively adsorbing Sm3?/Eu3? to form unstable outer-sphere complexes, while Tb3? was adsorbed to form stable inner-sphere complexes. Furthermore, the quantity of REEs adsorbed by sepiolite far exceeded that incorporated into the carbonate lattice, significantly exacerbating the relative depletion of Eu. During the mudstone depositional stage, with paleo-seawater temperatures ranging from 8.15 to 14.25°C, sepiolite synergistically adsorbed Tb3? with organic matter, further intensifying the Eu negative anomaly. During the diagenetic stage, the stability of the complexes increased, preserving the Eu negative anomaly. Therefore, the Eu negative anomaly in the Mao-1 Member was formed by the differential complexation of REEs by sepiolite within a cool-water environment and was preserved from the depositional stage through the diagenetic stage.
Abstract:
[Objectives] The middle to late Early Carboniferous witnessed profound reorganization of Earth’s tectonic paleogeography, climate systems, marine environments, and ecosystems. To refine the initiation timing of the main-phase of the Late Paleozoic Ice Age (LPIA), [Methods] this study integrates high-resolution sedimentary facies and their stacking patterns analyses of the Du’an Formation in the Shuidong Section, Youjiang Basin, South China. A robust transgressive–regressive (T-R) sequence framework was established, enabling quantitative reconstruction of relative sea-level history for the basin during the middle-late Early Carboniferous. [Results] Twelve microfacies were identified across the section, spanning the middle Visean to upper Serpukhovian. These are grouped into four distinct facies associations: restricted platform, open platform, platform-margin shoal, and upper slope. Seven third-order sequences and at least twenty-six fourth-order T-R cycles were resolved within the upper Visean and Serpukhovian. [Conclusions] The onset of high-frequency, high-amplitude relative sea-level fluctuations in the Youjiang Basin coincides with the Aleksinian-Mikhailovian boundary. This is independently corroborated by other low-latitude far-field proxies, thereby providing robust stratigraphic evidence that the main-phase of the LPIA initiated during this interval.
[Objectives] The middle to late Early Carboniferous witnessed profound reorganization of Earth’s tectonic paleogeography, climate systems, marine environments, and ecosystems. To refine the initiation timing of the main-phase of the Late Paleozoic Ice Age (LPIA), [Methods] this study integrates high-resolution sedimentary facies and their stacking patterns analyses of the Du’an Formation in the Shuidong Section, Youjiang Basin, South China. A robust transgressive–regressive (T-R) sequence framework was established, enabling quantitative reconstruction of relative sea-level history for the basin during the middle-late Early Carboniferous. [Results] Twelve microfacies were identified across the section, spanning the middle Visean to upper Serpukhovian. These are grouped into four distinct facies associations: restricted platform, open platform, platform-margin shoal, and upper slope. Seven third-order sequences and at least twenty-six fourth-order T-R cycles were resolved within the upper Visean and Serpukhovian. [Conclusions] The onset of high-frequency, high-amplitude relative sea-level fluctuations in the Youjiang Basin coincides with the Aleksinian-Mikhailovian boundary. This is independently corroborated by other low-latitude far-field proxies, thereby providing robust stratigraphic evidence that the main-phase of the LPIA initiated during this interval.
Abstract:
[Objective]Studying lake-level changes is an important approach to understanding regional tectonic and climatic evolution, while how to accurately interpret or quantitatively reconstruct lake-level changes has always been a subject of ongoing exploration. [Methods]In this study, by utilizing environmental proxies and integrating traditional sedimentary environment analysis, observations and measurements of environmental indicators (chroma, magnetic susceptibility, CaCO3 content, and total organic carbon (TOC) content) were conducted on the approximately 10-meter-thick lacustrine sediments in the lower part of the Liwasxia Formation of the Liupanshan Group, where the tectonic setting of the Liupanshan Basin is relatively stable.This research investigated the lake-level changes recorded by these sediments and analyzed the applicability of environmental proxies. [Results]This research investigated the lake-level changes recorded by these sediments and analyzed the applicability of environmental proxies. The results indicate that from the lower to the upper part of the sedimentary sequence, the sedimentary subfacies develop in the order of shallow-water delta plain, shore-shallow lake, semi-deep lake, shore-shallow lake, semi-deep lake, and shore-shallow lake, forming two lake-level change processes characterized by a transition from shallow to deep water. Environmental proxies exhibit significant variations across different sedimentary subfacies: chroma parameters show the largest fluctuations in the shallow-water delta plain facies and gradually become stable upward; magnetic susceptibility demonstrates a gradual decrease from the bottom to the top of the sequence; CaCO3 and TOC contents fluctuate considerably but generally show an increasing trend. Specifically, in the microfacies dominated by argillaceous sediments within the shore-shallow lake facies, the a* value and TOC content can serve as indicators for determining the lake-level elevation, while the CaCO3 content can be used to identify the driving mechanisms of lake-level changes during this period. In contrast, in the microfacies dominated by marl-limestone sedimentary assemblages, the CaCO3 content and TOC value can indicate the lake-level elevation.[Conclusion]Based on the sedimentary facies and indicator results, it can be concluded that the aforementioned indicators are relatively sensitive to feedback on lake-level changes. However, a single indicator cannot be directly used as a basis for characterizing lake-level changes and their driving mechanisms; instead, a comprehensive discussion should be conducted in combination with sedimentary facies and lithological associations.
[Objective]Studying lake-level changes is an important approach to understanding regional tectonic and climatic evolution, while how to accurately interpret or quantitatively reconstruct lake-level changes has always been a subject of ongoing exploration. [Methods]In this study, by utilizing environmental proxies and integrating traditional sedimentary environment analysis, observations and measurements of environmental indicators (chroma, magnetic susceptibility, CaCO3 content, and total organic carbon (TOC) content) were conducted on the approximately 10-meter-thick lacustrine sediments in the lower part of the Liwasxia Formation of the Liupanshan Group, where the tectonic setting of the Liupanshan Basin is relatively stable.This research investigated the lake-level changes recorded by these sediments and analyzed the applicability of environmental proxies. [Results]This research investigated the lake-level changes recorded by these sediments and analyzed the applicability of environmental proxies. The results indicate that from the lower to the upper part of the sedimentary sequence, the sedimentary subfacies develop in the order of shallow-water delta plain, shore-shallow lake, semi-deep lake, shore-shallow lake, semi-deep lake, and shore-shallow lake, forming two lake-level change processes characterized by a transition from shallow to deep water. Environmental proxies exhibit significant variations across different sedimentary subfacies: chroma parameters show the largest fluctuations in the shallow-water delta plain facies and gradually become stable upward; magnetic susceptibility demonstrates a gradual decrease from the bottom to the top of the sequence; CaCO3 and TOC contents fluctuate considerably but generally show an increasing trend. Specifically, in the microfacies dominated by argillaceous sediments within the shore-shallow lake facies, the a* value and TOC content can serve as indicators for determining the lake-level elevation, while the CaCO3 content can be used to identify the driving mechanisms of lake-level changes during this period. In contrast, in the microfacies dominated by marl-limestone sedimentary assemblages, the CaCO3 content and TOC value can indicate the lake-level elevation.[Conclusion]Based on the sedimentary facies and indicator results, it can be concluded that the aforementioned indicators are relatively sensitive to feedback on lake-level changes. However, a single indicator cannot be directly used as a basis for characterizing lake-level changes and their driving mechanisms; instead, a comprehensive discussion should be conducted in combination with sedimentary facies and lithological associations.
Abstract:
[Objectives] The lower Cambrian Qiongzhusi Formation in the western Yangtze Block is a good reservoir for shale gas, and it has been well targeted for shale gas exploration in the Sichuan Basin. On the contrary, the shale gas exploration in the central Yunnan is underestimated. To provide geological supports for shale gas exploration in the central Yunnan, this study focuses on siliciclastic rocks of the Lower Cambrian Qiongzhusi Formation, and investigates its sedimentary and diagenetic environment, source rocks, nature of weathering and paleo-climate, tectonic setting of source regions. [Methods] Targeted on the Yinmachi section of the Liangwangshan area, it conducts stratigraphic logging of the section, microscopic petrography and whole-rock geochemistry works on the Qiongzhusi Formation. [Results] The Qiongzhusi siliciclastic samples are mainly siltstones with low compositional maturity. All analysed siltstone samples yield U/Th ratios of 0.23-0.81, V/Cr ratios of 1.38-3.56, V/Sc ratios of 7.36-18.9, Ni/Co ratios of 2.34-4.85, Ba/Sr ratios of 9.04-16.2, La/Ce ratios of 0.48-0.57, (La/Yb)n ratios of 0.54-1.25 and δEu values of 0.62-0.67. [Conclusions] Integrated with published data of the Qiongzhusi siltstone and mudstone samples in central Yunnan, it concludes that the Qiongzhusi Formation in the central Yunnan was predominantly sourced from felsic magmatic rocks and siliceous sedimentary rocks, with minor mafic components. After chemical weathering and transportation, the detritus were deposited on an oxic shallow marine continental shelf, and were influenced by hydrothermal fluids during its sedimentation and diagenesis processes. Source regions of the Qiongzhusi detritus possibly include local Kangdian oldland and exotic Gondwana continents. During the early Cambrian, source regions (include the western Yangtze) were likely sitting on an active continental margin with a warm and humid weather condition.
[Objectives] The lower Cambrian Qiongzhusi Formation in the western Yangtze Block is a good reservoir for shale gas, and it has been well targeted for shale gas exploration in the Sichuan Basin. On the contrary, the shale gas exploration in the central Yunnan is underestimated. To provide geological supports for shale gas exploration in the central Yunnan, this study focuses on siliciclastic rocks of the Lower Cambrian Qiongzhusi Formation, and investigates its sedimentary and diagenetic environment, source rocks, nature of weathering and paleo-climate, tectonic setting of source regions. [Methods] Targeted on the Yinmachi section of the Liangwangshan area, it conducts stratigraphic logging of the section, microscopic petrography and whole-rock geochemistry works on the Qiongzhusi Formation. [Results] The Qiongzhusi siliciclastic samples are mainly siltstones with low compositional maturity. All analysed siltstone samples yield U/Th ratios of 0.23-0.81, V/Cr ratios of 1.38-3.56, V/Sc ratios of 7.36-18.9, Ni/Co ratios of 2.34-4.85, Ba/Sr ratios of 9.04-16.2, La/Ce ratios of 0.48-0.57, (La/Yb)n ratios of 0.54-1.25 and δEu values of 0.62-0.67. [Conclusions] Integrated with published data of the Qiongzhusi siltstone and mudstone samples in central Yunnan, it concludes that the Qiongzhusi Formation in the central Yunnan was predominantly sourced from felsic magmatic rocks and siliceous sedimentary rocks, with minor mafic components. After chemical weathering and transportation, the detritus were deposited on an oxic shallow marine continental shelf, and were influenced by hydrothermal fluids during its sedimentation and diagenesis processes. Source regions of the Qiongzhusi detritus possibly include local Kangdian oldland and exotic Gondwana continents. During the early Cambrian, source regions (include the western Yangtze) were likely sitting on an active continental margin with a warm and humid weather condition.
Abstract:
[Objective] The Qinling Mountains serve as a crucial north-south geographical boundary in eastern China, characterized by diverse vegetation types that respond sensitively to climate change. However, systematic analyses of vegetation response mechanisms to natural and anthropogenic factors in this region are still limited, hindering a comprehensive understanding of ecological changes and human-environment interactions in the Qinling area. [Methods] This study is based on the sediment core from Daye Lake, an alpine lake on the Mount Taibai which is the highest mountains in the Qinling Mountain range. Using high-resolution pollen and charcoal records, we quantitatively reconstructed vegetation dynamics since the Last Glacial Maximum (LGM) through Biomisation and diversity indices, and extracted fire frequency and fire intensity sequences using the CharAnalysis software. We combined reliable climate and human activity records with fire activity sequences reconstructed from charcoal, this study systematically examined vegetation response mechanisms to climate change and human activities. Furthermore, statistical methods including redundancy analysis and correlation analysis reveal the contributions of various drivers to vegetation evolution across different time scales. [Results] Findings indicate that vegetation landscape in the Qinling region evolved from a coniferous forest dominated by Picea and Pinus during the LGM to a mixed coniferous-broadleaf forest during the Last Deglaciation, accompanied by pronounced fluctuations in vegetation diversity. During the Holocene, the vegetation transitioned to temperate deciduous broadleaf forests dominated by Corylus/Carpinus and Quercus, accompanied by a marked increase in vegetation diversity. The expansion of herbaceous plants such as Artemisia and Poaceae, along with land openness, occurred in the Late Holocene, while vegetation diversity showed a declining trend. Fire activity exhibited high frequency both in the Last Deglaciation and the Late Holocene, driven respectively by cold-dry climate and human activities. [Conclusion] Since the LGM, vegetation in the Qinling Mountains has shown a significant response to climate change. From the LGM to the Last Deglaciation, both broadleaf and coniferous forests showed significant correlation with temperature. Redundancy analysis confirmed temperature was the dominant factor, while fire activity promoted vegetation diversity by altering interspecific competition patterns and niche availability. However, precipitation strongly correlated with temperate deciduous broadleaf forests during the Holocene, and precipitation became a key controlling factor in vegetation succession with the intensification of the East Asian summer monsoon. Since the Middle Holocene, human activities have emerged as a third significant driving factor, with their influence steadily increasing over time. This study systematically reveals the response patterns of Qinling vegetation to climate change and the process of human intervention, providing a crucial case study for understanding the long-term interaction mechanisms among vegetation, climate, fire, and human activities in alpine regions.
Abstract:
【Objective】 Thick layers (8~30 m) of calcareous cement developed in deep sandstones (current burial depth 5500~6000 m) below the unconformity in the Yongjin area of the Junggar Basin show a significant spatial correlation with hydrocarbon distribution. However, their formation mechanisms and distribution patterns remain unclear, particularly lacking systematic understanding of the contribution of eogenetic diagenesis to calcareous cementation below the unconformity.【Methods】 Utilizing integrated analytical techniques including reservoir cast thin sections, fluid inclusion analysis, and cathodoluminescence microscopy, the stages of calcareous cementation were systematically classified. Physical simulation experiments were designed to analyze the genetic mechanisms and distribution patterns of eogenetic calcareous cements.【Results】 Three types of calcareous cement are developed within the Qigu Formation: Cement Type I: Dominated by poikilotopic cement of silt-sized calcite aggregates, exhibiting orange-red cathodoluminescence color. This CL color is consistent with that of micritic calcite within calcareous nodules in the paleosol layers of the Qigu Formation. Cement Type II: Primarily composed of pore-filling calcite, with fluid inclusion homogenization temperatures of 80~100°C. Cement Type III: Consists mainly of granular dolomite, incompletely filling pores, with fluid inclusion homogenization temperatures generally >100°C, formed subsequent to large-scale hydrocarbon charging. Cast thin section and X-ray diffraction analyses indicate that Calcareous Cements Type I and Type II are predominantly developed in the down-dip structural direction and in the middle-lower parts of sand bodies, exerting a significant impact on reservoir quality. Calcareous Cement Type III is developed in various locations but exhibits generally low abundance, exerting a weak impact on reservoir quality. The total calcareous cement content is low proximal to the denudation front and high distal to it. Physical simulation experiments demonstrate that eogenetic calcareous cement is generally located in the lower parts of sand bodies and in the down-dip direction of dipping strata. Concurrently, low-permeability fine-grained beds inhibit calcareous cement precipitation in underlying coarse-grained sand layers, consistent with observations in the study area.【Conclusion】 Thick calcareous cement layers primarily develop distal to the denudation front (down-dip), while calcareous cement content is low proximal to the denudation front. Their formation is primarily controlled by the superimposition of eogenetic calcareous cement (Type I) and Type II calcareous cement, influenced by dipping strata. The actual development of calcareous cement is also influenced by reservoir heterogeneity; calcareous cement is more prone to develop within fine-grained reservoir intervals and at interfaces between coarse- and fine-grained reservoir facies.
【Objective】 Thick layers (8~30 m) of calcareous cement developed in deep sandstones (current burial depth 5500~6000 m) below the unconformity in the Yongjin area of the Junggar Basin show a significant spatial correlation with hydrocarbon distribution. However, their formation mechanisms and distribution patterns remain unclear, particularly lacking systematic understanding of the contribution of eogenetic diagenesis to calcareous cementation below the unconformity.【Methods】 Utilizing integrated analytical techniques including reservoir cast thin sections, fluid inclusion analysis, and cathodoluminescence microscopy, the stages of calcareous cementation were systematically classified. Physical simulation experiments were designed to analyze the genetic mechanisms and distribution patterns of eogenetic calcareous cements.【Results】 Three types of calcareous cement are developed within the Qigu Formation: Cement Type I: Dominated by poikilotopic cement of silt-sized calcite aggregates, exhibiting orange-red cathodoluminescence color. This CL color is consistent with that of micritic calcite within calcareous nodules in the paleosol layers of the Qigu Formation. Cement Type II: Primarily composed of pore-filling calcite, with fluid inclusion homogenization temperatures of 80~100°C. Cement Type III: Consists mainly of granular dolomite, incompletely filling pores, with fluid inclusion homogenization temperatures generally >100°C, formed subsequent to large-scale hydrocarbon charging. Cast thin section and X-ray diffraction analyses indicate that Calcareous Cements Type I and Type II are predominantly developed in the down-dip structural direction and in the middle-lower parts of sand bodies, exerting a significant impact on reservoir quality. Calcareous Cement Type III is developed in various locations but exhibits generally low abundance, exerting a weak impact on reservoir quality. The total calcareous cement content is low proximal to the denudation front and high distal to it. Physical simulation experiments demonstrate that eogenetic calcareous cement is generally located in the lower parts of sand bodies and in the down-dip direction of dipping strata. Concurrently, low-permeability fine-grained beds inhibit calcareous cement precipitation in underlying coarse-grained sand layers, consistent with observations in the study area.【Conclusion】 Thick calcareous cement layers primarily develop distal to the denudation front (down-dip), while calcareous cement content is low proximal to the denudation front. Their formation is primarily controlled by the superimposition of eogenetic calcareous cement (Type I) and Type II calcareous cement, influenced by dipping strata. The actual development of calcareous cement is also influenced by reservoir heterogeneity; calcareous cement is more prone to develop within fine-grained reservoir intervals and at interfaces between coarse- and fine-grained reservoir facies.
Abstract:
[Objective]The reconstruction of atmospheric carbon dioxide concentrations (pCO2) during the Cenomanian-Turonian (C-T) has been previously limited to stage-scale temporal resolutions, which has greatly constrained its effectiveness in unveiling the extreme greenhouse climate of the C-T period and perturbations in the global carbon cycle. [Methods]To enhance our understanding of pCO2 fluctuations and the paleoclimatic response during the C-T "greenhouse climate," this study reconstructed high-resolution atmospheric pCO2 and mean annual precipitation (MAP) during the C-T period by analyzing the stable carbon and oxygen isotopes of calcareous nodules from the Upper Cretaceous Nanxin Formation in the Qamdo Basin, southeastern Tibet. [Results and Discussions]Our results reveal a gradual decline in pCO2 during the early Cenomanian period, followed by an increase in the middle and late Cenomanian stages. Moreover, we observed significant fluctuations in pCO2 during the early and middle Turonian stage. These findings align with the variations in pCO2 throughout the C-T period that have been estimated using the stomata ratio method of plant fossils and the geochemical models. In addition to the long-term pCO2 fluctuations, the paleosoils of the Nanxin Formation also documented two crucial carbon cycle perturbations during the Cenomanian period: Oceanic Anoxia Event 2 (OAE2) and Mid-Cenomanian Event (MCE). The atmospheric pCO2 rose by 309 ppmV and followed by a rapid decline of 520 ppmV across the initiation of MCE, accompanied by a positive shift of 1.25‰ in organic carbon isotopes (13Corg). During OAE2, there were two distinct CO2 pulses, with the maximum pCO2 concentration coeval with the lowest 13Corg. The pCO2 increased up to approximately 1300 ppmV at the OAE2 boundary, followed by a remarkably reduction of around 17% due to substantial organic carbon burial during OAE2. The increased concentration of CO2 throughout the mid-late Cenomanian intensified the greenhouse effect and elevated atmospheric humidity, resulting in significant negative shifts in 13Corg that were decoupled with inorganic carbon isotopes. Our conclusion further suggests that there would be an increase of 511 mm/yr in MAP as the atmospheric pCO2 elevated from 500 ppmV to 1000 ppmV during the C-T period. [Conclusions]We assumed that the elevated atmospheric pCO2 and climatic sensitivity during the mid-Cretaceous considerably intensified the hydrological cycle, contributing to an extreme greenhouse climate period.
[Objective]The reconstruction of atmospheric carbon dioxide concentrations (pCO2) during the Cenomanian-Turonian (C-T) has been previously limited to stage-scale temporal resolutions, which has greatly constrained its effectiveness in unveiling the extreme greenhouse climate of the C-T period and perturbations in the global carbon cycle. [Methods]To enhance our understanding of pCO2 fluctuations and the paleoclimatic response during the C-T "greenhouse climate," this study reconstructed high-resolution atmospheric pCO2 and mean annual precipitation (MAP) during the C-T period by analyzing the stable carbon and oxygen isotopes of calcareous nodules from the Upper Cretaceous Nanxin Formation in the Qamdo Basin, southeastern Tibet. [Results and Discussions]Our results reveal a gradual decline in pCO2 during the early Cenomanian period, followed by an increase in the middle and late Cenomanian stages. Moreover, we observed significant fluctuations in pCO2 during the early and middle Turonian stage. These findings align with the variations in pCO2 throughout the C-T period that have been estimated using the stomata ratio method of plant fossils and the geochemical models. In addition to the long-term pCO2 fluctuations, the paleosoils of the Nanxin Formation also documented two crucial carbon cycle perturbations during the Cenomanian period: Oceanic Anoxia Event 2 (OAE2) and Mid-Cenomanian Event (MCE). The atmospheric pCO2 rose by 309 ppmV and followed by a rapid decline of 520 ppmV across the initiation of MCE, accompanied by a positive shift of 1.25‰ in organic carbon isotopes (13Corg). During OAE2, there were two distinct CO2 pulses, with the maximum pCO2 concentration coeval with the lowest 13Corg. The pCO2 increased up to approximately 1300 ppmV at the OAE2 boundary, followed by a remarkably reduction of around 17% due to substantial organic carbon burial during OAE2. The increased concentration of CO2 throughout the mid-late Cenomanian intensified the greenhouse effect and elevated atmospheric humidity, resulting in significant negative shifts in 13Corg that were decoupled with inorganic carbon isotopes. Our conclusion further suggests that there would be an increase of 511 mm/yr in MAP as the atmospheric pCO2 elevated from 500 ppmV to 1000 ppmV during the C-T period. [Conclusions]We assumed that the elevated atmospheric pCO2 and climatic sensitivity during the mid-Cretaceous considerably intensified the hydrological cycle, contributing to an extreme greenhouse climate period.
Abstract:
[Objective] The origin of concretions in sedimentary rocks can not only reveal the depositional environment but also serve as key evidence for studying diagenetic evolution, holding significant importance for research on pore evolution in sedimentary rocks. [Methods] This study focuses on the Devonian Jinbaoshi Formation in the Gediba section of northwestern Sichuan Basin. Through detailed analysis of rock thin sections, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS), the characteristics of concretions in the Jinbaoshi Formation were examined, and their genetic mechanisms were explored. [Results] The quartz sandstone in the Jinbaoshi Formation of the Gediba section exhibits high maturity and well-developed pores, with numerous ellipsoidal to sub-spherical concretions distributed along the bedding planes, displaying brown to dark brown surfaces. Microscopic analysis reveals that the quartz grains within the concretions are consistent with the surrounding sandstone in terms of morphology, size, sorting, roundness, and texture. Compared to the quartz sandstone, the quartz grains within the concretions are sparsely arranged, primarily in point contact, and are predominantly cemented by silica, followed by iron and clay minerals. Residual pores locally filled with crude oil were observed. The concretion-bearing intervals exhibit small-scale dome structures and bioturbation, with filamentous microbial remnants and silicified bacterial colonies identified under SEM. [Conclusion] Comprehensive analysis suggests that the formation of concretions in the Jinbaoshi Formation of the Gediba section is closely related to microbial mats. Gases produced by the decomposition of organic matter migrated upward, forming dome structures on the sediment surface. When the microbial mats were too thin to produce sufficient gas for upward migration, localized sparse distribution of detrital grains occurred, accompanied by early siliceous and ferruginous cementation. These concretions resisted compaction during subsequent burial processes, maintaining their spherical morphology, and turned brown due to oxidation after being uplifted to the surface. This study establishes a genetic evolution model of the concretions, which can provide insights into the diagenetic evolution and pore structure adjustment of the quartz sandstone in the Jinbaoshi Formation, offering important geological significance for hydrocarbon exploration.
[Objective] The origin of concretions in sedimentary rocks can not only reveal the depositional environment but also serve as key evidence for studying diagenetic evolution, holding significant importance for research on pore evolution in sedimentary rocks. [Methods] This study focuses on the Devonian Jinbaoshi Formation in the Gediba section of northwestern Sichuan Basin. Through detailed analysis of rock thin sections, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS), the characteristics of concretions in the Jinbaoshi Formation were examined, and their genetic mechanisms were explored. [Results] The quartz sandstone in the Jinbaoshi Formation of the Gediba section exhibits high maturity and well-developed pores, with numerous ellipsoidal to sub-spherical concretions distributed along the bedding planes, displaying brown to dark brown surfaces. Microscopic analysis reveals that the quartz grains within the concretions are consistent with the surrounding sandstone in terms of morphology, size, sorting, roundness, and texture. Compared to the quartz sandstone, the quartz grains within the concretions are sparsely arranged, primarily in point contact, and are predominantly cemented by silica, followed by iron and clay minerals. Residual pores locally filled with crude oil were observed. The concretion-bearing intervals exhibit small-scale dome structures and bioturbation, with filamentous microbial remnants and silicified bacterial colonies identified under SEM. [Conclusion] Comprehensive analysis suggests that the formation of concretions in the Jinbaoshi Formation of the Gediba section is closely related to microbial mats. Gases produced by the decomposition of organic matter migrated upward, forming dome structures on the sediment surface. When the microbial mats were too thin to produce sufficient gas for upward migration, localized sparse distribution of detrital grains occurred, accompanied by early siliceous and ferruginous cementation. These concretions resisted compaction during subsequent burial processes, maintaining their spherical morphology, and turned brown due to oxidation after being uplifted to the surface. This study establishes a genetic evolution model of the concretions, which can provide insights into the diagenetic evolution and pore structure adjustment of the quartz sandstone in the Jinbaoshi Formation, offering important geological significance for hydrocarbon exploration.
Abstract:
Abstract: [Objective] The geochemical significance of phenanthrene series compounds within tricyclic aromatic hydrocarbons (TAHs) remains debated, particularly regarding the relative control of thermal maturity and depositional environment. This study focuses on variations in phenanthrene series parameters during thermal evolution and their controlling mechanisms.[Methods] Lacustrine shale samples from the Shahejie Formation (Es?) of Well X in the Bohai Bay Basin were subjected to closed-system pyrolysis using a DK-III thermal simulation apparatus at seven temperature points (275–450 °C) for 48 h. Aromatic fractions of expelled oil were analyzed via GC–MS to determine the distribution and evolution of polycyclic aromatic hydrocarbons (PAHs).[Results] The original shale extract was dominated by aromatic steroids, followed by tricyclic aromatics. With increasing simulation temperature, bicyclic aromatics first increased then decreased, tricyclic and heteroaromatic hydrocarbons remained relatively stable, and tetracyclic and pentacyclic aromatics increased, while aromatic steroids decreased sharply. In the phenanthrene series, methylphenanthrenes (MPs) were dominant at low temperatures, gradually replaced by dimethylphenanthrenes (DMPs) at 300–400 °C, and phenanthrene (P) became dominant above 400 °C. Anthracene series abundance increased with temperature up to 400 °C, then declined. Bicyclic aromatics showed higher thermal stability below 375 °C, while aromatic steroids transformed into tri- and tetracyclic aromatics between 300–400 °C. Two new maturity parameters, DPR-3 (2,7-DMP/1,2-DMP) and DPR-4 [2,7-DMP/(2,10+1,3+3,10+3,9-DMP)], exhibited strong positive correlation with maturity, supplementing existing F1, F2, DPR-1, and DPR-2 indices.[Conclusions] Methylphenanthrene distributions are suitable for depositional environment assessment in low-maturity samples but are more reliable maturity indicators at mature to overmature stages. Aromatic steroids and bicyclic aromatics are sensitive to early-stage thermal evolution, whereas phenanthrene and anthracene series compounds are better indicators for high maturity. The newly proposed DPR-3 and DPR-4 parameters improve the accuracy and applicability of maturity evaluation, especially in high- to overmature stages. This work refines the understanding of phenanthrene series geochemical significance and provides a robust aromatic hydrocarbon framework for deep hydrocarbon resource assessment and thermal maturity classification.
Abstract: [Objective] The geochemical significance of phenanthrene series compounds within tricyclic aromatic hydrocarbons (TAHs) remains debated, particularly regarding the relative control of thermal maturity and depositional environment. This study focuses on variations in phenanthrene series parameters during thermal evolution and their controlling mechanisms.[Methods] Lacustrine shale samples from the Shahejie Formation (Es?) of Well X in the Bohai Bay Basin were subjected to closed-system pyrolysis using a DK-III thermal simulation apparatus at seven temperature points (275–450 °C) for 48 h. Aromatic fractions of expelled oil were analyzed via GC–MS to determine the distribution and evolution of polycyclic aromatic hydrocarbons (PAHs).[Results] The original shale extract was dominated by aromatic steroids, followed by tricyclic aromatics. With increasing simulation temperature, bicyclic aromatics first increased then decreased, tricyclic and heteroaromatic hydrocarbons remained relatively stable, and tetracyclic and pentacyclic aromatics increased, while aromatic steroids decreased sharply. In the phenanthrene series, methylphenanthrenes (MPs) were dominant at low temperatures, gradually replaced by dimethylphenanthrenes (DMPs) at 300–400 °C, and phenanthrene (P) became dominant above 400 °C. Anthracene series abundance increased with temperature up to 400 °C, then declined. Bicyclic aromatics showed higher thermal stability below 375 °C, while aromatic steroids transformed into tri- and tetracyclic aromatics between 300–400 °C. Two new maturity parameters, DPR-3 (2,7-DMP/1,2-DMP) and DPR-4 [2,7-DMP/(2,10+1,3+3,10+3,9-DMP)], exhibited strong positive correlation with maturity, supplementing existing F1, F2, DPR-1, and DPR-2 indices.[Conclusions] Methylphenanthrene distributions are suitable for depositional environment assessment in low-maturity samples but are more reliable maturity indicators at mature to overmature stages. Aromatic steroids and bicyclic aromatics are sensitive to early-stage thermal evolution, whereas phenanthrene and anthracene series compounds are better indicators for high maturity. The newly proposed DPR-3 and DPR-4 parameters improve the accuracy and applicability of maturity evaluation, especially in high- to overmature stages. This work refines the understanding of phenanthrene series geochemical significance and provides a robust aromatic hydrocarbon framework for deep hydrocarbon resource assessment and thermal maturity classification.
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Geochemical proxies such as the Chemical Index of Alteration (CIA) are essential for reconstructing paleoweathering intensity and climate regimes. However, their reliability is fundamentally constrained by conventional analytical methods due to their inability to distinguish mineral phases with identical chemical compositions yet possessing distinct genetic origins—such as detrital mica and diagenetic illite—often leading to significant misinterpretations of continental weathering and paleoclimatic conditions. To address this persistent challenge, this study applied the advanced SEM-EDS-Nanomin system, which integrates high-resolution field-emission scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS) and an innovative mixed pixel deconvolution (Mixel) algorithm. This cutting-edge technology achieves nanoscale (<0.5 μm) mineral mapping and automatically correlates resolved compositional data with critical morphological characteristics (e.g., particle orientation, boundary sharpness, spatial relationships) to objectively classify clay minerals into detrital, authigenic, or diagenetic categories with high accuracy. The elevated bulk CIA values of the organic-rich shales from the Late Neoproterozoic intervals in South China (Cryogenian interglacial Datangpo Formation and Ediacaran Doushantuo Formation) and the Mesoproterozoic Velkerri Formation in northern Australia reflected intense chemical weathering under warm and humid climatic condition. However, the new generation of the SEM-EDS coupled with the Nanomin system in contrast revealed that these seemingly apparent chemical weathering signatures were substantially overprinted by the dominant contribution of diagenetic clay minerals to key element budgets. Specifically, aluminum-rich phases previously assumed to reflect pedogenic processes were identified as diagenetic products, especially illite and kaolinite formed via pseudomorphic replacement of precursor feldspar or mica grains during burial and diagenesis. After systematically excluding these secondary phases identified through Nanomin genetic discrimination and recalculating CIA based on primary mineral assemblages, the data in fact indicated physically dominated weathering operating under colder, more arid climatic conditions. This critical revision resolved long-standing paradoxes where high bulk CIA values conflicted with independent sedimentological, mineralogical, and isotopic evidence. Furthermore, beyond CIA correction, the technique demonstrated exceptional capability in identifying authigenic minerals such as saponite formed in restricted marine evaporitic settings during the Ediacaran biotic emergence. By providing essential mineralogical context, the SEM-EDS-Nanomin methodology fundamentally enhances the reliability of diverse geochemical proxies and analytical techniques including accurate neodymium isotope(εNd) analysis and high-precision Rb-Sr geochronology based on unaltered minerals (e.g., glauconite). By establishing the unambiguous determination of clay mineral origin as an indispensable prerequisite for robust paleoenvironmental interpretation, this SEM-EDS-Nanomin approach—through its unique synergy of high-resolution imaging, spectral deconvolution, and automated morphological correlation—provides a revolutionary framework for deciphering Earth's environmental dynamics during critical transitional periods marked by extreme climate shifts, tectonic reorganizations, and biological innovations. Its capacity to bridge nano-scale mineralogical observations with macro-scale geochemical signals represents a paradigm shift in reconstructing deep-time Earth system processes.
Geochemical proxies such as the Chemical Index of Alteration (CIA) are essential for reconstructing paleoweathering intensity and climate regimes. However, their reliability is fundamentally constrained by conventional analytical methods due to their inability to distinguish mineral phases with identical chemical compositions yet possessing distinct genetic origins—such as detrital mica and diagenetic illite—often leading to significant misinterpretations of continental weathering and paleoclimatic conditions. To address this persistent challenge, this study applied the advanced SEM-EDS-Nanomin system, which integrates high-resolution field-emission scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS) and an innovative mixed pixel deconvolution (Mixel) algorithm. This cutting-edge technology achieves nanoscale (<0.5 μm) mineral mapping and automatically correlates resolved compositional data with critical morphological characteristics (e.g., particle orientation, boundary sharpness, spatial relationships) to objectively classify clay minerals into detrital, authigenic, or diagenetic categories with high accuracy. The elevated bulk CIA values of the organic-rich shales from the Late Neoproterozoic intervals in South China (Cryogenian interglacial Datangpo Formation and Ediacaran Doushantuo Formation) and the Mesoproterozoic Velkerri Formation in northern Australia reflected intense chemical weathering under warm and humid climatic condition. However, the new generation of the SEM-EDS coupled with the Nanomin system in contrast revealed that these seemingly apparent chemical weathering signatures were substantially overprinted by the dominant contribution of diagenetic clay minerals to key element budgets. Specifically, aluminum-rich phases previously assumed to reflect pedogenic processes were identified as diagenetic products, especially illite and kaolinite formed via pseudomorphic replacement of precursor feldspar or mica grains during burial and diagenesis. After systematically excluding these secondary phases identified through Nanomin genetic discrimination and recalculating CIA based on primary mineral assemblages, the data in fact indicated physically dominated weathering operating under colder, more arid climatic conditions. This critical revision resolved long-standing paradoxes where high bulk CIA values conflicted with independent sedimentological, mineralogical, and isotopic evidence. Furthermore, beyond CIA correction, the technique demonstrated exceptional capability in identifying authigenic minerals such as saponite formed in restricted marine evaporitic settings during the Ediacaran biotic emergence. By providing essential mineralogical context, the SEM-EDS-Nanomin methodology fundamentally enhances the reliability of diverse geochemical proxies and analytical techniques including accurate neodymium isotope(εNd) analysis and high-precision Rb-Sr geochronology based on unaltered minerals (e.g., glauconite). By establishing the unambiguous determination of clay mineral origin as an indispensable prerequisite for robust paleoenvironmental interpretation, this SEM-EDS-Nanomin approach—through its unique synergy of high-resolution imaging, spectral deconvolution, and automated morphological correlation—provides a revolutionary framework for deciphering Earth's environmental dynamics during critical transitional periods marked by extreme climate shifts, tectonic reorganizations, and biological innovations. Its capacity to bridge nano-scale mineralogical observations with macro-scale geochemical signals represents a paradigm shift in reconstructing deep-time Earth system processes.
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[Objective]The characteristics and types of stromatolites are of great significance for revealing the co-evolution of early life and the geological environment on Earth. The Neoproterozoic Wangshan Formation in the Xuhuai area on the southeastern margin of the North China Craton is widely developed with stromatolites, but the macroscopic and microscopic characteristics of the stromatolites and their response mechanisms to the sedimentary environment and paleoclimate significance remain unclear. [Methods]This study takes the stromatolites of the Wangshan Formation as the research object. Through the analysis of the macroscopic and microscopic characteristics of the stromatolites, combined with the analysis of major elements and carbon-oxygen isotopes, the sedimentary facies zones of the stromatolites in the Wangshan Formation are systematically divided, the main controlling factors of stromatolite growth are discussed, and the co-evolution relationship between stromatolites and the paleoenvironment is reconstructed.[Conclusions]The research results show that the Wangshan Formation mainly contains three types of stromatolites: columnar, wavy, and laminated. Columnar and wavy stromatolites are concentrated in high-energy intertidal zones, while laminated stromatolites are stably distributed in low-energy supratidal zones. During the sedimentary period of the Wangshan Formation, the climate changed frequently, and the sea level experienced three complete transgression-regression cycles, showing a general trend of gradual shallowing. The prosperous period of stromatolites is coupled with the shallow water environment under the background of an arid climate. By comparing the stromatolites in this study area with modern stromatolites, it is found that there may be multiple microorganisms on the microbial mats of the stromatolites in the Wangshan Formation, and their metabolic activities and interactions to some extent restrict the microscopic morphology of stromatolites. The intensity of terrigenous clastic input and storm events to some extent regulate the microbial ecological succession; accommodation space, light intensity and hydrodynamic environmental factors affect the macroscopic morphology of stromatolites. This study can provide new evidence for the co-evolution of life and environment in the Neoproterozoic.
[Objective]The characteristics and types of stromatolites are of great significance for revealing the co-evolution of early life and the geological environment on Earth. The Neoproterozoic Wangshan Formation in the Xuhuai area on the southeastern margin of the North China Craton is widely developed with stromatolites, but the macroscopic and microscopic characteristics of the stromatolites and their response mechanisms to the sedimentary environment and paleoclimate significance remain unclear. [Methods]This study takes the stromatolites of the Wangshan Formation as the research object. Through the analysis of the macroscopic and microscopic characteristics of the stromatolites, combined with the analysis of major elements and carbon-oxygen isotopes, the sedimentary facies zones of the stromatolites in the Wangshan Formation are systematically divided, the main controlling factors of stromatolite growth are discussed, and the co-evolution relationship between stromatolites and the paleoenvironment is reconstructed.[Conclusions]The research results show that the Wangshan Formation mainly contains three types of stromatolites: columnar, wavy, and laminated. Columnar and wavy stromatolites are concentrated in high-energy intertidal zones, while laminated stromatolites are stably distributed in low-energy supratidal zones. During the sedimentary period of the Wangshan Formation, the climate changed frequently, and the sea level experienced three complete transgression-regression cycles, showing a general trend of gradual shallowing. The prosperous period of stromatolites is coupled with the shallow water environment under the background of an arid climate. By comparing the stromatolites in this study area with modern stromatolites, it is found that there may be multiple microorganisms on the microbial mats of the stromatolites in the Wangshan Formation, and their metabolic activities and interactions to some extent restrict the microscopic morphology of stromatolites. The intensity of terrigenous clastic input and storm events to some extent regulate the microbial ecological succession; accommodation space, light intensity and hydrodynamic environmental factors affect the macroscopic morphology of stromatolites. This study can provide new evidence for the co-evolution of life and environment in the Neoproterozoic.
Abstract:
[Objective] The Songliao Basin is a major hydrocarbon-producing region in China. After more than 60 years of exploration, conventional oil and gas development has entered a phase characterized by high cost and high technical difficulty, and the basin has now fully transitioned into the stage of unconventional hydrocarbon exploration. During the deposition of the Upper Cretaceous Qingshankou and Nenjiang formations, two large-scale lacustrine transgressions occurred, creating vast potential resources for shale oil. The first and second members of the Nenjiang Formation host thick intervals of dark, organic-rich fine-grained sedimentary rocks deposited in deep to semi-deep lacustrine environments. However, current understanding of these deposits remains limited, necessitating a systematic characterization of their lithofacies and organic matter enrichment mechanisms. [Methods] Based on core data from Well A34, centimeter-scale high-resolution core descriptions were conducted for the first and second members of the Upper Cretaceous Nenjiang Formation in the Songliao Basin. Microscopic petrographic analysis, total organic carbon (TOC) content measurements, and elemental line and area scanning techniques were applied to further refine the classification of the structures and fabrics of fine-grained sedimentary rocks. [Results and Discussions] A total of 16 lithofacies and 6 facies assemblages of fine-grained sedimentary rocks were identified in the first and second members of the Nenjiang Formation. Based on sedimentological characteristics and elemental geochemistry, a clear distinction was observed between organic-rich intervals (TOC>6%) and those with lower organic content. Organic-rich intervals are predominantly composed of medium- to fine-grained dark mudstones with continuous lamination. The most favorable conditions for organic matter accumulation and lamina development occur at the base of the second member of the Nenjiang Formation. [Conclusions] Synthesizing previously published stable isotope data, it is proposed that a humid climate and nutrient influx from sediment sources were key factors in creating the high biological productivity and strongly reducing conditions in the Songliao Basin during the deposition of the first and second members of the Nenjiang Formation. These environmental conditions promoted the development of high TOC values and well lamination, leading to the widespread formation of organic-rich fine-grained sedimentary rocks.
[Objective] The Songliao Basin is a major hydrocarbon-producing region in China. After more than 60 years of exploration, conventional oil and gas development has entered a phase characterized by high cost and high technical difficulty, and the basin has now fully transitioned into the stage of unconventional hydrocarbon exploration. During the deposition of the Upper Cretaceous Qingshankou and Nenjiang formations, two large-scale lacustrine transgressions occurred, creating vast potential resources for shale oil. The first and second members of the Nenjiang Formation host thick intervals of dark, organic-rich fine-grained sedimentary rocks deposited in deep to semi-deep lacustrine environments. However, current understanding of these deposits remains limited, necessitating a systematic characterization of their lithofacies and organic matter enrichment mechanisms. [Methods] Based on core data from Well A34, centimeter-scale high-resolution core descriptions were conducted for the first and second members of the Upper Cretaceous Nenjiang Formation in the Songliao Basin. Microscopic petrographic analysis, total organic carbon (TOC) content measurements, and elemental line and area scanning techniques were applied to further refine the classification of the structures and fabrics of fine-grained sedimentary rocks. [Results and Discussions] A total of 16 lithofacies and 6 facies assemblages of fine-grained sedimentary rocks were identified in the first and second members of the Nenjiang Formation. Based on sedimentological characteristics and elemental geochemistry, a clear distinction was observed between organic-rich intervals (TOC>6%) and those with lower organic content. Organic-rich intervals are predominantly composed of medium- to fine-grained dark mudstones with continuous lamination. The most favorable conditions for organic matter accumulation and lamina development occur at the base of the second member of the Nenjiang Formation. [Conclusions] Synthesizing previously published stable isotope data, it is proposed that a humid climate and nutrient influx from sediment sources were key factors in creating the high biological productivity and strongly reducing conditions in the Songliao Basin during the deposition of the first and second members of the Nenjiang Formation. These environmental conditions promoted the development of high TOC values and well lamination, leading to the widespread formation of organic-rich fine-grained sedimentary rocks.
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Abstract: [Objective] Carbonate outcrops are often affected by tectonic fragmentation, weathering, and vegetation cover, making it difficult to obtain complete and precise sedimentary and reservoir characteristics by traditional field survey methods. This study introduces terrestrial laser scanning (TLS) to address the challenges in high-resolution characterization of carbonate outcrops, improve lithofacies recognition accuracy, and explore its applicability in sedimentology and reservoir studies. [Methods] Taking the Gaojiashan section of the Dengying Formation in Ningqiang, Shaanxi as an example, high-precision point cloud data were acquired using TLS, and laser intensity information was analyzed quantitatively and qualitatively. Three-dimensional digital outcrop models were constructed through point cloud modeling and texture mapping. Combined with thin-section petrography, rock physical property tests, and field sedimentological investigations, lithofacies recognition, sedimentary facies classification, and depositional model reconstruction were carried out. [Results and Discussions] (1) A high-resolution digital outcrop model was established, which enables full-coverage 3D visualization and interactive analysis of the outcrop, allowing geological features such as lithology and stratigraphic geometry to be accurately preserved and studied. (2) A lithology recognition atlas based on laser intensity was established, clarifying the intensity features and thickness proportions of different lithologies within the profile. Dolostone facies such as algal dolostone and crystalline dolostone correspond to high laser intensity and relatively high porosity–permeability values, while micritic dolostone and argillaceous dolostone correspond to low intensity and poor reservoir quality, indicating a good correlation between laser intensity and reservoir properties. (3) Using the upper sub-member of the second member of the Dengying Formation as an example, sub-layer division was refined based on laser intensity, and the geometric characteristics of the 14th sub-layer were deeply characterized. Analysis of depositional patterns indicated that variations in laser intensity are closely related to sea-level fluctuations, revealing cyclic depositional evolution. [Conclusions] The integration of TLS with traditional geological methods provides a more intuitive representation of carbonate outcrop features and effectively preserves primary field data. The digital outcrop model established in this study offers valuable data support for hydrocarbon exploration and reservoir evaluation, demonstrating the broad prospects and significant potential of TLS applications in geological research.
Abstract: [Objective] Carbonate outcrops are often affected by tectonic fragmentation, weathering, and vegetation cover, making it difficult to obtain complete and precise sedimentary and reservoir characteristics by traditional field survey methods. This study introduces terrestrial laser scanning (TLS) to address the challenges in high-resolution characterization of carbonate outcrops, improve lithofacies recognition accuracy, and explore its applicability in sedimentology and reservoir studies. [Methods] Taking the Gaojiashan section of the Dengying Formation in Ningqiang, Shaanxi as an example, high-precision point cloud data were acquired using TLS, and laser intensity information was analyzed quantitatively and qualitatively. Three-dimensional digital outcrop models were constructed through point cloud modeling and texture mapping. Combined with thin-section petrography, rock physical property tests, and field sedimentological investigations, lithofacies recognition, sedimentary facies classification, and depositional model reconstruction were carried out. [Results and Discussions] (1) A high-resolution digital outcrop model was established, which enables full-coverage 3D visualization and interactive analysis of the outcrop, allowing geological features such as lithology and stratigraphic geometry to be accurately preserved and studied. (2) A lithology recognition atlas based on laser intensity was established, clarifying the intensity features and thickness proportions of different lithologies within the profile. Dolostone facies such as algal dolostone and crystalline dolostone correspond to high laser intensity and relatively high porosity–permeability values, while micritic dolostone and argillaceous dolostone correspond to low intensity and poor reservoir quality, indicating a good correlation between laser intensity and reservoir properties. (3) Using the upper sub-member of the second member of the Dengying Formation as an example, sub-layer division was refined based on laser intensity, and the geometric characteristics of the 14th sub-layer were deeply characterized. Analysis of depositional patterns indicated that variations in laser intensity are closely related to sea-level fluctuations, revealing cyclic depositional evolution. [Conclusions] The integration of TLS with traditional geological methods provides a more intuitive representation of carbonate outcrop features and effectively preserves primary field data. The digital outcrop model established in this study offers valuable data support for hydrocarbon exploration and reservoir evaluation, demonstrating the broad prospects and significant potential of TLS applications in geological research.
Abstract:
[Objective] This study aims to establish a quantitative model of downstream grain size diminution in large-scale Distributive Fluvial Systems(fluvial fans) to provide a knowledge base for predicting the distribution of sedimentary systems in intelligent oil and gas exploration and development. [Methods] The study conducted detailed descriptions of channel morphology and sediment grain size in the Dagle fluvial fan of the Qaidam Basin, Qinghai. with using geographic information software such as Google Earth , 91 Wei Tu and ImageJ combined with field investigations. Appropriate methods were selected to fit the grain size variation curves, and the longitudinal changes in gravel size were summarized. [Results] The findings are as follows: The sediments grain size of the Dagle Fluvial Fan are generally coarse, dominated by gravels, with the maximum clast size reaching 90 cm at the apex,showing a clear diminishing trend from the apex to the distal end. Exponential functions better fit the trend of gravel size diminution in the fluvial fan. The fitting function for maximum clast size vs. distance is Dmax= 90e-0.264x, and for average clast size vs. distance is D ?= 7.04 e-0.11x. By comparing the exponential models of maximum and average clast sizes, the critical role of hydraulic sorting in grain size diminution is highlighted. The longitudinal decay model of maximum clast size is deemed effective in reflecting the grain size variation trends of the entire distributive fluvial system (within the studied area), leading to the establishment of a sediment diminution model for the Dagle Distributive Fluvial System.[Conclusions] Sediment grain size variations are close related to the source , the sediment supply, the hydrodynamic conditions, and the depositional environments etc. This study provides a knowledge base for quantitatively sedimentary facies prediction and depositional system scale estimation of large-scale Distributive Fluvial Systems.
[Objective] This study aims to establish a quantitative model of downstream grain size diminution in large-scale Distributive Fluvial Systems(fluvial fans) to provide a knowledge base for predicting the distribution of sedimentary systems in intelligent oil and gas exploration and development. [Methods] The study conducted detailed descriptions of channel morphology and sediment grain size in the Dagle fluvial fan of the Qaidam Basin, Qinghai. with using geographic information software such as Google Earth , 91 Wei Tu and ImageJ combined with field investigations. Appropriate methods were selected to fit the grain size variation curves, and the longitudinal changes in gravel size were summarized. [Results] The findings are as follows: The sediments grain size of the Dagle Fluvial Fan are generally coarse, dominated by gravels, with the maximum clast size reaching 90 cm at the apex,showing a clear diminishing trend from the apex to the distal end. Exponential functions better fit the trend of gravel size diminution in the fluvial fan. The fitting function for maximum clast size vs. distance is Dmax= 90e-0.264x, and for average clast size vs. distance is D ?= 7.04 e-0.11x. By comparing the exponential models of maximum and average clast sizes, the critical role of hydraulic sorting in grain size diminution is highlighted. The longitudinal decay model of maximum clast size is deemed effective in reflecting the grain size variation trends of the entire distributive fluvial system (within the studied area), leading to the establishment of a sediment diminution model for the Dagle Distributive Fluvial System.[Conclusions] Sediment grain size variations are close related to the source , the sediment supply, the hydrodynamic conditions, and the depositional environments etc. This study provides a knowledge base for quantitatively sedimentary facies prediction and depositional system scale estimation of large-scale Distributive Fluvial Systems.
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[Objective] The Early Cambrian Longmenshan tectonic belt hosts a series of sedimentary manganese deposits; however, their material sources, sedimentary environment, and precipitation mechanisms remain controversial. This study takes the Magong manganese deposit as an example, aiming to provide an in-depth discussion on the metallogenic model and precipitation mechanisms of this ore district, thereby offering a theoretical basis for research on sedimentary manganese deposits within this tectonic belt. [Methods] Electron microscopy and electron probe microanalysis were employed to analyze the characteristics of mineral assemblages and the particle size distribution of framboidal pyrite. Inductively coupled plasma optical emission spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS) were used to determine the whole-rock major and trace element contents. Based on these analyses, the material sources, sedimentary environment, manganese precipitation mechanisms, and metallogenic processes of the Early Cambrian Magong manganese deposit on the northern margin of the Longmenshan tectonic belt were systematically investigated. [Result] Petrographic and mineralogical studies reveal that the manganese carbonate ore layers, pyrite layers, and siliceous rocks in the Magong manganese deposit are rhythmically interbedded. The primary manganese-bearing minerals include kutnohorite with microbial textures and alabandite, followed by psilomelane and pyrolusite. The gangue minerals consist of framboidal pyrite, organic matter, quartz, and dolomite. Major and trace element analyses show no significant positive correlation between MnO and Al2O3. Samples fall into the hydrothermal end-member in discrimination diagrams such as SiO2-Al2O3, (Co+Cu+Ni)×10-Fe-Mn, and (Zr+Y+Ce)×100-(Cu+Ni)×15-(Fe+Mn)/4. Most samples exhibit Fe/Ti>20 and Al/(Al+Fe+Mn)<0.35, further indicating that the ore-forming materials were primarily derived from submarine hydrothermal activity. Enriched EFU and EFMo values, along with EFMo/EFU>1, suggest a weakly restricted to restricted paleo-hydrodynamic environment. The stringent conditions required for the formation of alabandite, the presence of synsedimentary framboidal pyrite (<6 μm), and multiple geochemical indicators (such as V/Mo-Mo, V/Cr, Ni/Co, and V/(V+Ni)) collectively reflect that the Magong Manganese Deposit was formed in a relatively dynamic dysoxic-suboxic-euxinic environment. Euhedral to subhedral organic matter and extremely high EFMo, EFP, and EFCd values indicate high paleoproductivity conditions. [Conclusion] The geochemical characteristics suggest that the ore-forming materials of the Magong manganese deposit were mainly derived from submarine hydrothermal activity. The manganese carbonate ores were enriched in a relatively dynamic shelf-slope environment under dysoxic-suboxic-euxinic conditions. High paleoproductivity provided the material basis for the mineralization through water column stratification under the "sulfide wedge" model. The presence of microbial-textured kutnohorite, abundant organic matter, and framboidal pyrite indicates that the manganese precipitation process was significantly influenced by bacterial sulfate reduction (BSR) and extracellular polymeric substances (EPS): the BSR process provided HCO3- for the formation of manganese carbonates and H?S for the generation of pyrite and alabandite; the EPS secreted by bacteria provided adsorption sites for Mn2+, Mg2+, and Ca2+, promoting the precipitation of kutnohorite. During the mineralization process, iron and manganese were separated due to differences in their geochemical behaviors. This study provides new evidence for understanding the genesis of sedimentary manganese deposits in the Longmenshan tectonic belt and further refines the precipitation mechanism of sedimentary manganese ores within this metallogenic belt.
[Objective] The Early Cambrian Longmenshan tectonic belt hosts a series of sedimentary manganese deposits; however, their material sources, sedimentary environment, and precipitation mechanisms remain controversial. This study takes the Magong manganese deposit as an example, aiming to provide an in-depth discussion on the metallogenic model and precipitation mechanisms of this ore district, thereby offering a theoretical basis for research on sedimentary manganese deposits within this tectonic belt. [Methods] Electron microscopy and electron probe microanalysis were employed to analyze the characteristics of mineral assemblages and the particle size distribution of framboidal pyrite. Inductively coupled plasma optical emission spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS) were used to determine the whole-rock major and trace element contents. Based on these analyses, the material sources, sedimentary environment, manganese precipitation mechanisms, and metallogenic processes of the Early Cambrian Magong manganese deposit on the northern margin of the Longmenshan tectonic belt were systematically investigated. [Result] Petrographic and mineralogical studies reveal that the manganese carbonate ore layers, pyrite layers, and siliceous rocks in the Magong manganese deposit are rhythmically interbedded. The primary manganese-bearing minerals include kutnohorite with microbial textures and alabandite, followed by psilomelane and pyrolusite. The gangue minerals consist of framboidal pyrite, organic matter, quartz, and dolomite. Major and trace element analyses show no significant positive correlation between MnO and Al2O3. Samples fall into the hydrothermal end-member in discrimination diagrams such as SiO2-Al2O3, (Co+Cu+Ni)×10-Fe-Mn, and (Zr+Y+Ce)×100-(Cu+Ni)×15-(Fe+Mn)/4. Most samples exhibit Fe/Ti>20 and Al/(Al+Fe+Mn)<0.35, further indicating that the ore-forming materials were primarily derived from submarine hydrothermal activity. Enriched EFU and EFMo values, along with EFMo/EFU>1, suggest a weakly restricted to restricted paleo-hydrodynamic environment. The stringent conditions required for the formation of alabandite, the presence of synsedimentary framboidal pyrite (<6 μm), and multiple geochemical indicators (such as V/Mo-Mo, V/Cr, Ni/Co, and V/(V+Ni)) collectively reflect that the Magong Manganese Deposit was formed in a relatively dynamic dysoxic-suboxic-euxinic environment. Euhedral to subhedral organic matter and extremely high EFMo, EFP, and EFCd values indicate high paleoproductivity conditions. [Conclusion] The geochemical characteristics suggest that the ore-forming materials of the Magong manganese deposit were mainly derived from submarine hydrothermal activity. The manganese carbonate ores were enriched in a relatively dynamic shelf-slope environment under dysoxic-suboxic-euxinic conditions. High paleoproductivity provided the material basis for the mineralization through water column stratification under the "sulfide wedge" model. The presence of microbial-textured kutnohorite, abundant organic matter, and framboidal pyrite indicates that the manganese precipitation process was significantly influenced by bacterial sulfate reduction (BSR) and extracellular polymeric substances (EPS): the BSR process provided HCO3- for the formation of manganese carbonates and H?S for the generation of pyrite and alabandite; the EPS secreted by bacteria provided adsorption sites for Mn2+, Mg2+, and Ca2+, promoting the precipitation of kutnohorite. During the mineralization process, iron and manganese were separated due to differences in their geochemical behaviors. This study provides new evidence for understanding the genesis of sedimentary manganese deposits in the Longmenshan tectonic belt and further refines the precipitation mechanism of sedimentary manganese ores within this metallogenic belt.
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[Purpose]As a new type of unconventional natural gas, coal and rock gas has made a major breakthrough in the exploration of the central and western basins in recent years. At present, the research on coal and rock gas mainly focuses on the gas generation capacity and gas accumulation capacity of coal and rock, but the formation mechanism and distribution law of thick coal seams, the carrier of coal and rock gas, are obviously insufficient, which leads to the optimization of favorable coal and rock gas zones, especially the optimization of drilling intervals in thick coal seams of medium and low coal rank in the western basin.[Methods]Based on the description of the coring and macroscopic characteristics of underground coal and rock in the Tuha Basin, the microscopic component identification, major and trace element analysis and rock pyrolysis analysis of coal and rock samples of Xishanyao Formation were systematically carried out, and on this basis, the sedimentary environment, coal facies characteristics and characteristics of coal and rock gas sweet spot sections in thick coal seams were compared.[Results]The analysis of coal and rock microscopic components shows that there are not only various types of coal facies in the Xishanyao Formation in the Tuha Basin, but also various types of coal facies superimposed in the thick coal seam at the same location. The comparative analysis of the main and trace elements in coal and rock shows that there are multi-stage water inflow and water retreat cyclic characteristics in thick coal seams.[Conclusions]The results show that the coal-forming environment is different at different stages in the process of water inflow and retreat, resulting in the difference in the quality of coal and rock between different coal phases, and the cause of the thick coal seam is the longitudinal superposition of coal and rock of different types of coal facies caused by the rise and fall of water bodies in multiple stages. During the Jurassic period, the Tuha Basin was an intermountain continental basin, and different sedimentary facies zones were affected by the depth of the water body, and the ratio of peat production rate to the growth rate of the accommodating space was obviously different, resulting in the difference in the development and distribution of coal seams. In the thick coal seams in this area, the flooded interface and the tolerable spatial transition surface in the process of water retreat can be identified, among which the water degradation transition surface is the layer with the highest organic matter content in coal and rock, and it is also the sweet spot section of coal and rock gas exploration.
[Purpose]As a new type of unconventional natural gas, coal and rock gas has made a major breakthrough in the exploration of the central and western basins in recent years. At present, the research on coal and rock gas mainly focuses on the gas generation capacity and gas accumulation capacity of coal and rock, but the formation mechanism and distribution law of thick coal seams, the carrier of coal and rock gas, are obviously insufficient, which leads to the optimization of favorable coal and rock gas zones, especially the optimization of drilling intervals in thick coal seams of medium and low coal rank in the western basin.[Methods]Based on the description of the coring and macroscopic characteristics of underground coal and rock in the Tuha Basin, the microscopic component identification, major and trace element analysis and rock pyrolysis analysis of coal and rock samples of Xishanyao Formation were systematically carried out, and on this basis, the sedimentary environment, coal facies characteristics and characteristics of coal and rock gas sweet spot sections in thick coal seams were compared.[Results]The analysis of coal and rock microscopic components shows that there are not only various types of coal facies in the Xishanyao Formation in the Tuha Basin, but also various types of coal facies superimposed in the thick coal seam at the same location. The comparative analysis of the main and trace elements in coal and rock shows that there are multi-stage water inflow and water retreat cyclic characteristics in thick coal seams.[Conclusions]The results show that the coal-forming environment is different at different stages in the process of water inflow and retreat, resulting in the difference in the quality of coal and rock between different coal phases, and the cause of the thick coal seam is the longitudinal superposition of coal and rock of different types of coal facies caused by the rise and fall of water bodies in multiple stages. During the Jurassic period, the Tuha Basin was an intermountain continental basin, and different sedimentary facies zones were affected by the depth of the water body, and the ratio of peat production rate to the growth rate of the accommodating space was obviously different, resulting in the difference in the development and distribution of coal seams. In the thick coal seams in this area, the flooded interface and the tolerable spatial transition surface in the process of water retreat can be identified, among which the water degradation transition surface is the layer with the highest organic matter content in coal and rock, and it is also the sweet spot section of coal and rock gas exploration.
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Abstract: [Objective] Accurate identification of carbonate sedimentary facies forms the basis for lithofacies paleogeographic reconstruction. However, challenges arise from the ambiguity of facies characteristics and the subjectivity of manual interpretation, while traditional well-log facies interpretation methods suffer from inefficiency and experience-dependent limitations. This study aims to construct a reusable intelligent interpretation model using machine learning algorithms to overcome the limitations in well-log facies interpretation and enhance the accuracy and efficiency of carbonate facies identification. [Methods] Five boreholes from the reef-shoal facies belt of the Changxing Formation in the Yuanba area of northeastern Sichuan Basin were selected as the study objects. Feature parameters, including acoustic log, gamma-ray well log , and other well-log curves, were used to establish a random forest algorithm-based model for carbonate well-log facies identification. To address class imbalance, the SMOTE-Nearmiss-1 resampling algorithm was implemented, while optimal hyperparameters were determined through grid search coupled with K-fold cross-validation. [Results] The improved model demonstrated significantly enhanced recognition capability for minority classes, achieving an accuracy of 0.87 in single well testing. This case highlights the significant potential of machine learning models in accurately identifying carbonate sedimentary facies.
Abstract: [Objective] Accurate identification of carbonate sedimentary facies forms the basis for lithofacies paleogeographic reconstruction. However, challenges arise from the ambiguity of facies characteristics and the subjectivity of manual interpretation, while traditional well-log facies interpretation methods suffer from inefficiency and experience-dependent limitations. This study aims to construct a reusable intelligent interpretation model using machine learning algorithms to overcome the limitations in well-log facies interpretation and enhance the accuracy and efficiency of carbonate facies identification. [Methods] Five boreholes from the reef-shoal facies belt of the Changxing Formation in the Yuanba area of northeastern Sichuan Basin were selected as the study objects. Feature parameters, including acoustic log, gamma-ray well log , and other well-log curves, were used to establish a random forest algorithm-based model for carbonate well-log facies identification. To address class imbalance, the SMOTE-Nearmiss-1 resampling algorithm was implemented, while optimal hyperparameters were determined through grid search coupled with K-fold cross-validation. [Results] The improved model demonstrated significantly enhanced recognition capability for minority classes, achieving an accuracy of 0.87 in single well testing. This case highlights the significant potential of machine learning models in accurately identifying carbonate sedimentary facies.
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[Objective]Contourites well developed along southern and western margin of Ordos Basin, where sedimentary characteristics very abundant. It is ideal area of research contourites. The mounded contourite, a special type of contourite formed in Kelimoli formation of Ordovician in western margin of Ordos Basin. [Methods] Using outcrop, thin section, and paleacurrent, mechanism of mounded contourites had been researched. [Results] (1) Micrite-, calcisiltite-, and muddy contourites are well favour in the study area. The lithology is mainly dark grey motted thin micrtici and calsisiltitic limestone with wavy boundary. They are lenticular-shape with unstabe along layer. Horizontal-, wavy- bedding, erosive surfaces are usually founded. Bioclastic fragments are relative rich. Bioturbation and burrow are well developed. And the paleocurrent direction is northeast, paralleling to slope. (2) Mounded contourites are mounded shape, and NW-SE orientation, about parallel to slope. (3) Contourites dunes developed in mounded contourites showing mounded shape are micrite and calcisiltite, some muddy limestone interbedded very thin mudstone in lower and upper parts. The thickness of mounded contourites being fine-coarse-fine cycle, is 152 cm, can be divided into 11 units. 4 small mounded contourites developed with a series progradation. Bioturbation and burrows are usually formed. (4) The mounded contourite can be divided into early initial, middle formative and late decline stages. They were resulting from secondary flow showing helical feature when high energy contour current moved northeastward. Helical flow can lead to erosion and deposition and form moat. Anticlockwise secondary flow also can transport sediment, and develop mounded contourite on the side of moat facing to deep sea basin. [Conclusion] This study demonstrate characteristics, processes, and depositional model of mounded contourite, not only add one example of mounded contourites, but also help to reconstruct contour current circulation and recover the palaeoenvironment.
[Objective]Contourites well developed along southern and western margin of Ordos Basin, where sedimentary characteristics very abundant. It is ideal area of research contourites. The mounded contourite, a special type of contourite formed in Kelimoli formation of Ordovician in western margin of Ordos Basin. [Methods] Using outcrop, thin section, and paleacurrent, mechanism of mounded contourites had been researched. [Results] (1) Micrite-, calcisiltite-, and muddy contourites are well favour in the study area. The lithology is mainly dark grey motted thin micrtici and calsisiltitic limestone with wavy boundary. They are lenticular-shape with unstabe along layer. Horizontal-, wavy- bedding, erosive surfaces are usually founded. Bioclastic fragments are relative rich. Bioturbation and burrow are well developed. And the paleocurrent direction is northeast, paralleling to slope. (2) Mounded contourites are mounded shape, and NW-SE orientation, about parallel to slope. (3) Contourites dunes developed in mounded contourites showing mounded shape are micrite and calcisiltite, some muddy limestone interbedded very thin mudstone in lower and upper parts. The thickness of mounded contourites being fine-coarse-fine cycle, is 152 cm, can be divided into 11 units. 4 small mounded contourites developed with a series progradation. Bioturbation and burrows are usually formed. (4) The mounded contourite can be divided into early initial, middle formative and late decline stages. They were resulting from secondary flow showing helical feature when high energy contour current moved northeastward. Helical flow can lead to erosion and deposition and form moat. Anticlockwise secondary flow also can transport sediment, and develop mounded contourite on the side of moat facing to deep sea basin. [Conclusion] This study demonstrate characteristics, processes, and depositional model of mounded contourite, not only add one example of mounded contourites, but also help to reconstruct contour current circulation and recover the palaeoenvironment.
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The rapid advancement of information, intelligence, and automation technologies has significantly expanded the use of Unmanned Aerial Vehicle (UAV)-based oblique photogrammetry in geology. In recent years, its applications in field geology have advanced swiftly. This paper reviews the principles and developments of UAV-based photogrammetry, the associated primary products and modeling technologies, and its applications in sedimentology, stratigraphy, and reservoir geology. Key advancements include digital outcrop dataset collection paradigms, visual representation and digital twin technologies, sequence stratigraphy and modern sedimentary source-sink analyses, quantitative reservoir sedimentology, and 3D geological modeling. Based on a comprehensive literature review and current technological trends, future developments are expected to focus on intelligent identification and modeling of sedimentary geological features, enhancements to digital twin systems and visualization, deeper exploration of reservoir genesis and prototype modeling, multi-scale data calibration and integration, and the expansion of geological research directions.
The rapid advancement of information, intelligence, and automation technologies has significantly expanded the use of Unmanned Aerial Vehicle (UAV)-based oblique photogrammetry in geology. In recent years, its applications in field geology have advanced swiftly. This paper reviews the principles and developments of UAV-based photogrammetry, the associated primary products and modeling technologies, and its applications in sedimentology, stratigraphy, and reservoir geology. Key advancements include digital outcrop dataset collection paradigms, visual representation and digital twin technologies, sequence stratigraphy and modern sedimentary source-sink analyses, quantitative reservoir sedimentology, and 3D geological modeling. Based on a comprehensive literature review and current technological trends, future developments are expected to focus on intelligent identification and modeling of sedimentary geological features, enhancements to digital twin systems and visualization, deeper exploration of reservoir genesis and prototype modeling, multi-scale data calibration and integration, and the expansion of geological research directions.
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Abstract: [Objective] To study a large number of Rhizocorallium in Guanling Formation in Xingyi area, and analyze the morphological and structural characteristics of its burrows, so as to reveal the behavioral habits of trace-making organisms and their indicative significance to the ancient sedimentary environment. [Methods] The cross-sectional diameter of the relict tube and the distance between the two wing tubes of Rhizocorallium U-shaped burrows were measured. The correlation of parameters was analyzed by linear regression, and the morphological structure and trace-making process were analyzed. The disturbance intensity was evaluated by bioturbation index (BI), and the sedimentary environment was revealed by combining lithology and other characteristics. [Results](1) The basic structure of Rhizocorallium is composed of U-shaped and linear burrows, and its formation mechanism is the mutual transformation of foraging-crawling behavior of trace-making organisms. (2) The cross-sectional diameter (0.1~2.6 cm) of the U-shaped trace tube was highly positively correlated with the distance between the two wing tubes (0.5~11.9 cm) (R2=0.92688), indicating that the behavior patterns of trace-building organisms from larvae to adults were highly consistent, which was consistent with the survival strategy characteristics of opportunistic species (r-selected).The bioclastic limestone and micritic limestone in the lower Guanling Formation are characterized by high-density Rhizocorallium ( BI = 3 ~ 5 ), mainly distributed in low-energy open platforms. The bioclastic limestone in the upper section of the is characterized by sparse Rhizocorallium ( BI = 1 ~ 3 ), which is mainly distributed in the high-energy intertidal zone. [ Conclusion ] In this study, the morphological identification and identification of Rhizocorallium revealed the characteristics and formation mechanism of its burrow structure, and further confirmed the consistency of behavior and its r-selection strategy in the process of individual development. More importantly, the study clarified the behavioral response and adaptive characteristics of Rhizocorallium to different sedimentary environments. These findings together indicate that Rhizocorallium can be used as a reliable ichnological marker for the reconstruction of paleoecology and paleosedimentary environment.
Abstract: [Objective] To study a large number of Rhizocorallium in Guanling Formation in Xingyi area, and analyze the morphological and structural characteristics of its burrows, so as to reveal the behavioral habits of trace-making organisms and their indicative significance to the ancient sedimentary environment. [Methods] The cross-sectional diameter of the relict tube and the distance between the two wing tubes of Rhizocorallium U-shaped burrows were measured. The correlation of parameters was analyzed by linear regression, and the morphological structure and trace-making process were analyzed. The disturbance intensity was evaluated by bioturbation index (BI), and the sedimentary environment was revealed by combining lithology and other characteristics. [Results](1) The basic structure of Rhizocorallium is composed of U-shaped and linear burrows, and its formation mechanism is the mutual transformation of foraging-crawling behavior of trace-making organisms. (2) The cross-sectional diameter (0.1~2.6 cm) of the U-shaped trace tube was highly positively correlated with the distance between the two wing tubes (0.5~11.9 cm) (R2=0.92688), indicating that the behavior patterns of trace-building organisms from larvae to adults were highly consistent, which was consistent with the survival strategy characteristics of opportunistic species (r-selected).The bioclastic limestone and micritic limestone in the lower Guanling Formation are characterized by high-density Rhizocorallium ( BI = 3 ~ 5 ), mainly distributed in low-energy open platforms. The bioclastic limestone in the upper section of the is characterized by sparse Rhizocorallium ( BI = 1 ~ 3 ), which is mainly distributed in the high-energy intertidal zone. [ Conclusion ] In this study, the morphological identification and identification of Rhizocorallium revealed the characteristics and formation mechanism of its burrow structure, and further confirmed the consistency of behavior and its r-selection strategy in the process of individual development. More importantly, the study clarified the behavioral response and adaptive characteristics of Rhizocorallium to different sedimentary environments. These findings together indicate that Rhizocorallium can be used as a reliable ichnological marker for the reconstruction of paleoecology and paleosedimentary environment.
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The Late Ordovician - Early Silurian witness changes in climate, however, the triggering mechanism are still under debate. In this study, the Longmaxi Formation in the Wanhe section, located in southwestern Sichuan Province, are selected to analyze the depositional and diagenetic processes using sedimentology, mineralogy and geochemistry. Furthermore, this study evaluates the formation mechanism of pyrite in the Longmaxi Formation/carbonate lenticle, and reveals the influences of early diagenesis on the changes in climate. The results show that the black shales in Longmaxi Formation are mainly deposited under euxinic anoxic water conditions, considering the high Corg/P ratios, high UEF and VEF values. The organic matter in the sediments is anaerobically oxidized by the microbial sulfate reduction (MSR) process due to anoxia of waters above the water?rock interface. In addition, the sulfate-driven anaerobic oxidation of methane (SD-AOM) resulted in the precipitation of euhedral pyrite with high Co contents and δ34Spyr values (>10‰) and low V contents. Of course, the influences of TSR in late diagenesis on the formation of pyrite in studied areas need more work in the future. Given that methane is a greenhouse gas, this study suggested that the release of methane formed in early diagenesis has the potential to influence the hydrochemical conditions in seawater or warming climate in early Silurian.
The Late Ordovician - Early Silurian witness changes in climate, however, the triggering mechanism are still under debate. In this study, the Longmaxi Formation in the Wanhe section, located in southwestern Sichuan Province, are selected to analyze the depositional and diagenetic processes using sedimentology, mineralogy and geochemistry. Furthermore, this study evaluates the formation mechanism of pyrite in the Longmaxi Formation/carbonate lenticle, and reveals the influences of early diagenesis on the changes in climate. The results show that the black shales in Longmaxi Formation are mainly deposited under euxinic anoxic water conditions, considering the high Corg/P ratios, high UEF and VEF values. The organic matter in the sediments is anaerobically oxidized by the microbial sulfate reduction (MSR) process due to anoxia of waters above the water?rock interface. In addition, the sulfate-driven anaerobic oxidation of methane (SD-AOM) resulted in the precipitation of euhedral pyrite with high Co contents and δ34Spyr values (>10‰) and low V contents. Of course, the influences of TSR in late diagenesis on the formation of pyrite in studied areas need more work in the future. Given that methane is a greenhouse gas, this study suggested that the release of methane formed in early diagenesis has the potential to influence the hydrochemical conditions in seawater or warming climate in early Silurian.
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Abstract: [Objective] The Wangka section in the Changdu area, North Qiangtang terrane, records a critical lithological transition from limestone to clastic rocks in the Lower-Middle Permian Jiaoga Formation. The timing of this transition may hold significant implications for understanding the driving mechanisms of the Permian P3 glaciation. [Methods] We conducted an integrated study of the Jiaoga Formation tuffs, including petrography, zircon U-Pb geochronology, zircon trace element analysis, and whole-rock major/trace element geochemistry, to constrain their emplacement age and provenance. [Results] Zircon U-Pb dating yielded a weighted mean age of 268.2 ± 1.9 Ma for the tuffs. Zircon trace elements exhibit light rare earth element (LREE) depletion and heavy REE (HREE) enrichment. Whole-rock geochemistry shows High Al?O?/TiO?, Zr/TiO?, and Th/Sc ratios. Primitive mantle-normalized patterns with Nb-Ta-Ti depletion and Th-U enrichment. [Conclusions] The age of the Jiaoga Formation tuffs overlaps with the onset of the Permian P3 glaciation within analytical uncertainty. Their geochemical signatures suggest derivation from a volcanic island arc spanning the North Qiangtang-Simao terranes. Intensive weathering of this arc system may have triggered or amplified the P3 glaciation.
Abstract: [Objective] The Wangka section in the Changdu area, North Qiangtang terrane, records a critical lithological transition from limestone to clastic rocks in the Lower-Middle Permian Jiaoga Formation. The timing of this transition may hold significant implications for understanding the driving mechanisms of the Permian P3 glaciation. [Methods] We conducted an integrated study of the Jiaoga Formation tuffs, including petrography, zircon U-Pb geochronology, zircon trace element analysis, and whole-rock major/trace element geochemistry, to constrain their emplacement age and provenance. [Results] Zircon U-Pb dating yielded a weighted mean age of 268.2 ± 1.9 Ma for the tuffs. Zircon trace elements exhibit light rare earth element (LREE) depletion and heavy REE (HREE) enrichment. Whole-rock geochemistry shows High Al?O?/TiO?, Zr/TiO?, and Th/Sc ratios. Primitive mantle-normalized patterns with Nb-Ta-Ti depletion and Th-U enrichment. [Conclusions] The age of the Jiaoga Formation tuffs overlaps with the onset of the Permian P3 glaciation within analytical uncertainty. Their geochemical signatures suggest derivation from a volcanic island arc spanning the North Qiangtang-Simao terranes. Intensive weathering of this arc system may have triggered or amplified the P3 glaciation.
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In order to break through the bottleneck of high cost of oil and gas exploration due to the difficulty of accurate prediction of sedimentary system, the study is based on the theory of distributive fluvial system, and takes the Jurassic Shaximiao Formation in Northwest Sichuan Basin as the sample, synthesizes the data of field outcrops, cores, and logging-recording wells, etc., and utilizes the research methods combining the analysis of the ground surface and the subsurface, and the analysis of the quantitative and qualitative analyses, so as to achieve the quantitative characterization of the sedimentary system of the Shaximiao Formation in the Northwest Sichuan Basin. The results of the study showed that: (1) The Shaximiao Formation in the central and northwest Sichuan basin has developed three types of sedimentary systems: distributive fluvial system, delta system, and lacustrine system. The distributive fluvial system of Shaximiao Formation in the study area can be divided into 3 subfacies and 6 microfacies types, the delta system can be divided into 2 subfacies and 4 microfacies types, and the lacustrine system can be further divided into shore-shallow lake beach-bar deposits. (2) The ratio of subfacies net-to-gross ratio is 50%-70% in the proximal, and the sand bodies are superimposed with thick layers. The medial subfacies net-to-gross ratio is between 30%-50%, and the sand body is distributed in thick layers. The distal subfacies net-to-gross ratio is 20%-30%, and the sand body is isolated. The delta and lacustrine facies systems are distributed in the middle Sichuan basin, and the net-to-gross ratio change is generally less than 15%. The sand bodies are also distributed in continuous sheets, but the maximum thickness of single sand bodies is thinner than that of the medial facies belt. (3) The Shaximiao Formation in the study area is mainly influenced by fluvial action, with the Sha 1 Member showing the distributive fluvial system terminating in a larger range of lacustrine deposits, and the Sha 2 Member showing the distributive fluvial system terminating in some small temporary lakes. (4) According to the modern sedimentary reference, in addition to the development of large high-quality reservoirs in the delta system, the medial and distal facies of the distributive fluvial system can also form continuous channel sand reservoirs. The results of the research are of great significance in characterizing the development scale and overlay style of different sand bodies, and also provide a reference basis for the study of the distribution of sedimentary systems in the context of similar fluvial deposits.
In order to break through the bottleneck of high cost of oil and gas exploration due to the difficulty of accurate prediction of sedimentary system, the study is based on the theory of distributive fluvial system, and takes the Jurassic Shaximiao Formation in Northwest Sichuan Basin as the sample, synthesizes the data of field outcrops, cores, and logging-recording wells, etc., and utilizes the research methods combining the analysis of the ground surface and the subsurface, and the analysis of the quantitative and qualitative analyses, so as to achieve the quantitative characterization of the sedimentary system of the Shaximiao Formation in the Northwest Sichuan Basin. The results of the study showed that: (1) The Shaximiao Formation in the central and northwest Sichuan basin has developed three types of sedimentary systems: distributive fluvial system, delta system, and lacustrine system. The distributive fluvial system of Shaximiao Formation in the study area can be divided into 3 subfacies and 6 microfacies types, the delta system can be divided into 2 subfacies and 4 microfacies types, and the lacustrine system can be further divided into shore-shallow lake beach-bar deposits. (2) The ratio of subfacies net-to-gross ratio is 50%-70% in the proximal, and the sand bodies are superimposed with thick layers. The medial subfacies net-to-gross ratio is between 30%-50%, and the sand body is distributed in thick layers. The distal subfacies net-to-gross ratio is 20%-30%, and the sand body is isolated. The delta and lacustrine facies systems are distributed in the middle Sichuan basin, and the net-to-gross ratio change is generally less than 15%. The sand bodies are also distributed in continuous sheets, but the maximum thickness of single sand bodies is thinner than that of the medial facies belt. (3) The Shaximiao Formation in the study area is mainly influenced by fluvial action, with the Sha 1 Member showing the distributive fluvial system terminating in a larger range of lacustrine deposits, and the Sha 2 Member showing the distributive fluvial system terminating in some small temporary lakes. (4) According to the modern sedimentary reference, in addition to the development of large high-quality reservoirs in the delta system, the medial and distal facies of the distributive fluvial system can also form continuous channel sand reservoirs. The results of the research are of great significance in characterizing the development scale and overlay style of different sand bodies, and also provide a reference basis for the study of the distribution of sedimentary systems in the context of similar fluvial deposits.
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[Objective] The carbonate content is high in the source rocks of the Permian Pusige Formation in the southwestern depression of Tarim Basin, and the relationship between carbonate and organic matter remains unclear. This has hindered the evaluation of their potential for hydrocarbon generation.[Methods] Through analysis using techniques such as core examination, thin section analysis, scanning electron microscopy (SEM), whole-rock X-ray diffraction (XRD), and geochemical studies, this research investigates the storage state and origin of carbonate, the distribution patterns of organic matter, and the relationship between carbonate and organic matter in the source rocks of the Permian Pusige Formation located at the peripheral areas of Kekeya in the southwestern depression of Tarim Basin. [Results] The source rocks of Permian Pusige Formation primarily exhibit three lithofacies associations: sand-mud, gray-mud, and sand-gray-mud. Calcite in the source rock mainly exists as cement and particles, displaying a multi-scale cycloidal feature of increasing and then decreasing from bottom to top within each “sand-gray-mud” cycle. The sources of carbonate include: 1) detrital particles derived from ancient continental sediments; 2) intra-depositional detritus or carbonate laminae; and 3) cementation or replacement products formed during the diagenetic phase. By conducting detailed analysis of the multi-scale cyclothem sequences, it was observed that organic matter tends to accumulate at the tops of these sequences. The main hydrocarbon-bearing intervals could be the laminites at the top of each cyclothem. Geochemical analysis revealed a strong negative correlation between carbonate minerals like calcite and Rock-Eval parameters such as TOC (Total Organic Carbon), PG (Pyrolytic Hydrocarbon Generation Potential), and HI (Hydrogen Index), reflecting a pattern of high calcium and low Rock-Eval parameters. The underlying cause is that calcium primarily resides in sandstone and calcitic layers at the cycle's base and middle sections, where stronger hydrodynamic conditions during deposition were unfavorable for organic accumulation and preservation. [Conclusion] The high-calcium source rocks within multi-scale "sand-gray-mud" cycles may possess unique characteristics pertaining to hydrocarbon generation conditions, expulsion efficiency, and oil/gas migration processes, indicating a substantial potential for hydrocarbon generation in the source rocks of Permian Pusige Formation in the Kekeya.
[Objective] The carbonate content is high in the source rocks of the Permian Pusige Formation in the southwestern depression of Tarim Basin, and the relationship between carbonate and organic matter remains unclear. This has hindered the evaluation of their potential for hydrocarbon generation.[Methods] Through analysis using techniques such as core examination, thin section analysis, scanning electron microscopy (SEM), whole-rock X-ray diffraction (XRD), and geochemical studies, this research investigates the storage state and origin of carbonate, the distribution patterns of organic matter, and the relationship between carbonate and organic matter in the source rocks of the Permian Pusige Formation located at the peripheral areas of Kekeya in the southwestern depression of Tarim Basin. [Results] The source rocks of Permian Pusige Formation primarily exhibit three lithofacies associations: sand-mud, gray-mud, and sand-gray-mud. Calcite in the source rock mainly exists as cement and particles, displaying a multi-scale cycloidal feature of increasing and then decreasing from bottom to top within each “sand-gray-mud” cycle. The sources of carbonate include: 1) detrital particles derived from ancient continental sediments; 2) intra-depositional detritus or carbonate laminae; and 3) cementation or replacement products formed during the diagenetic phase. By conducting detailed analysis of the multi-scale cyclothem sequences, it was observed that organic matter tends to accumulate at the tops of these sequences. The main hydrocarbon-bearing intervals could be the laminites at the top of each cyclothem. Geochemical analysis revealed a strong negative correlation between carbonate minerals like calcite and Rock-Eval parameters such as TOC (Total Organic Carbon), PG (Pyrolytic Hydrocarbon Generation Potential), and HI (Hydrogen Index), reflecting a pattern of high calcium and low Rock-Eval parameters. The underlying cause is that calcium primarily resides in sandstone and calcitic layers at the cycle's base and middle sections, where stronger hydrodynamic conditions during deposition were unfavorable for organic accumulation and preservation. [Conclusion] The high-calcium source rocks within multi-scale "sand-gray-mud" cycles may possess unique characteristics pertaining to hydrocarbon generation conditions, expulsion efficiency, and oil/gas migration processes, indicating a substantial potential for hydrocarbon generation in the source rocks of Permian Pusige Formation in the Kekeya.
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Abstract: In order to study the characteristics of hydrocarbon enrichment at oil source faults above mudstone caprocks in petroliferous basins with developed lower source rocks, based on the research on the mechanism and degree of weakening of hydrocarbon transport capacity of oil source faults by mudstone caprocks, we use the paleo-fault throw, dip angle, paleo-normal pressure generated by the weight of overlying sedimentary load, paleo-formation pore fluid pressure difference of source rocks, and hydrocarbon supply capacity index within the mudstone caprock to calculate the hydrocarbon transport capacity index of oil source faults that do not penetrate the mudstone caprock. Then, we divide this value by the paleo-connection thickness of the mudstone caprock to obtain the hydrocarbon transport capacity index of oil source faults that penetrate the mudstone caprock. According to the relative magnitudes of the two indices, we establish a method for assessing the degree of weakening of hydrocarbon transport capacity of oil source faults by mudstone caprocks. We apply this method to study the degree of weakening of hydrocarbon transport capacity from the Gangdong fault to the third member of the Dongying Formation (Ed3) by the mudstone caprock of the middle part of the first member of Shahejie Formation (Es1m) in the Qikou Sag of the Bohai Bay Basin. The results indicate that the degree of weakening of hydrocarbon transport capacity from the mudstone caprock of Es1m affecting the Gangdong fault to Ed3 is zero at measurement points 4-10, 100% at measurement points 1-3 and 11-12, and between 0% and 100% at measurement points 3-4 and 10-11. The measurement points 4-10, where the degree of weakening of the hydrocarbon transport capacity from the Es1m mudstone caprock to the Gangdong Fault for transporting hydrocarbons to Ed3 is relatively small, are favorable locations for the accumulation and reservoir formation of hydrocarbons supplied by the underlying third member of the Shahejie Formation (Es3) source rock in Ed3. This is consistent with the current distribution of hydrocarbons discovered in Ed3 at the Gangdong Fault, which are mainly concentrated at measurement points 6-10, indicating that the method is feasible for studying the degree of weakening of the hydrocarbon transport capacity of oil - source faults by mudstone caprocks.
Abstract: In order to study the characteristics of hydrocarbon enrichment at oil source faults above mudstone caprocks in petroliferous basins with developed lower source rocks, based on the research on the mechanism and degree of weakening of hydrocarbon transport capacity of oil source faults by mudstone caprocks, we use the paleo-fault throw, dip angle, paleo-normal pressure generated by the weight of overlying sedimentary load, paleo-formation pore fluid pressure difference of source rocks, and hydrocarbon supply capacity index within the mudstone caprock to calculate the hydrocarbon transport capacity index of oil source faults that do not penetrate the mudstone caprock. Then, we divide this value by the paleo-connection thickness of the mudstone caprock to obtain the hydrocarbon transport capacity index of oil source faults that penetrate the mudstone caprock. According to the relative magnitudes of the two indices, we establish a method for assessing the degree of weakening of hydrocarbon transport capacity of oil source faults by mudstone caprocks. We apply this method to study the degree of weakening of hydrocarbon transport capacity from the Gangdong fault to the third member of the Dongying Formation (Ed3) by the mudstone caprock of the middle part of the first member of Shahejie Formation (Es1m) in the Qikou Sag of the Bohai Bay Basin. The results indicate that the degree of weakening of hydrocarbon transport capacity from the mudstone caprock of Es1m affecting the Gangdong fault to Ed3 is zero at measurement points 4-10, 100% at measurement points 1-3 and 11-12, and between 0% and 100% at measurement points 3-4 and 10-11. The measurement points 4-10, where the degree of weakening of the hydrocarbon transport capacity from the Es1m mudstone caprock to the Gangdong Fault for transporting hydrocarbons to Ed3 is relatively small, are favorable locations for the accumulation and reservoir formation of hydrocarbons supplied by the underlying third member of the Shahejie Formation (Es3) source rock in Ed3. This is consistent with the current distribution of hydrocarbons discovered in Ed3 at the Gangdong Fault, which are mainly concentrated at measurement points 6-10, indicating that the method is feasible for studying the degree of weakening of the hydrocarbon transport capacity of oil - source faults by mudstone caprocks.
Abstract:
The Late Triassic period in the Western Sichuan Foreland Basin represents a critical phase of the Indosinian orogeny. The sedimentary strata formed during this period not only preserve the geological records of the basin's tectonic evolution but also contain key information about volcanic activities during that time. Analyzing the material sources of tuff in the strata can provide important evidence for revealing the tectonic-magmatic activity patterns of the surrounding orogenic belts during the Indosinian Orogeny. This study selected the tuff of the Xujia Formation in the Jiguan Mountain section of the Western Sichuan Foreland Basin as the research object. Through a combination of petrological analysis, geochemical characteristic analysis, and zircon U-Pb isotopic dating techniques, the sedimentary age of this tuff layer and the source of its magma were revealed. Zircon U-Pb dating yielded ages of 213.5 Ma and 208.5 Ma, corresponding to the Late Triassic Norian-Rhaetian stage. The rare earth element (REE) patterns of the tuffaceous rocks exhibit a right-leaning distribution characterized by enriched light rare earth elements (LREEs) and relatively flat heavy rare earth elements (HREEs), with pronounced negative Eu anomalies. Primitive mantle-normalized trace element spider diagrams show enrichment in large-ion lithophile elements (LILEs, e.g., Rb, Ba) and depletion in high-field-strength elements (HFSEs, e.g., Ta), consistent with geochemical signatures of continental lower crust. Zircon Hf isotopic data indicate that the magmas originated from a mixture of crustal contamination and depleted mantle sources. Building upon this foundation, and in consideration of the regional distribution and formation age of Late Triassic igneous rocks in the periphery of the western Sichuan foreland basin, integrated with geochemical characteristics of magmatic rocks, it is concluded that the volcanic material of tuff shares a common magmatic origin with the Late Triassic andesite of the Songpan-Ganzi terrane. This magmatic activity is closely associated with the melting of continental crust under the context of bidirectional plate subduction.
The Late Triassic period in the Western Sichuan Foreland Basin represents a critical phase of the Indosinian orogeny. The sedimentary strata formed during this period not only preserve the geological records of the basin's tectonic evolution but also contain key information about volcanic activities during that time. Analyzing the material sources of tuff in the strata can provide important evidence for revealing the tectonic-magmatic activity patterns of the surrounding orogenic belts during the Indosinian Orogeny. This study selected the tuff of the Xujia Formation in the Jiguan Mountain section of the Western Sichuan Foreland Basin as the research object. Through a combination of petrological analysis, geochemical characteristic analysis, and zircon U-Pb isotopic dating techniques, the sedimentary age of this tuff layer and the source of its magma were revealed. Zircon U-Pb dating yielded ages of 213.5 Ma and 208.5 Ma, corresponding to the Late Triassic Norian-Rhaetian stage. The rare earth element (REE) patterns of the tuffaceous rocks exhibit a right-leaning distribution characterized by enriched light rare earth elements (LREEs) and relatively flat heavy rare earth elements (HREEs), with pronounced negative Eu anomalies. Primitive mantle-normalized trace element spider diagrams show enrichment in large-ion lithophile elements (LILEs, e.g., Rb, Ba) and depletion in high-field-strength elements (HFSEs, e.g., Ta), consistent with geochemical signatures of continental lower crust. Zircon Hf isotopic data indicate that the magmas originated from a mixture of crustal contamination and depleted mantle sources. Building upon this foundation, and in consideration of the regional distribution and formation age of Late Triassic igneous rocks in the periphery of the western Sichuan foreland basin, integrated with geochemical characteristics of magmatic rocks, it is concluded that the volcanic material of tuff shares a common magmatic origin with the Late Triassic andesite of the Songpan-Ganzi terrane. This magmatic activity is closely associated with the melting of continental crust under the context of bidirectional plate subduction.
Abstract:
[Objective] The successful breakthrough in the Jurassic of Well Fusha 8 in the piedmont area of the southwest Tarim Basin marks the emergence of a new oil-bearing series, demonstrating promising prospects for oil and gas exploration. Currently, the insufficient understanding of the provenance characteristics and sedimentary evolution of the Jurassic system has become a critical issue constraining oil and gas exploration. [Methods] Taking the Jurassic in the piedmont area of the southwest Tarim Basin as the research object, this study investigates the provenance systems, sedimentary facies types, and sedimentary evolution processes based on field outcrops, drilling data, and other relevant materials. [Results] The Jurassic were primarily sourced from the West Kunlun Mountains and the West Tianshan Mountains, with weaker contributions from the slope area. Both the Fusha and Aketao Fault Depressions developed two major provenance systems: the southern (West Kunlun Mountains) and the northern (slope area). In contrast, the Kuzigongsu Fault Depression was mainly sourced from the West Tianshan Mountains. Fourteen lithofacies and fourteen lithofacies associations were found in the Jurassic, and seven sedimentary facies types were determined. The Fusha and Aketao Fault depressions in the piedmont area of the West Kunlun Mountains underwent a evolution during the Early to Middle Jurassic, progressing from the initial rifting stage to the intense rifting stage and finally to the rift-sag transition stage. The steep slope zone in the south primarily developed alluvial fan-fan delta sedimentary systems, while the gentle slope zone in the north developed small-scale braided river deltas and fan deltas. In the Kuzigongsu Fault Depression in front of the West Tianshan Mountains, the boundary faults remained active during the Early to Middle Jurassic. The main sub-depression developed an alluvial fan-fan delta sedimentary system, while the secondary sub-depression was initially filled and leveled by alluvial fan conglomerates. In the later stage, the rifting activity expanded westward, predominantly forming braided river-meandering river deposits. During the Late Jurassic, compression and regional aridification triggered basin basement uplift and enhanced sediment supply, resulting in widespread development of alluvial fan deposits across the study area, marking the terminal stage of rifted lake basin evolution. [Conclusions] Overall, the Jurassic strata in the piedmont area of the southwestern Tarim Basin experienced a transgression during the Early to Middle Jurassic and a regression during the Middle to Late Jurassic, reflecting a multi-stage sedimentary evolution. This study not only reconstructs the filling and evolution process of the Jurassic in the southwestern Tarim Basin but also provides valuable insights for the paleogeographic reconstruction of sedimentary areas in residual basins.
[Objective] The successful breakthrough in the Jurassic of Well Fusha 8 in the piedmont area of the southwest Tarim Basin marks the emergence of a new oil-bearing series, demonstrating promising prospects for oil and gas exploration. Currently, the insufficient understanding of the provenance characteristics and sedimentary evolution of the Jurassic system has become a critical issue constraining oil and gas exploration. [Methods] Taking the Jurassic in the piedmont area of the southwest Tarim Basin as the research object, this study investigates the provenance systems, sedimentary facies types, and sedimentary evolution processes based on field outcrops, drilling data, and other relevant materials. [Results] The Jurassic were primarily sourced from the West Kunlun Mountains and the West Tianshan Mountains, with weaker contributions from the slope area. Both the Fusha and Aketao Fault Depressions developed two major provenance systems: the southern (West Kunlun Mountains) and the northern (slope area). In contrast, the Kuzigongsu Fault Depression was mainly sourced from the West Tianshan Mountains. Fourteen lithofacies and fourteen lithofacies associations were found in the Jurassic, and seven sedimentary facies types were determined. The Fusha and Aketao Fault depressions in the piedmont area of the West Kunlun Mountains underwent a evolution during the Early to Middle Jurassic, progressing from the initial rifting stage to the intense rifting stage and finally to the rift-sag transition stage. The steep slope zone in the south primarily developed alluvial fan-fan delta sedimentary systems, while the gentle slope zone in the north developed small-scale braided river deltas and fan deltas. In the Kuzigongsu Fault Depression in front of the West Tianshan Mountains, the boundary faults remained active during the Early to Middle Jurassic. The main sub-depression developed an alluvial fan-fan delta sedimentary system, while the secondary sub-depression was initially filled and leveled by alluvial fan conglomerates. In the later stage, the rifting activity expanded westward, predominantly forming braided river-meandering river deposits. During the Late Jurassic, compression and regional aridification triggered basin basement uplift and enhanced sediment supply, resulting in widespread development of alluvial fan deposits across the study area, marking the terminal stage of rifted lake basin evolution. [Conclusions] Overall, the Jurassic strata in the piedmont area of the southwestern Tarim Basin experienced a transgression during the Early to Middle Jurassic and a regression during the Middle to Late Jurassic, reflecting a multi-stage sedimentary evolution. This study not only reconstructs the filling and evolution process of the Jurassic in the southwestern Tarim Basin but also provides valuable insights for the paleogeographic reconstruction of sedimentary areas in residual basins.
Abstract:
[Objective] Pore-laminar fracture system is an important reservoir space and seepage channel of shale oil, which controls the distribution of shale oil. Due to the internal relationship between pores and lamellar fractures in genesis, this paper will take the pore-laminar fracture system as a whole to be researched. [Methods] The shale of the Es3x-Es4s of the Paleogene in Jiyang Depression is selected as an example. Taking the lamina as a unit, a series of experiments were carried out on the two typical lithofacies of organic-rich laminated limestone and organic-rich laminated mudstone by means of microscope observation, scanning electron microscope and SEM-EDS-AMICS analysis technology, and the development distribution mode and control factors of pore-lamellar fracture system were discussed. [Results and Discussions] The results show that the organic-rich laminated limestone mainly developed calcite-associated pores (intercrystalline pores, intergranular pores and dissolved pores), accounting for 32.48% of the total surface pore rate. The boundary between the two laminae is clear and the laminar fractures are well developed. The organic-rich laminated mudstone is most developed with clay mineral-associated pores (intercrystalline pores and intergranular pores), accounting for 37.51 % of the total surface pore rate, and there is also a good development of laminar fractures. [Conclusions] The pore-laminate fracture system of organic-rich laminated limestone is mainly composed of calcite intercrystalline pores, clay mineral intercrystalline pores and laminar fractures. The organic-rich laminated mudstone is mainly composed of clay mineral intercrystalline pores, organic pores and laminar fractures. The development of different pore-laminar fracture system in the two lithofacies is controlled by sedimentation and diagenesis. The depositional environments control the types of laminae and their combinatorial methods. In the process of diagenesis, the organic-rich laminated limestone is mainly affected by calcite recrystallization and dissolution, and the organic-rich laminated mudstone is dominated by clay mineral transformation and thermal evolution of organic matter. It is of great significance to clarify the distribution pattern and development control factors of pore-laminar fracture system for guiding the exploration and development of shale oil in Jiyang Depression.
[Objective] Pore-laminar fracture system is an important reservoir space and seepage channel of shale oil, which controls the distribution of shale oil. Due to the internal relationship between pores and lamellar fractures in genesis, this paper will take the pore-laminar fracture system as a whole to be researched. [Methods] The shale of the Es3x-Es4s of the Paleogene in Jiyang Depression is selected as an example. Taking the lamina as a unit, a series of experiments were carried out on the two typical lithofacies of organic-rich laminated limestone and organic-rich laminated mudstone by means of microscope observation, scanning electron microscope and SEM-EDS-AMICS analysis technology, and the development distribution mode and control factors of pore-lamellar fracture system were discussed. [Results and Discussions] The results show that the organic-rich laminated limestone mainly developed calcite-associated pores (intercrystalline pores, intergranular pores and dissolved pores), accounting for 32.48% of the total surface pore rate. The boundary between the two laminae is clear and the laminar fractures are well developed. The organic-rich laminated mudstone is most developed with clay mineral-associated pores (intercrystalline pores and intergranular pores), accounting for 37.51 % of the total surface pore rate, and there is also a good development of laminar fractures. [Conclusions] The pore-laminate fracture system of organic-rich laminated limestone is mainly composed of calcite intercrystalline pores, clay mineral intercrystalline pores and laminar fractures. The organic-rich laminated mudstone is mainly composed of clay mineral intercrystalline pores, organic pores and laminar fractures. The development of different pore-laminar fracture system in the two lithofacies is controlled by sedimentation and diagenesis. The depositional environments control the types of laminae and their combinatorial methods. In the process of diagenesis, the organic-rich laminated limestone is mainly affected by calcite recrystallization and dissolution, and the organic-rich laminated mudstone is dominated by clay mineral transformation and thermal evolution of organic matter. It is of great significance to clarify the distribution pattern and development control factors of pore-laminar fracture system for guiding the exploration and development of shale oil in Jiyang Depression.
Abstract:
[Objective] The Permian represents a critical period of tectonic-sedimentary transition in the northern Ordos Basin. A detailed characterization of its complex source-to-sink processes is essential for revealing multi-provenance differential supply patterns and paleogeographic evolution.[Methods] This study systematically analyzes provenance signals and sedimentary filling processes in the northern Ordos Basin through outcrop observations, well-logging data, and detrital zircon U-Pb geochronology, aiming to clarify the paleogeographic framework constrained by source-to-sink systems and its implications for the closure of the Paleo-Asian Ocean. [Results](1) Detrital zircon U-Pb ages reveal that the Alxa Block, Xing-Meng Orogenic Belt, and North China Craton contributed differentially during the Permian, with marked east-west variations. Multidimensional scaling (MDS) analysis divides the northern basin into three provenance zones: Alxa, western Yinshan, and eastern Yinshan. (2) The Permian depositional systems evolved from tide-dominated deltas to braided-river deltas, with sediment thickness and sandbody distribution showing a "thicker in the west and stronger in the north" pattern. Provenance supply intensity closely correlated with environmental changes. Enhanced lateral continuity of sandbodies in the Shanxi and Shihezi Formations reflects braided channel development under strong sediment supply.(3) Source-to-sink analysis indicates:Taiyuan Stage: Dominated by proximal weak supply.Shanxi Stage: Increased provenance mixing formed an east-west differentiated depositional pattern.Shihezi Stage: Significant uplift of the North China Craton basement led to dominant distal supply, with maximum sandbody thickness and spatial extent. [Conclusions]The inhomogeneity of the subduction/closure of the Paleo-Asian Ocean drove the differential uplift characterized by "strong in the west and weak in the east" in the Xing'an-Mongolian Orogenic Belt and "strong in the east and weak in the west" in the basement of the North China Craton. This controlled the east-west differentiation pattern and evolution of the provenance in the northern part of the basin, which showed "mixed provenance in the west and cratonic basement in the east". The sedimentary system transformed from a tide-dominated delta in the marine-continental transitional environment to a continental braided river delta from the Taiyuan Stage to the Shihezi Stage. [Significance] The established paleogeographic evolution model, based on source-to-sink processes, provides a dynamic coupling framework for understanding multi-provenance-sedimentation responses and hydrocarbon exploration in large cratonic basins.
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Abstract: [Objective] The Oceanic Anoxic Event 1d (OAE 1d), took place during the late Albian of the Early Cretaceous, represents a significant perturbation in the global carbon cycle associated with a greenhouse climate. Although extensive research has been conducted on the characteristics and origin of OAE 1d in the western Tethys and North Atlantic regions, the paleoenvironmental response to this event in the eastern Tethys remains poorly understood. [Methods] This study focuses on the Lower Cretaceous Lengqingre Formation at the Youxia section in Dingri (southern Tibet) within the eastern Tethyan domain. Here, an integrated approach of mineralogical, petrological, sedimentological, isotopic, and elemental geochemical analyses, was employed to evaluated the paleoenvironmental response to OAE 1d and its distinction from coeval global patterns compared to contemporaneous global records. [Results] (1) Both bulk carbonate and organic carbon isotope data confirm that the Youxia section records the OAE 1d excursion, supporting the global signal of this phenomenon. (2) Thin-section observations and terrigenous input proxies (e.g., Si/Al and Ti/Al ratios) indicate enhanced input of siliciclastic materials during OAE 1d, likely linked to accelerated hydrological cycling and intensified continental weathering related to coeval volcanic activity. (3) Corg/P ratios and framboidal pyrite characteristics reveal that the Youxia section was deposited under oxic to suboxic bottom-water conditions during OAE 1d, markedly different from the anoxic conditions observed in the western Tethys. This highlights the spatial heterogeneity in marine redox conditions during this event. (4) The absence of organic carbon-rich sediments at Youxia is attributed to a combination of oxic environmental conditions, intense terrigenous input, and dilution effects induced by rapid sedimentation rates. This suggests that organic carbon burial was suppressed by regional environmental controls in the Dingri region during OAE 1d. [Conclusion] The oxic-suboxic conditions and lack of organic-rich sediments in the eastern Tethys during OAE 1d underscore the regional heterogeneity in sedimentary and oceanographic responses to global Oceanic Anoxic Events. These findings provide critical insights into complexity of paleoenvironmental feedbacks during global Oceanic Anoxic Events.
Abstract: [Objective] The Oceanic Anoxic Event 1d (OAE 1d), took place during the late Albian of the Early Cretaceous, represents a significant perturbation in the global carbon cycle associated with a greenhouse climate. Although extensive research has been conducted on the characteristics and origin of OAE 1d in the western Tethys and North Atlantic regions, the paleoenvironmental response to this event in the eastern Tethys remains poorly understood. [Methods] This study focuses on the Lower Cretaceous Lengqingre Formation at the Youxia section in Dingri (southern Tibet) within the eastern Tethyan domain. Here, an integrated approach of mineralogical, petrological, sedimentological, isotopic, and elemental geochemical analyses, was employed to evaluated the paleoenvironmental response to OAE 1d and its distinction from coeval global patterns compared to contemporaneous global records. [Results] (1) Both bulk carbonate and organic carbon isotope data confirm that the Youxia section records the OAE 1d excursion, supporting the global signal of this phenomenon. (2) Thin-section observations and terrigenous input proxies (e.g., Si/Al and Ti/Al ratios) indicate enhanced input of siliciclastic materials during OAE 1d, likely linked to accelerated hydrological cycling and intensified continental weathering related to coeval volcanic activity. (3) Corg/P ratios and framboidal pyrite characteristics reveal that the Youxia section was deposited under oxic to suboxic bottom-water conditions during OAE 1d, markedly different from the anoxic conditions observed in the western Tethys. This highlights the spatial heterogeneity in marine redox conditions during this event. (4) The absence of organic carbon-rich sediments at Youxia is attributed to a combination of oxic environmental conditions, intense terrigenous input, and dilution effects induced by rapid sedimentation rates. This suggests that organic carbon burial was suppressed by regional environmental controls in the Dingri region during OAE 1d. [Conclusion] The oxic-suboxic conditions and lack of organic-rich sediments in the eastern Tethys during OAE 1d underscore the regional heterogeneity in sedimentary and oceanographic responses to global Oceanic Anoxic Events. These findings provide critical insights into complexity of paleoenvironmental feedbacks during global Oceanic Anoxic Events.
Abstract:
The tight sandstone interlayer of the Chang 71-2 sub-member of the Yanchang Formation in the Qingcheng Oilfield is a typical sublacustrine fan deposit. However, the research on the difference of sandstone reservoir performance on the sedimentary microfacies scale is insufficient, which restricts the screening of high-yield blocks and the difference analysis of horizontal well production effects. Based on core, logging and seismic data, using thin section identification, scanning electron microscopy, micro-CT, nuclear magnetic resonance, high-pressure mercury injection and constant-rate mercury injection tests, combined with oil test and production test data, the differences in tight sandstone rock and ore, microscopic pore structure, oil content and mobility, and oil test in different sedimentary microfacies are analyzed. The results show that under the control of multi-stage slope break zones, multi-stage sublacustrine fan sand bodies accumulate in the relatively gentle semi-deep lake-deep lake slope toe zone, and frequently interact with organic-rich shale to form a large area of ' sandwich ' source-reservoir symbiosis, which constitutes the main geological dessert of interlayer shale oil in Qingcheng Oilfield. The sandstone interlayer can be divided into four sedimentary microfacies : channel, lobe, lobe side edge and slump body. The sandstone superimposed thickness of channel, lobe and lobe side edge is relatively large, and the logging interpretation is generally oil layer, which is the main target of horizontal well development. The reservoir space of channel, lobe body and lobe lateral margin is dominated by dissolution pores and intergranular pores. The cements are mainly ferrodolomite, ferrocalcite and quartz. The channel and lobe body have the characteristics of low content of interstitial materials and cements, high surface porosity, good physical properties and microscopic pore structure, high proportion of connected pore volume, large pore throat radius, and relatively high movable fluid saturation and oiliness. The saturation of movable fluid is closely related to the pore structure. Controlled by the microscopic pore structure, the channel has better microscopic pore throat parameters than the lobe, which is more conducive to the filling and seepage of oil and gas. Under the same thickness scale, the channel is easier to obtain high-yield industrial oil flow than the lobed body vertical well test, and the vertical superposition of the channel and the lobed body is a high-yield sedimentary combination box ; fine sublacustrine fan tight sandstone sedimentary microfacies characterization, optimizing the orientation and length of horizontal wells, and optimizing favorable microfacies combinations for development are the key to improving the drilling rate and development effect of horizontal well reservoirs.
The tight sandstone interlayer of the Chang 71-2 sub-member of the Yanchang Formation in the Qingcheng Oilfield is a typical sublacustrine fan deposit. However, the research on the difference of sandstone reservoir performance on the sedimentary microfacies scale is insufficient, which restricts the screening of high-yield blocks and the difference analysis of horizontal well production effects. Based on core, logging and seismic data, using thin section identification, scanning electron microscopy, micro-CT, nuclear magnetic resonance, high-pressure mercury injection and constant-rate mercury injection tests, combined with oil test and production test data, the differences in tight sandstone rock and ore, microscopic pore structure, oil content and mobility, and oil test in different sedimentary microfacies are analyzed. The results show that under the control of multi-stage slope break zones, multi-stage sublacustrine fan sand bodies accumulate in the relatively gentle semi-deep lake-deep lake slope toe zone, and frequently interact with organic-rich shale to form a large area of ' sandwich ' source-reservoir symbiosis, which constitutes the main geological dessert of interlayer shale oil in Qingcheng Oilfield. The sandstone interlayer can be divided into four sedimentary microfacies : channel, lobe, lobe side edge and slump body. The sandstone superimposed thickness of channel, lobe and lobe side edge is relatively large, and the logging interpretation is generally oil layer, which is the main target of horizontal well development. The reservoir space of channel, lobe body and lobe lateral margin is dominated by dissolution pores and intergranular pores. The cements are mainly ferrodolomite, ferrocalcite and quartz. The channel and lobe body have the characteristics of low content of interstitial materials and cements, high surface porosity, good physical properties and microscopic pore structure, high proportion of connected pore volume, large pore throat radius, and relatively high movable fluid saturation and oiliness. The saturation of movable fluid is closely related to the pore structure. Controlled by the microscopic pore structure, the channel has better microscopic pore throat parameters than the lobe, which is more conducive to the filling and seepage of oil and gas. Under the same thickness scale, the channel is easier to obtain high-yield industrial oil flow than the lobed body vertical well test, and the vertical superposition of the channel and the lobed body is a high-yield sedimentary combination box ; fine sublacustrine fan tight sandstone sedimentary microfacies characterization, optimizing the orientation and length of horizontal wells, and optimizing favorable microfacies combinations for development are the key to improving the drilling rate and development effect of horizontal well reservoirs.
Abstract:
Abstract: In order to deeply understand the source properties of Quaternary mudstones in Sebei area of the Sanhu Depression in the Qaidam Basin, this paper focuses on sedimentary geochemistry and conducts analysis and testing of major, trace, and rare earth elements on core samples from different depths of the Quaternary in the Sebei area's Setan 1 and Setan2 wells (ST1 and ST2 wells). In general, the samples has low silicon (SiO2=48.17%), high aluminum (Al2O3=15.23%, aluminum index A / NK=2.54, aluminum saturation index A / NK = 1.59), rich magnesium and calcium (MgO=3.81%, CaO=7.91%), relatively rich iron (Fe2O3=4.99), low total alkali content (K2O + Na2O=5.94%), significant enrichment of Ba and Cu compared to UCC, Nb depletion, and other trace and rare earth elements comparable to the upper crust. Quaternary mudstones in Sebu area are less affected by sedimentary sorting, rewinding, weathering and diagenesis and its elemental features can effectively indicate the source and tectonic background. The results show that the characteristics of the two well samples are relatively consistent. The F1-F2, Co / Th-La / Sc, La / Th-Hf, La / Yb-Σ REE discrimination diagrams, rare earth right-dip flat distribution pattern and related parameters all show that the sedimentary source of Quaternary mudstones in the study area is mainly derived from the upper crust felsic sedimentary rocks and igneous rock sources. (SiO2/ Al2O3) - (K2O / Na2O), (K2O / Na2O) -SiO2, [K2O / (Na2O + CaO)] - (SiO2 / Al2O3), Ti / Zr-La / Sc, La-Th-Sc, Th-Co-Zr / 10 and Th-Zr / 10 ternary phase diagrams and related parameters determine that the source area is an active continental margin and continental island arc environment. The potential source areas of Quaternary sediments in the Sebei area of Sanhu Depression mainly include the eastern Kunlun orogenic belt on the south side of the depression and the south Qilian orogenic belt on the north side of the depression.
Abstract: In order to deeply understand the source properties of Quaternary mudstones in Sebei area of the Sanhu Depression in the Qaidam Basin, this paper focuses on sedimentary geochemistry and conducts analysis and testing of major, trace, and rare earth elements on core samples from different depths of the Quaternary in the Sebei area's Setan 1 and Setan2 wells (ST1 and ST2 wells). In general, the samples has low silicon (SiO2=48.17%), high aluminum (Al2O3=15.23%, aluminum index A / NK=2.54, aluminum saturation index A / NK = 1.59), rich magnesium and calcium (MgO=3.81%, CaO=7.91%), relatively rich iron (Fe2O3=4.99), low total alkali content (K2O + Na2O=5.94%), significant enrichment of Ba and Cu compared to UCC, Nb depletion, and other trace and rare earth elements comparable to the upper crust. Quaternary mudstones in Sebu area are less affected by sedimentary sorting, rewinding, weathering and diagenesis and its elemental features can effectively indicate the source and tectonic background. The results show that the characteristics of the two well samples are relatively consistent. The F1-F2, Co / Th-La / Sc, La / Th-Hf, La / Yb-Σ REE discrimination diagrams, rare earth right-dip flat distribution pattern and related parameters all show that the sedimentary source of Quaternary mudstones in the study area is mainly derived from the upper crust felsic sedimentary rocks and igneous rock sources. (SiO2/ Al2O3) - (K2O / Na2O), (K2O / Na2O) -SiO2, [K2O / (Na2O + CaO)] - (SiO2 / Al2O3), Ti / Zr-La / Sc, La-Th-Sc, Th-Co-Zr / 10 and Th-Zr / 10 ternary phase diagrams and related parameters determine that the source area is an active continental margin and continental island arc environment. The potential source areas of Quaternary sediments in the Sebei area of Sanhu Depression mainly include the eastern Kunlun orogenic belt on the south side of the depression and the south Qilian orogenic belt on the north side of the depression.
Abstract:
[Objective] The shale gas of the Cambrian Qiongzhusi Formation in the Sichuan Basin has good prospects for exploration and development. It is of great significance to reveal the characteristics of its sedimentary environment for the evaluation of shale gas resources in the Qiongzhusi Formation. [Methods] Taking the shale series of Qiongzhusi Formation in Chengjiaba section of northern Sichuan as the research object, based on detailed field geological survey, the whole rock X-ray diffraction and geochemical analysis were carried out, and the characteristic elements and ratio parameters were optimized to reveal the sedimentary paleoenvironment and conditions of organic-rich shale in Qiongzhusi Formation in northern Sichuan. [Results] (1) The first and second members of Qiongzhusi Formation are mainly gray-black or black mudstone and silty mudstone, with siliceous rock facies and siliceous shale facies, with horizontal bedding, which is a deep-water shelf facies. (2) The shale TOC content of Qiongzhusi Formation is relatively high, ranging from 0.22 % to 4.34 %, with an average of 2.68 %. (3) The geochemical characteristics of the elements show that the climate was warm and humid during the deposition of the black shale of the Qiongzhusi Formation, the paleoproductivity and deposition rate were high, and the water body was anoxic and highly retained. [Conclusions] It is considered that the formation of organic-rich shale in Qiongzhusi Formation is controlled by redox conditions, paleoproductivity, paleoclimate and deposition rate.
[Objective] The shale gas of the Cambrian Qiongzhusi Formation in the Sichuan Basin has good prospects for exploration and development. It is of great significance to reveal the characteristics of its sedimentary environment for the evaluation of shale gas resources in the Qiongzhusi Formation. [Methods] Taking the shale series of Qiongzhusi Formation in Chengjiaba section of northern Sichuan as the research object, based on detailed field geological survey, the whole rock X-ray diffraction and geochemical analysis were carried out, and the characteristic elements and ratio parameters were optimized to reveal the sedimentary paleoenvironment and conditions of organic-rich shale in Qiongzhusi Formation in northern Sichuan. [Results] (1) The first and second members of Qiongzhusi Formation are mainly gray-black or black mudstone and silty mudstone, with siliceous rock facies and siliceous shale facies, with horizontal bedding, which is a deep-water shelf facies. (2) The shale TOC content of Qiongzhusi Formation is relatively high, ranging from 0.22 % to 4.34 %, with an average of 2.68 %. (3) The geochemical characteristics of the elements show that the climate was warm and humid during the deposition of the black shale of the Qiongzhusi Formation, the paleoproductivity and deposition rate were high, and the water body was anoxic and highly retained. [Conclusions] It is considered that the formation of organic-rich shale in Qiongzhusi Formation is controlled by redox conditions, paleoproductivity, paleoclimate and deposition rate.
Abstract:
Gulong shale oil is an important replacement resource in the Songliao Basin, with an estimated resource amount of 15.1 billion tons. From the perspective of sedimentary and petrological characteristics, the most prominent feature of Gulong Qingshankou Formation shale is the extensive development of sand dikes, which is not found in shale oil reservoirs in other basins. Therefore, this is also an important information to explore the formation environment of Gulong Qingshankou Formation shale. The most peculiar thing is that in addition to sand dikes, there are two other special dikes in Gulong Qingshankou shale: pyrite dikes and cementite dikes. Pyrite dikes and cementite dikes are generally small in scale, with widths of 1-2 mm. Visible length several centimeters; Mostly curved like intestine, a few slightly curved. Multi-tilt output, tilt direction has a certain rule, 180° symmetry. The pyrite dikes in the shale oil reservoir of Qingshankou Formation in Gulong are formed by late pyritization of sand dikes. The cementation dikes are formed by gravity subsidence caused by density inversion and the formation kinetics of pyrite and tuff dike are also studied. The study of pyrite dikes and puffite dikes in the Gulong shale oil reservoir has three significances: first, the formation time, sedimentary environment, sedimentary process and sediment state of pyrite dikes and tuff dikes can be known; Second, the diagenetic compaction rate of shale can be known. Third, it can assist reservoir evaluation.
Gulong shale oil is an important replacement resource in the Songliao Basin, with an estimated resource amount of 15.1 billion tons. From the perspective of sedimentary and petrological characteristics, the most prominent feature of Gulong Qingshankou Formation shale is the extensive development of sand dikes, which is not found in shale oil reservoirs in other basins. Therefore, this is also an important information to explore the formation environment of Gulong Qingshankou Formation shale. The most peculiar thing is that in addition to sand dikes, there are two other special dikes in Gulong Qingshankou shale: pyrite dikes and cementite dikes. Pyrite dikes and cementite dikes are generally small in scale, with widths of 1-2 mm. Visible length several centimeters; Mostly curved like intestine, a few slightly curved. Multi-tilt output, tilt direction has a certain rule, 180° symmetry. The pyrite dikes in the shale oil reservoir of Qingshankou Formation in Gulong are formed by late pyritization of sand dikes. The cementation dikes are formed by gravity subsidence caused by density inversion and the formation kinetics of pyrite and tuff dike are also studied. The study of pyrite dikes and puffite dikes in the Gulong shale oil reservoir has three significances: first, the formation time, sedimentary environment, sedimentary process and sediment state of pyrite dikes and tuff dikes can be known; Second, the diagenetic compaction rate of shale can be known. Third, it can assist reservoir evaluation.
Abstract:
[Objective] Shallowing-upward sequences are extensively developed in shallow-water carbonate environments, and the identification of shallowing-upward sequences in deep-water carbonate rocks holds a unique advantage for analyzing the evolution of shallow-water carbonate sedimentary environments. [Method] This study focuses on the Late Cambrian Xixiangchi Formation in Xiuqi Town, Chengkou County, on the northern margin of the Yangtze Platform. Through detailed field sedimentological analysis and laboratory microfacies analysis, shallowing-upward sequences in carbonate rocks were identified. The compositional characteristics and vertical stacking relationships were analyzed, thereby exploring the Late Cambrian sedimentary environment evolution in the upper Yangtze region. [Results and Discussions] In the Xixiangchi Formation, 14 sedimentary microfacies have been identified, including MF0 argillaceous shale, MF1 micrite, MF2 bright crystal gravel limestone, MF3 bright crystal sand limestone, MF4 muddy siltstone, MF5 dolomitic microcrystalline limestone, MF6 gravel dolomite, MF7 bright crystal sand dolomite, MF8 sandy gravel grain dolomite, MF9 sandy sandy lithic grain dolomite, MF10 sandy dolomite, MF11 laminated dolomite, MF12 bamboo leaf-like gravel dolomite, and MF13 grain dolomite with dissolution pores. Based on the microfacies associations, six decimeter-scale upward-shallowing tidal sub-environments (C1-1 to C1-6) have been recognized within the Xixiangchi Formation, representing subtidal deposition. Additionally, six decimeter-scale upward-shallowing tidal-subtidal transition sub-environments (C2-1 to C2-6) and two decimeter-scale upward-shallowing intertidal-subtidal transition sub-environments (C3-1 and C3-2) have been identified. These decimeter-scale upward-shallowing cycles constitute three secondary sea-level cycles. These three secondary cycles, from bottom to top, combine to form one upward-shallowing sedimentary cycle within the Xixiangchi Formation, representing a rapid transgression followed by gradual regression. [Conclusions] Based on the identification of shallowing-upward sedimentary sequences and the analysis of their compositional characteristics and vertical stacking relationships, it is believed that the Late Cambrian Xixiangchi Formation was significantly influenced by high-frequency sea-level changes. It as a whole underwent an early rapid marine transgression, followed by a late slow regression, and experienced multiple episodes of secondary sea-level fluctuations. This resulted in a restricted carbonate platform sedimentary model characterized by mixed intertidal subfacies, chalky lagoon subfacies, and inner-shelf grainy shoal subfacies. This study further confirms a global late Cambrian sea-level regression event.
[Objective] Shallowing-upward sequences are extensively developed in shallow-water carbonate environments, and the identification of shallowing-upward sequences in deep-water carbonate rocks holds a unique advantage for analyzing the evolution of shallow-water carbonate sedimentary environments. [Method] This study focuses on the Late Cambrian Xixiangchi Formation in Xiuqi Town, Chengkou County, on the northern margin of the Yangtze Platform. Through detailed field sedimentological analysis and laboratory microfacies analysis, shallowing-upward sequences in carbonate rocks were identified. The compositional characteristics and vertical stacking relationships were analyzed, thereby exploring the Late Cambrian sedimentary environment evolution in the upper Yangtze region. [Results and Discussions] In the Xixiangchi Formation, 14 sedimentary microfacies have been identified, including MF0 argillaceous shale, MF1 micrite, MF2 bright crystal gravel limestone, MF3 bright crystal sand limestone, MF4 muddy siltstone, MF5 dolomitic microcrystalline limestone, MF6 gravel dolomite, MF7 bright crystal sand dolomite, MF8 sandy gravel grain dolomite, MF9 sandy sandy lithic grain dolomite, MF10 sandy dolomite, MF11 laminated dolomite, MF12 bamboo leaf-like gravel dolomite, and MF13 grain dolomite with dissolution pores. Based on the microfacies associations, six decimeter-scale upward-shallowing tidal sub-environments (C1-1 to C1-6) have been recognized within the Xixiangchi Formation, representing subtidal deposition. Additionally, six decimeter-scale upward-shallowing tidal-subtidal transition sub-environments (C2-1 to C2-6) and two decimeter-scale upward-shallowing intertidal-subtidal transition sub-environments (C3-1 and C3-2) have been identified. These decimeter-scale upward-shallowing cycles constitute three secondary sea-level cycles. These three secondary cycles, from bottom to top, combine to form one upward-shallowing sedimentary cycle within the Xixiangchi Formation, representing a rapid transgression followed by gradual regression. [Conclusions] Based on the identification of shallowing-upward sedimentary sequences and the analysis of their compositional characteristics and vertical stacking relationships, it is believed that the Late Cambrian Xixiangchi Formation was significantly influenced by high-frequency sea-level changes. It as a whole underwent an early rapid marine transgression, followed by a late slow regression, and experienced multiple episodes of secondary sea-level fluctuations. This resulted in a restricted carbonate platform sedimentary model characterized by mixed intertidal subfacies, chalky lagoon subfacies, and inner-shelf grainy shoal subfacies. This study further confirms a global late Cambrian sea-level regression event.
Abstract:
The formation and evolution of the Earth's crust is an important aspect of studying the early evolution of the Earth towards livable planets. The successive discovery of early basement rocks and their zircon age information in the Dabie region has made it an important place to study the early Earth. Compared with the limited exposure of basement rocks, the detrital zircons in river sediments can comprehensively reflect the geological evolution information of the source area, and are an effective means to explore the growth and evolution of continental crust. This article conducts U-Pb Hf isotope analysis of zircons in detrital sediments from three different rivers in the Dabie region. The U-Pb zircon age peaks obtained can be divided into four groups: 2676-2424 Ma, 2035-1812 Ma, 890-748 Ma, and 140-120 Ma. Based on the data in this article and previous research results, the following conclusions can be drawn: (1) The magmatic activity records in the Dabie region can be roughly divided into 10 stages and 5 cycles. The quiet periods are 3.0-2.9Ga, 2.4-2.1Ga, 1.8-0.9Ga, 0.70-0.14Ga, and the explosive periods are 4.0-3.0Ga, 2.9-2.4Ga, 2.1-1.8Ga, 0.9-0.7Ga, 0.14-0.12Ga, respectively. (2) The Dabie region mainly experienced four stages of metamorphic events, which occurred in the late Neoarchean (~2.5Ga), middle Paleoproterozoic (~2.0Ga), Triassic (0.24-0.20Ga), and Cretaceous (0.14-0.12Ga). (3) The Dabie region mainly experienced four stages of crustal growth events: initial crustal growth during the Mesozoic era, and massive crustal growth at 4.4-4.1Ga; The crustal growth at 4.1~3.9Ga is relatively slow, with no significant crustal growth; 3.9~2.8Ga sustained crustal growth; The crustal growth rate is relatively slow at 2.8~1.0Ga. (4) Compared to the magmatic, metamorphic, and crustal growth events in the Dabie and Huangling regions, there is a significant difference between the two before~2.0Ga, suggesting that the two were more likely independent micro landmasses before this time.
The formation and evolution of the Earth's crust is an important aspect of studying the early evolution of the Earth towards livable planets. The successive discovery of early basement rocks and their zircon age information in the Dabie region has made it an important place to study the early Earth. Compared with the limited exposure of basement rocks, the detrital zircons in river sediments can comprehensively reflect the geological evolution information of the source area, and are an effective means to explore the growth and evolution of continental crust. This article conducts U-Pb Hf isotope analysis of zircons in detrital sediments from three different rivers in the Dabie region. The U-Pb zircon age peaks obtained can be divided into four groups: 2676-2424 Ma, 2035-1812 Ma, 890-748 Ma, and 140-120 Ma. Based on the data in this article and previous research results, the following conclusions can be drawn: (1) The magmatic activity records in the Dabie region can be roughly divided into 10 stages and 5 cycles. The quiet periods are 3.0-2.9Ga, 2.4-2.1Ga, 1.8-0.9Ga, 0.70-0.14Ga, and the explosive periods are 4.0-3.0Ga, 2.9-2.4Ga, 2.1-1.8Ga, 0.9-0.7Ga, 0.14-0.12Ga, respectively. (2) The Dabie region mainly experienced four stages of metamorphic events, which occurred in the late Neoarchean (~2.5Ga), middle Paleoproterozoic (~2.0Ga), Triassic (0.24-0.20Ga), and Cretaceous (0.14-0.12Ga). (3) The Dabie region mainly experienced four stages of crustal growth events: initial crustal growth during the Mesozoic era, and massive crustal growth at 4.4-4.1Ga; The crustal growth at 4.1~3.9Ga is relatively slow, with no significant crustal growth; 3.9~2.8Ga sustained crustal growth; The crustal growth rate is relatively slow at 2.8~1.0Ga. (4) Compared to the magmatic, metamorphic, and crustal growth events in the Dabie and Huangling regions, there is a significant difference between the two before~2.0Ga, suggesting that the two were more likely independent micro landmasses before this time.
Abstract:
[Objective]The limestone-marlstone alterations (LMAs) are the important archives of climatic, hydrological, and geological events for geologists. But as a typical endmember of the LMAs, the knowledge about the genesis of the LMAs with eyelid-eyeball structure of the Member I of the Maokou Formation in the Sichuan Basin has not come to agreement widely. This not only seriously affects environmental researching of the LMAs with eyelid-eyeball structure, but also affects the efficiency of its exploration. Dolomitization is developed in the LMAs with eyelid-eyeball structure (mainly in the eyelids) of the Member I of the Maokou Formation of the DB1 well during the penecontemporaneous and early diagenetic phase can be used to constrain the depositional environments of carbonate rocks. [Methods] In this paper, petrological and geochemical analyses have been carried out in detailed, the process and the fluid properties of dolomitization has been discussed to constrain the process of LMAs with eyelid-eyeball structure. [Results and Discussions]Three types of dolomitization are developed in the Member I of the Maokou Formation, which are muddy dolomitic (EDM-I); limy dolomitic (EDM-II) and dolomitic marlstone(EDL). EDM-I has the highest degree of dolomitization, distinct bands of cathodoluminescence, a flat rare earth partitioning pattern, higher δ13C and δ18O values but lower Y/Ho values relative to seawater, it was dolomitized by penecontemporaneous seawater mixed with a small amount of meteoric freshwater during the penecontemporaneous phase. EDM-II has the middle degree of dolomitization, a left leaning rare earth partitioning pattern and values of δ13C, δ18O, 87Sr/86Sr, and Y/Ho similar to seawater, it was dolomitized by seawater with high Mg/Ca, which was induced by frequent sea-level rise and fall changes during the syn-sedimentary and penecontemporaneous phase. EDL has the lowest degree of dolomitization, a left leaning rare earth partitioning pattern, higher δ13C and δ18O values relative to EDM-II, and seawater-like Y/Ho values. EDL was dolomitized by the sealed seawater of the syn-sedimentary phase which has been alterated with the wall rocks in the early diagenetic stage. [Conclusions] The dolomitization of the LMAs with eyelid-eyeball structure by weakly evaporating seawater and seawater mixed with freshwater during the penecontemporaneous and early diagenetic phase requires that the LMAs deposited in a shallower water. The high mud content, high strontium isotope and rare earth element values, low carbon and oxygen isotope values and Y/Ho values of the eyelid-like marlstone relative to the eyeball-like limestone indicate that the depth of water for the eyelid-like marlstone is shallower than that of the eyeball-like limestone during they deposited. Therefore, it is inferred that the LMAs in the Member I of the Maokou Formation were produced under cyclic oscillation in the depositional environment, and their genesis may have been mainly influenced by sedimentary process.
[Objective]The limestone-marlstone alterations (LMAs) are the important archives of climatic, hydrological, and geological events for geologists. But as a typical endmember of the LMAs, the knowledge about the genesis of the LMAs with eyelid-eyeball structure of the Member I of the Maokou Formation in the Sichuan Basin has not come to agreement widely. This not only seriously affects environmental researching of the LMAs with eyelid-eyeball structure, but also affects the efficiency of its exploration. Dolomitization is developed in the LMAs with eyelid-eyeball structure (mainly in the eyelids) of the Member I of the Maokou Formation of the DB1 well during the penecontemporaneous and early diagenetic phase can be used to constrain the depositional environments of carbonate rocks. [Methods] In this paper, petrological and geochemical analyses have been carried out in detailed, the process and the fluid properties of dolomitization has been discussed to constrain the process of LMAs with eyelid-eyeball structure. [Results and Discussions]Three types of dolomitization are developed in the Member I of the Maokou Formation, which are muddy dolomitic (EDM-I); limy dolomitic (EDM-II) and dolomitic marlstone(EDL). EDM-I has the highest degree of dolomitization, distinct bands of cathodoluminescence, a flat rare earth partitioning pattern, higher δ13C and δ18O values but lower Y/Ho values relative to seawater, it was dolomitized by penecontemporaneous seawater mixed with a small amount of meteoric freshwater during the penecontemporaneous phase. EDM-II has the middle degree of dolomitization, a left leaning rare earth partitioning pattern and values of δ13C, δ18O, 87Sr/86Sr, and Y/Ho similar to seawater, it was dolomitized by seawater with high Mg/Ca, which was induced by frequent sea-level rise and fall changes during the syn-sedimentary and penecontemporaneous phase. EDL has the lowest degree of dolomitization, a left leaning rare earth partitioning pattern, higher δ13C and δ18O values relative to EDM-II, and seawater-like Y/Ho values. EDL was dolomitized by the sealed seawater of the syn-sedimentary phase which has been alterated with the wall rocks in the early diagenetic stage. [Conclusions] The dolomitization of the LMAs with eyelid-eyeball structure by weakly evaporating seawater and seawater mixed with freshwater during the penecontemporaneous and early diagenetic phase requires that the LMAs deposited in a shallower water. The high mud content, high strontium isotope and rare earth element values, low carbon and oxygen isotope values and Y/Ho values of the eyelid-like marlstone relative to the eyeball-like limestone indicate that the depth of water for the eyelid-like marlstone is shallower than that of the eyeball-like limestone during they deposited. Therefore, it is inferred that the LMAs in the Member I of the Maokou Formation were produced under cyclic oscillation in the depositional environment, and their genesis may have been mainly influenced by sedimentary process.
Abstract:
The saturated hydrocarbons of 26 coal measure source rock samples in the Upper Paleozoic of Ordos Basin were analyzed by gas chromatography and chromatography-mass spectrometry. According to the distribution characteristics of tricyclic terpanes (TT), the source rock samples in the study area were divided into three different types of distribution patterns. The C19TT is the main peak of the type I source rock, and the content of C19TT, C20TT and C21TT gradually decreases. At the same time, the Pr/Ph ratio is high and the content of C24 tetracyclic terpane is also rich in this type of sample. This kind of samples are mainly humic coal and carbonaceous mudstone, revealing the characteristics of lacustrine facies sedimentary environment. On the contrary, the tricyclic terpanes of type II source rocks are dominated by C23TT, and the contents of C19TT, C20TT and C21TT are gradually increasing, while the Pr/Ph ratio is low, and the content of C24 tetracyclic terpane is not abundant. In addition to coal-bearing mudstone, there are also humic coal and carbonaceous mudstone in these samples, and their sedimentary environment is quite different from that of type I source rocks. The distribution of tricyclic terpanes in type III source rocks is between type I and type II source rocks. On the one hand, the main peak carbon of tricyclic terpane is C23TT, on the other hand, the relative content of C19TT, C20TT and C21TT decreases in turn and shows the characteristics of stepwise distribution. In addition, the Pr/Ph ratio and the content of C24 tetracyclic terpane are between type I and type II source rocks. The results also reveal that with the increase of the thermal evolution degree of source rocks, the total amount of tricyclic terpanes shows an gradually increasing trend, but the distribution patterns of tricyclic terpanes in different types of source rocks have not changed significantly, in other words, the different distribution patterns of tricyclic terpanes in coal-bearing source rocks in the study area are less affected by the thermal evolution degree of organic matter, but mainly controlled by sedimentary environment and parent material type.
The saturated hydrocarbons of 26 coal measure source rock samples in the Upper Paleozoic of Ordos Basin were analyzed by gas chromatography and chromatography-mass spectrometry. According to the distribution characteristics of tricyclic terpanes (TT), the source rock samples in the study area were divided into three different types of distribution patterns. The C19TT is the main peak of the type I source rock, and the content of C19TT, C20TT and C21TT gradually decreases. At the same time, the Pr/Ph ratio is high and the content of C24 tetracyclic terpane is also rich in this type of sample. This kind of samples are mainly humic coal and carbonaceous mudstone, revealing the characteristics of lacustrine facies sedimentary environment. On the contrary, the tricyclic terpanes of type II source rocks are dominated by C23TT, and the contents of C19TT, C20TT and C21TT are gradually increasing, while the Pr/Ph ratio is low, and the content of C24 tetracyclic terpane is not abundant. In addition to coal-bearing mudstone, there are also humic coal and carbonaceous mudstone in these samples, and their sedimentary environment is quite different from that of type I source rocks. The distribution of tricyclic terpanes in type III source rocks is between type I and type II source rocks. On the one hand, the main peak carbon of tricyclic terpane is C23TT, on the other hand, the relative content of C19TT, C20TT and C21TT decreases in turn and shows the characteristics of stepwise distribution. In addition, the Pr/Ph ratio and the content of C24 tetracyclic terpane are between type I and type II source rocks. The results also reveal that with the increase of the thermal evolution degree of source rocks, the total amount of tricyclic terpanes shows an gradually increasing trend, but the distribution patterns of tricyclic terpanes in different types of source rocks have not changed significantly, in other words, the different distribution patterns of tricyclic terpanes in coal-bearing source rocks in the study area are less affected by the thermal evolution degree of organic matter, but mainly controlled by sedimentary environment and parent material type.
