2024 Vol. 42, No. 2
Display Method:
2024, 42(2): 343-349.
doi: 10.14027/j.issn.1000-0550.2024.050
Abstract:
Significance The study of the conceptual framework of the “Carbonate Factory” has been hailed by Reijmer as “one of the main advances in carbonate sedimentology achieved during the last three decades”. It has a unique value in reconstructing deep time carbonate production processes and their controlling factors, as well as in predicting the impact of future climate and environmental changes on the sedimentary system. [Progress] The study of carbonate factories focuses on the major factors influencing the sedimentary system, including producers, production processes and controls, and ecological accommodation. This is different from the traditional “facies model” that is based on lithofacies and sedimentary environment. As the related conceptual framework has matured, it is worth exploring new directions and research values of the carbonate factories to guide the innovation of theoretical and applied sedimentology. This study reviews the background of the formation of the conceptual framework of the carbonate factories, introduces their compositions and main influencing factors, and recommends the review and research papers in this special issue. [Prospects] The authors hope that through the introduction of these papers, domestic colleagues can be attracted to pay attention and interest to this field, and jointly promote the in-depth research and development of the carbonate factories.
Significance The study of the conceptual framework of the “Carbonate Factory” has been hailed by Reijmer as “one of the main advances in carbonate sedimentology achieved during the last three decades”. It has a unique value in reconstructing deep time carbonate production processes and their controlling factors, as well as in predicting the impact of future climate and environmental changes on the sedimentary system. [Progress] The study of carbonate factories focuses on the major factors influencing the sedimentary system, including producers, production processes and controls, and ecological accommodation. This is different from the traditional “facies model” that is based on lithofacies and sedimentary environment. As the related conceptual framework has matured, it is worth exploring new directions and research values of the carbonate factories to guide the innovation of theoretical and applied sedimentology. This study reviews the background of the formation of the conceptual framework of the carbonate factories, introduces their compositions and main influencing factors, and recommends the review and research papers in this special issue. [Prospects] The authors hope that through the introduction of these papers, domestic colleagues can be attracted to pay attention and interest to this field, and jointly promote the in-depth research and development of the carbonate factories.
Detailed Characterization of Carbonate Factories from the Perspective of Quantitative Reconstruction
2024, 42(2): 351-370.
doi: 10.14027/j.issn.1000-0550.2024.047
Abstract:
Significance Carbonate factories provide the foundation for forming carbonate depositional systems. They are closely related to marine evolution, elemental cycling, and earth surface processes and are an essential part reflecting the evolution of the Earth system. The transition of carbonate factories often coincides with biological and environmental changes. Over geological time, the change of carbonate factories frequently occurs along with biological or environmental crises such as mass extinction or the initiation of new life forms such as life explosion or biological recovery after crisis. Therefore, quantifying the characterization of carbonate factories and their controlling factors is for a deeper understanding of the geological information recorded in carbonate rocks. [ Progress ] However, most studies on carbonate factories, particularly those from the geological records, are qualitative based on the lithological and microfacies analysis. This study summarizes the research progress in the semi-quantitative to quantitative characterization of carbonate factories, combining the introduction of the methods for forward modelling of sedimentary processes to provide a perspective for the detailed characterization of carbonate factories from a quantitative reconstruction perspective. In addition to providing information on rock components that indicate the ingredients of carbonate factories, statistical analysis of carbonate grains can offer insights into the sedimentary environment by examining parameters such as size, roundness, and sorting. These shape characteristics can serve as quantitative indicators of water energy and grain transportation processes. Elemental geochemical proxies enable the assessment of environmental parameters such as redox conditions, nutrient levels, and climatic conditions. Isotopic geochemical proxies play a crucial role in reconstructing the evolution of environmental factors such as temperature and seawater carbonate saturation. By a combination of multiproxies and sedimentary process modelling, a comprehensive analysis can identify the production process, controlling factors, and evolution of carbonate factories. [ Prospects ] Based on traditional carbonate sedimentology, studying carbonate factories should deepen our understanding of their controlling factors, particularly the quantitative assessment of factors directly impacting ecosystems. When applying quantitative analysis methods such as sediment transport patterns or hydrodynamic analysis that are established in siliciclastic sedimentary systems, the differences caused by the biogenic nature of certain carbonate sediments compared to siliciclastic sediments need to be considered. Furthermore, models and analysis methods applicable to carbonate grains should be further refined. In addition, when quantitatively evaluating the development of carbonate factories in ancient marine using quantitative carbonate grains statistics or geochemical indicators, there should be a strengthened focus on developing quantitative assessment methods for the level of diagenetic alteration. Further efforts should be directed toward developing and applying in-situ elemental and isotopic testing methods to reduce the impact of diagenesis on geochemical signals. While developing and applying new geochemical proxies for carbonate rocks, the mechanisms of occurrence or fractionation need to be clearly understood to interpret the reflected paleoclimate and paleoenvironmental information accurately. Simultaneously, when studying deep-time evolution and production mechanisms of carbonate factories through sedimentary process forward modelling, it is necessary to consider the applicability of parameters obtained from modern environments in deep-time records and account for the impact of syndepositional-early diagenesis and chemical evolution of seawater geochemical evolution in the modelling process.
Significance Carbonate factories provide the foundation for forming carbonate depositional systems. They are closely related to marine evolution, elemental cycling, and earth surface processes and are an essential part reflecting the evolution of the Earth system. The transition of carbonate factories often coincides with biological and environmental changes. Over geological time, the change of carbonate factories frequently occurs along with biological or environmental crises such as mass extinction or the initiation of new life forms such as life explosion or biological recovery after crisis. Therefore, quantifying the characterization of carbonate factories and their controlling factors is for a deeper understanding of the geological information recorded in carbonate rocks. [ Progress ] However, most studies on carbonate factories, particularly those from the geological records, are qualitative based on the lithological and microfacies analysis. This study summarizes the research progress in the semi-quantitative to quantitative characterization of carbonate factories, combining the introduction of the methods for forward modelling of sedimentary processes to provide a perspective for the detailed characterization of carbonate factories from a quantitative reconstruction perspective. In addition to providing information on rock components that indicate the ingredients of carbonate factories, statistical analysis of carbonate grains can offer insights into the sedimentary environment by examining parameters such as size, roundness, and sorting. These shape characteristics can serve as quantitative indicators of water energy and grain transportation processes. Elemental geochemical proxies enable the assessment of environmental parameters such as redox conditions, nutrient levels, and climatic conditions. Isotopic geochemical proxies play a crucial role in reconstructing the evolution of environmental factors such as temperature and seawater carbonate saturation. By a combination of multiproxies and sedimentary process modelling, a comprehensive analysis can identify the production process, controlling factors, and evolution of carbonate factories. [ Prospects ] Based on traditional carbonate sedimentology, studying carbonate factories should deepen our understanding of their controlling factors, particularly the quantitative assessment of factors directly impacting ecosystems. When applying quantitative analysis methods such as sediment transport patterns or hydrodynamic analysis that are established in siliciclastic sedimentary systems, the differences caused by the biogenic nature of certain carbonate sediments compared to siliciclastic sediments need to be considered. Furthermore, models and analysis methods applicable to carbonate grains should be further refined. In addition, when quantitatively evaluating the development of carbonate factories in ancient marine using quantitative carbonate grains statistics or geochemical indicators, there should be a strengthened focus on developing quantitative assessment methods for the level of diagenetic alteration. Further efforts should be directed toward developing and applying in-situ elemental and isotopic testing methods to reduce the impact of diagenesis on geochemical signals. While developing and applying new geochemical proxies for carbonate rocks, the mechanisms of occurrence or fractionation need to be clearly understood to interpret the reflected paleoclimate and paleoenvironmental information accurately. Simultaneously, when studying deep-time evolution and production mechanisms of carbonate factories through sedimentary process forward modelling, it is necessary to consider the applicability of parameters obtained from modern environments in deep-time records and account for the impact of syndepositional-early diagenesis and chemical evolution of seawater geochemical evolution in the modelling process.
2024, 42(2): 371-386.
doi: 10.14027/j.issn.1000-0550.2024.006
Abstract:
Significance The formation and preservation of large-scale oolites in marine environments are valuable tools in climate and environmental evolution studies, and also hold considerable importance as petroleum reservoirs. However, traditional studies on oolites have focused primarily on their depositional sequences, patterns and spatial distribution, almost overlooking the dynamic processes of ooid shoal accumulation, burial and diagenesis, as well as the influence of environmental controls on their development. [Progress] This study aims to address this gap by a comprehensive analysis of typical ooid factories （rimmed shelves and ramps） in modern marine environments. It examines their producers, composition, distribution, construction process, relationship with other carbonate factories and the physical, chemical and biological factors influencing their development. It was found that the key limiting factors on the development of ooid factories include strong turbulent water conditions, high carbonate mineral saturation, and effective active-stationary stage transitions. Additionally, the developmental environment and scale of these factories are determined by their specific depositional backgrounds. [Conclusions and Prospects] From a carbonate factory perspective, this study provides important insights into the initiation-flourishing-decline processes of large-scale ooid factories, and offers multidimensional evidence for evaluating their value in oil and gas exploration. Furthermore, it provides new insights for exploring the deep-time environment and predicting future changes on carbonate depositional systems.
Significance The formation and preservation of large-scale oolites in marine environments are valuable tools in climate and environmental evolution studies, and also hold considerable importance as petroleum reservoirs. However, traditional studies on oolites have focused primarily on their depositional sequences, patterns and spatial distribution, almost overlooking the dynamic processes of ooid shoal accumulation, burial and diagenesis, as well as the influence of environmental controls on their development. [Progress] This study aims to address this gap by a comprehensive analysis of typical ooid factories （rimmed shelves and ramps） in modern marine environments. It examines their producers, composition, distribution, construction process, relationship with other carbonate factories and the physical, chemical and biological factors influencing their development. It was found that the key limiting factors on the development of ooid factories include strong turbulent water conditions, high carbonate mineral saturation, and effective active-stationary stage transitions. Additionally, the developmental environment and scale of these factories are determined by their specific depositional backgrounds. [Conclusions and Prospects] From a carbonate factory perspective, this study provides important insights into the initiation-flourishing-decline processes of large-scale ooid factories, and offers multidimensional evidence for evaluating their value in oil and gas exploration. Furthermore, it provides new insights for exploring the deep-time environment and predicting future changes on carbonate depositional systems.
2024, 42(2): 387-402.
doi: 10.14027/j.issn.1000-0550.2023.092
Abstract:
Significance The proposal of a carbonate factory and its classification study are of great significance for promoting the development of carbonate sedimentology. However, the current classification scheme is not sufficient to cover all carbonate sediments, so it is urgent to clarify different carbonate factories at the level of mechanism and process. [Progress] The carbon used by traditional carbonate factories mainly comes from atmospheric and oceanic inorganic carbon pools, whereas certain marine and continental carbonate factories mainly fix carbon from exogenous carbon pools, so the two are fundamentally different. The latter is often highly significant for tracing deep water environments and climate events. [Conclusions and prospects] As an example, this study takes hydrocarbon seep carbonate, and formally puts forward the concept of a “cold seep carbonate factory”. In addition, its sedimentary characteristics, biological composition and biogeochemical processes are summarized, and an example is discussed to address its geological significance. The type of carbonate factory based on an exogenous carbon pool deserves much attention.
Significance The proposal of a carbonate factory and its classification study are of great significance for promoting the development of carbonate sedimentology. However, the current classification scheme is not sufficient to cover all carbonate sediments, so it is urgent to clarify different carbonate factories at the level of mechanism and process. [Progress] The carbon used by traditional carbonate factories mainly comes from atmospheric and oceanic inorganic carbon pools, whereas certain marine and continental carbonate factories mainly fix carbon from exogenous carbon pools, so the two are fundamentally different. The latter is often highly significant for tracing deep water environments and climate events. [Conclusions and prospects] As an example, this study takes hydrocarbon seep carbonate, and formally puts forward the concept of a “cold seep carbonate factory”. In addition, its sedimentary characteristics, biological composition and biogeochemical processes are summarized, and an example is discussed to address its geological significance. The type of carbonate factory based on an exogenous carbon pool deserves much attention.
2024, 42(2): 403-414.
doi: 10.14027/j.issn.1000-0550.2023.059
Abstract:
Objective The study of the "Carbonate Factory" emphasizes the specific effects of biological communities on the carbonate production process and scale moderation, contributing to our understanding of large-scale carbonate sedimentation. Although the concept and application of the "Carbonate Factory" have gained attention in carbonate sedimentology, there is still a lack of systematic understanding of how to interpret the origin, developmental processes, and influencing factors of specific types of carbonate sedimentation in geological history, particularly in deep time. Methods This study investigated the Gushan Formation of the Cambrian Miaolingian in the Yunmengshan area （Henan Province） as an example of a microbial carbonate factory. A multi-scale grid-based quantitative evaluation method was used to describe the specific form, internal structure, and construction process of the carbonate factory. Results Our results show that benthic microbial communities were the primary builders of the microbial carbonate factory in the study area. They formed unique structures, including the stromatolite, thrombolite, leiolite, and microbial-induced sedimentary structure （MISS）. Various particle types, including ooids, flat pebbles, and microbialite breccias, were preserved within the microbial buildups. The development of the microbial carbonate factories occurred episodically during the Guzhangian Age （Miaolingian） at relatively small scales （height of <1 m）； however, they extended widely in space and exhibited different macroscopic shapes such as subspherical, domical, and tabular. The internal structures of the microbial carbonate factories were made up of individual forms and vertical combinations of composite forms. Conclusions By analyzing various factors influencing the development of these factories, we found that higher nutrient levels may have played a crucial role in their development in the tide-flat environment. Additionally, the flooding event during the Guzhangian led to the eventual closure of these factories. This study provides an ancient example, similar to the modern microbial carbonate factory of the Hamelin Pool in Australia, which is a good reference for understanding the types, characteristics and evolutionary patterns of Cambrian carbonate factories.
Objective The study of the "Carbonate Factory" emphasizes the specific effects of biological communities on the carbonate production process and scale moderation, contributing to our understanding of large-scale carbonate sedimentation. Although the concept and application of the "Carbonate Factory" have gained attention in carbonate sedimentology, there is still a lack of systematic understanding of how to interpret the origin, developmental processes, and influencing factors of specific types of carbonate sedimentation in geological history, particularly in deep time. Methods This study investigated the Gushan Formation of the Cambrian Miaolingian in the Yunmengshan area （Henan Province） as an example of a microbial carbonate factory. A multi-scale grid-based quantitative evaluation method was used to describe the specific form, internal structure, and construction process of the carbonate factory. Results Our results show that benthic microbial communities were the primary builders of the microbial carbonate factory in the study area. They formed unique structures, including the stromatolite, thrombolite, leiolite, and microbial-induced sedimentary structure （MISS）. Various particle types, including ooids, flat pebbles, and microbialite breccias, were preserved within the microbial buildups. The development of the microbial carbonate factories occurred episodically during the Guzhangian Age （Miaolingian） at relatively small scales （height of <1 m）； however, they extended widely in space and exhibited different macroscopic shapes such as subspherical, domical, and tabular. The internal structures of the microbial carbonate factories were made up of individual forms and vertical combinations of composite forms. Conclusions By analyzing various factors influencing the development of these factories, we found that higher nutrient levels may have played a crucial role in their development in the tide-flat environment. Additionally, the flooding event during the Guzhangian led to the eventual closure of these factories. This study provides an ancient example, similar to the modern microbial carbonate factory of the Hamelin Pool in Australia, which is a good reference for understanding the types, characteristics and evolutionary patterns of Cambrian carbonate factories.
2024, 42(2): 415-433.
doi: 10.14027/j.issn.1000-0550.2023.078
Abstract:
Objective During the Early Jurassic, multiple significant perturbations of the carbon-cycle occurred that coincided with abrupt and extreme changes in climate and environment. However, existing research has primarily focused on the western Tethys and northern Europe, with limited attention given to carbon cycle disturbances in the eastern Tethys and the quantitative assessment of their impact on biota. Methods This study examined the biological （larger benthic foraminifera）, sedimentological, and carbon-isotope （δ13C） stratigraphy from the southern Tibetan Kioto Platform formed in the southeastern Tethys during the Late Pliensbachian⁃Earliest Toarcian interval to investigate the influence of carbon-cycle perturbations and corresponding climatic and environmental variations on the evolution of carbonate-platform biota in the region. In addition, this study quantitatively analyzed the content of carbonate skeletal grains, the number and species changes of lager benthic foraminifera, and the distribution of Lithiotis Fauna to explore the impact of these events on biota. Results Three foraminiferal zones were recognized： Pliensbachian Bosniella oenensis⁃Cyclorbitosella tibetica （?） and Streptocyclammina liasica, as well as the Earliest Toarcian Siphovalvulina sp. A. According to the sedimentary structure and the quantitative analysis of grains composition and content, this study identified nine carbonate microfacies （MF） from the Pupuga Formation in the Dongqiu section. These microfacies included mudstone （MF1）, dolomitic peloidal wackestone （MF2）, bioclastic peloidal packstone （MF3）, bioclastic wackestone/packstone （MF4）, intraclastic packstone⁃grainstone （MF5）, lump grainstone （MF6）, Lithiotis rudstone （MF7）, bioclastic grainstone （MF8）, and oolitic grainstone （MF9）. The sedimentary microfacies and microfacies assemblages reveal that the Pupuga Formation in the Dongqiu section represented the shallow⁃water carbonate platform deposit, characterized by minimal influence from terrigenous input. Furthermore, temporal variation in microfacies demonstrated that the Dongqiu section provided a comprehensive record of a sedimentary succession, delineating a gradual transition from the open platform and platform margin to the inner platform, succeeded by a swift reversion towards the open platform and platform margin. Based on biostratigraphy, two carbon isotope excursion events in the Late Pliensbachian were identified： the margaritatus zone event （ME） characterized by positive carbon isotope excursion and the margaritatus⁃spinatum zone boundary event （MSBE） with negative carbon isotope excursion as a feature. Conclusions During the ME period, there was an increase in skeletal grain content, maintenance of high species diversity and abundance of larger benthic foraminifera, and the occurrence and spread of Lithiotis Fauna. This may indicate that the persistent burial of organic matter consumed atmospheric pCO2 generated during the Late Sinemurian to Early Pliensbachian interval, creating more suitable marine environmental conditions for biotic survival. In contrast, during the MSBE period, there was a decrease in the abundance and size of Lithiotis Fauna and skeletal grain content, along with the extinction of several index larger benthic foraminifera. This evidence indicates a possible relationship between biotic crises and sea-level fall. Although there were changes in the size and frequency of occurrence of the Lithiotis Fauna, it is undeniable that they flourished throughout the Tethyan shallow carbonate platform during the Pliensbachian interval, and the conditions for skeletal production of carbonates were not completely destroyed until the Toarcian oceanic anoxic event.
Objective During the Early Jurassic, multiple significant perturbations of the carbon-cycle occurred that coincided with abrupt and extreme changes in climate and environment. However, existing research has primarily focused on the western Tethys and northern Europe, with limited attention given to carbon cycle disturbances in the eastern Tethys and the quantitative assessment of their impact on biota. Methods This study examined the biological （larger benthic foraminifera）, sedimentological, and carbon-isotope （δ13C） stratigraphy from the southern Tibetan Kioto Platform formed in the southeastern Tethys during the Late Pliensbachian⁃Earliest Toarcian interval to investigate the influence of carbon-cycle perturbations and corresponding climatic and environmental variations on the evolution of carbonate-platform biota in the region. In addition, this study quantitatively analyzed the content of carbonate skeletal grains, the number and species changes of lager benthic foraminifera, and the distribution of Lithiotis Fauna to explore the impact of these events on biota. Results Three foraminiferal zones were recognized： Pliensbachian Bosniella oenensis⁃Cyclorbitosella tibetica （?） and Streptocyclammina liasica, as well as the Earliest Toarcian Siphovalvulina sp. A. According to the sedimentary structure and the quantitative analysis of grains composition and content, this study identified nine carbonate microfacies （MF） from the Pupuga Formation in the Dongqiu section. These microfacies included mudstone （MF1）, dolomitic peloidal wackestone （MF2）, bioclastic peloidal packstone （MF3）, bioclastic wackestone/packstone （MF4）, intraclastic packstone⁃grainstone （MF5）, lump grainstone （MF6）, Lithiotis rudstone （MF7）, bioclastic grainstone （MF8）, and oolitic grainstone （MF9）. The sedimentary microfacies and microfacies assemblages reveal that the Pupuga Formation in the Dongqiu section represented the shallow⁃water carbonate platform deposit, characterized by minimal influence from terrigenous input. Furthermore, temporal variation in microfacies demonstrated that the Dongqiu section provided a comprehensive record of a sedimentary succession, delineating a gradual transition from the open platform and platform margin to the inner platform, succeeded by a swift reversion towards the open platform and platform margin. Based on biostratigraphy, two carbon isotope excursion events in the Late Pliensbachian were identified： the margaritatus zone event （ME） characterized by positive carbon isotope excursion and the margaritatus⁃spinatum zone boundary event （MSBE） with negative carbon isotope excursion as a feature. Conclusions During the ME period, there was an increase in skeletal grain content, maintenance of high species diversity and abundance of larger benthic foraminifera, and the occurrence and spread of Lithiotis Fauna. This may indicate that the persistent burial of organic matter consumed atmospheric pCO2 generated during the Late Sinemurian to Early Pliensbachian interval, creating more suitable marine environmental conditions for biotic survival. In contrast, during the MSBE period, there was a decrease in the abundance and size of Lithiotis Fauna and skeletal grain content, along with the extinction of several index larger benthic foraminifera. This evidence indicates a possible relationship between biotic crises and sea-level fall. Although there were changes in the size and frequency of occurrence of the Lithiotis Fauna, it is undeniable that they flourished throughout the Tethyan shallow carbonate platform during the Pliensbachian interval, and the conditions for skeletal production of carbonates were not completely destroyed until the Toarcian oceanic anoxic event.
2024, 42(2): 434-444.
doi: 10.14027/j.issn.1000-0550.2023.094
Abstract:
Objective The shrinkage and demise of the Late Ordovician carbonate platform in northeastern Yunnan have been subject to various interpretations. To delve deeper into its origins, this paper intends to investigate the Late Ordovician paleoenvironmental evolution in northeastern Yunnan and analyze the influence of paleoenvironmental factors on the demise of the Late Ordovician carbonate platform in this region. Methods The Wanhe section in the northeast of Yunnan province was selected as the research object. Using methods such as indoor mineralogy, petrology, and sedimentary geochemistry analysis, a systematic study was conducted on the microfacies characteristics of the Ordovician Linxiang-Silurian Longmaxi Formations in northeastern Yunnan province. During this period, an analysis of paleoenvironmental changes was conducted to explore the paleoenvironmental impacts during the contraction and extinction of carbonate platforms in the Late Ordovician in northeastern Yunnan province. Results Through the analysis of lithological characteristics, microscopic mineral composition, and paleontological features within the Linxiang-Longmaxi Formations at the Wanhe section, six sedimentary microfacies were identified： nodular limestone, laminated microcrystalline limestone, microcrystalline fossiliferous limestone, laminated calcareous mudstone, sandy mudstone and silty sandstone, and calcareous shale and calcareous carbonaceous mudstone. These were further divided into three sedimentary subfacies： shallow carbonate ramp, deep carbonate ramp, and restricted retention shelf. By utilizing sedimentary geochemical methods, we analyzed the paleoceanographic geochemical characteristics and established a geochemical profile. This analysis revealed frequent and significant changes in the paleoceanographic indicators, including paleoproductivity, paleoredox conditions, terrestrial detrital input, and paleosea level fluctuations, between the Ordovician Daduhe and Silurian Longmaxi Formations. Conclusions Through the analysis of paleoceanographic changes around the Late Ordovician carbonate platform in northeastern Yunnan province, the extinction of the Late Ordovician carbonate platform in this region can be attributed to oceanic hypoxia. This hypoxia led to the decline of photosynthetic organisms and subsequently inhibited carbonate production, resulting in the contraction and eventual extinction of the carbonate platform. Additionally, sea-level changes, climate fluctuations, increased input of terrigenous debris, and the Late Ordovician mass extinction event played crucial roles in the process of the carbonate platform disappearance.
Objective The shrinkage and demise of the Late Ordovician carbonate platform in northeastern Yunnan have been subject to various interpretations. To delve deeper into its origins, this paper intends to investigate the Late Ordovician paleoenvironmental evolution in northeastern Yunnan and analyze the influence of paleoenvironmental factors on the demise of the Late Ordovician carbonate platform in this region. Methods The Wanhe section in the northeast of Yunnan province was selected as the research object. Using methods such as indoor mineralogy, petrology, and sedimentary geochemistry analysis, a systematic study was conducted on the microfacies characteristics of the Ordovician Linxiang-Silurian Longmaxi Formations in northeastern Yunnan province. During this period, an analysis of paleoenvironmental changes was conducted to explore the paleoenvironmental impacts during the contraction and extinction of carbonate platforms in the Late Ordovician in northeastern Yunnan province. Results Through the analysis of lithological characteristics, microscopic mineral composition, and paleontological features within the Linxiang-Longmaxi Formations at the Wanhe section, six sedimentary microfacies were identified： nodular limestone, laminated microcrystalline limestone, microcrystalline fossiliferous limestone, laminated calcareous mudstone, sandy mudstone and silty sandstone, and calcareous shale and calcareous carbonaceous mudstone. These were further divided into three sedimentary subfacies： shallow carbonate ramp, deep carbonate ramp, and restricted retention shelf. By utilizing sedimentary geochemical methods, we analyzed the paleoceanographic geochemical characteristics and established a geochemical profile. This analysis revealed frequent and significant changes in the paleoceanographic indicators, including paleoproductivity, paleoredox conditions, terrestrial detrital input, and paleosea level fluctuations, between the Ordovician Daduhe and Silurian Longmaxi Formations. Conclusions Through the analysis of paleoceanographic changes around the Late Ordovician carbonate platform in northeastern Yunnan province, the extinction of the Late Ordovician carbonate platform in this region can be attributed to oceanic hypoxia. This hypoxia led to the decline of photosynthetic organisms and subsequently inhibited carbonate production, resulting in the contraction and eventual extinction of the carbonate platform. Additionally, sea-level changes, climate fluctuations, increased input of terrigenous debris, and the Late Ordovician mass extinction event played crucial roles in the process of the carbonate platform disappearance.
2024, 42(2): 445-465.
doi: 10.14027/j.issn.1000-0550.2023.121
Abstract:
Objective The precipitation and accumulation of massive carbonates are called the "carbonate factory". Environmental and biological factors, such as water depth, light intensity, nutrients, hydrodynamics, climatic conditions, and sea-water chemistry control the composition of different carbonate factories. To properly understand the evolution of the carbonate sedimentary system, it is essential to accurately identify different types of carbonate factories and the factors controlling their development. To achieve this, the quantitative evaluation of the environmental and biological parameters in the transition of sedimentary systems is key. Methods In this study, the composition, characteristics, type transitions, and driving forces of tropical shallow-water carbonate factories within the Ma'antang Formation were studied in detail. We applied a systematic microfacies analysis in the deposition of the Ma'antang Formation at the Guanyinya section, Hanwang town, Mianzhu city, northwest Sichuan. Microfacies characteristics, including types and percentages of carbonate grains, matrix, and fossils, combined with quantitative analysis of the size and morphological parameters of 20,391 carbonate grains, including circularity, roundness, solidity, and aspect ratio, were analyzed from the thin sections. The size data were corrected and then used to calculate the average particle sizes, degree of sorting, skewness, and kurtosis following the Folk and Ward method. The analyses of the above-mentioned morphological parameters were accomplished in ImageJ using high-resolution scanning photos of the studied thin sections. Results Eleven carbonate microfacies are recognized in the study area. Based on the microfacies characteristics, the 11 microfacies types are further classified into five lithofacies associations （LA）： LA-peloids, LA-ooids, LA-skeletal grain, LA-sponge reef, and LA-shale, correlating to four specific carbonate factories, including peloids, ooids, skeletal grains, and sponge reef. The statistical data reveal that the LA-peloids exhibit the smallest average grain size, characterized by regular shapes and good roundness. However, the sedimentation was a carbonate sediment mixture with relatively poor sorting, and the shape coefficient had significant fluctuations, suggesting a medium to high-energy environment. Carbonate grains are relatively pure and slightly larger in the LA-ooids than the LA-peloids. Multiple samples demonstrate a concentrated average grain size distribution with predominantly near-circular grains. The ooids are well cemented with sparry calcite, indicating the strongest hydrodynamic condition over the studied sedimentary succession. The LA-skeletal grain contains various carbonate grains, displaying poor sorting and roundness in the statistical results. Furthermore, part of the carbonate grains of the LA-skeletal grains is deposited in the micritic matrix, indicating relatively deep and quiet water with weak hydrodynamic energy. Conclusions The primary carbonate grain types in the different specific factories indicated different depositional mechanisms that drive different carbonate production pathways. By analyzing the composition, depositional environment, hydrodynamic, and nutrient level of tropical shallow water factories in the study area, each specific factory has strong "habitat" characteristics. The rapid transition of different specific carbonate factories in the sedimentary succession indicates that they coexisted at the horizontal level. Frequent sea level changes can lead to rapid changes in hydrodynamic level, carbonate saturation, and other factors, driving the transition of factory types. Additionally, the nutrient level is crucial in regulating carbonate production and may control the formation of different specific types of carbonate factories in shallow-water environments. Therefore, the rapid transformation of subdivided specific carbonate factories on a short time scale is primarily related to the horizontal migration of factory types controlled by regional sea level and may not be related to extreme climate events and the transformation of sedimentary systems.
Objective The precipitation and accumulation of massive carbonates are called the "carbonate factory". Environmental and biological factors, such as water depth, light intensity, nutrients, hydrodynamics, climatic conditions, and sea-water chemistry control the composition of different carbonate factories. To properly understand the evolution of the carbonate sedimentary system, it is essential to accurately identify different types of carbonate factories and the factors controlling their development. To achieve this, the quantitative evaluation of the environmental and biological parameters in the transition of sedimentary systems is key. Methods In this study, the composition, characteristics, type transitions, and driving forces of tropical shallow-water carbonate factories within the Ma'antang Formation were studied in detail. We applied a systematic microfacies analysis in the deposition of the Ma'antang Formation at the Guanyinya section, Hanwang town, Mianzhu city, northwest Sichuan. Microfacies characteristics, including types and percentages of carbonate grains, matrix, and fossils, combined with quantitative analysis of the size and morphological parameters of 20,391 carbonate grains, including circularity, roundness, solidity, and aspect ratio, were analyzed from the thin sections. The size data were corrected and then used to calculate the average particle sizes, degree of sorting, skewness, and kurtosis following the Folk and Ward method. The analyses of the above-mentioned morphological parameters were accomplished in ImageJ using high-resolution scanning photos of the studied thin sections. Results Eleven carbonate microfacies are recognized in the study area. Based on the microfacies characteristics, the 11 microfacies types are further classified into five lithofacies associations （LA）： LA-peloids, LA-ooids, LA-skeletal grain, LA-sponge reef, and LA-shale, correlating to four specific carbonate factories, including peloids, ooids, skeletal grains, and sponge reef. The statistical data reveal that the LA-peloids exhibit the smallest average grain size, characterized by regular shapes and good roundness. However, the sedimentation was a carbonate sediment mixture with relatively poor sorting, and the shape coefficient had significant fluctuations, suggesting a medium to high-energy environment. Carbonate grains are relatively pure and slightly larger in the LA-ooids than the LA-peloids. Multiple samples demonstrate a concentrated average grain size distribution with predominantly near-circular grains. The ooids are well cemented with sparry calcite, indicating the strongest hydrodynamic condition over the studied sedimentary succession. The LA-skeletal grain contains various carbonate grains, displaying poor sorting and roundness in the statistical results. Furthermore, part of the carbonate grains of the LA-skeletal grains is deposited in the micritic matrix, indicating relatively deep and quiet water with weak hydrodynamic energy. Conclusions The primary carbonate grain types in the different specific factories indicated different depositional mechanisms that drive different carbonate production pathways. By analyzing the composition, depositional environment, hydrodynamic, and nutrient level of tropical shallow water factories in the study area, each specific factory has strong "habitat" characteristics. The rapid transition of different specific carbonate factories in the sedimentary succession indicates that they coexisted at the horizontal level. Frequent sea level changes can lead to rapid changes in hydrodynamic level, carbonate saturation, and other factors, driving the transition of factory types. Additionally, the nutrient level is crucial in regulating carbonate production and may control the formation of different specific types of carbonate factories in shallow-water environments. Therefore, the rapid transformation of subdivided specific carbonate factories on a short time scale is primarily related to the horizontal migration of factory types controlled by regional sea level and may not be related to extreme climate events and the transformation of sedimentary systems.
2024, 42(2): 466-485.
doi: 10.14027/j.issn.1000-0550.2022.044
Abstract:
Objective Jurassic strata are well exposed in the northeastern Qaidam Basin. However, the formation and scale of the Jurassic prototype-basin in this area has been obscured by intense tectonism, and currently remains unclear. Different views have been expressed regarding the original sedimentary relationships of the residual Jurassic depression. Methods An integrated analysis of the heavy-mineral assemblages comprising whole-rock geochemistry and U-Pb chronology of detrital zircon was conducted to determine the provenance of the Middle Jurassic in this area. Further, the evolution of Jurassic prototype basin in the study area combined with its paleo-tectonic setting and stratigraphic and sedimentary facies research were discussed. [Results and Conclusions] （1） During the Middle Jurassic, five distinct depositional areas existed in the study area, here designated as areas A, B, C, D and E. （2） The primary source of sediments in the study area was intermediate-to-acidic igneous rock, partially mixed with sedimentary rock. （3） The tectonic setting of the provenance area is dominated by continental island arcs and active continental margin. It is considered that the provenance area was affected by two periods of magmatic events （200⁃300 and 400⁃500 Ma）. （4） A comparison of the potential source areas and the depositional areas led to the following conclusions： Area A was mainly sourced from the North Qaidam UHP belt near the Xitie Mountain, with minor supplementary material from the Oulongbuluke Block. Area B was derived from the South Qilian Mountain in the northeast of the study area. Areas C and D, which have relatively low ZTR, are believed to be mainly sourced from near-source paleo-uplift. Area E was mainly sourced from the Ela Mountain in the east of the study area. （5） During the Middle Jurassic, the study area developed as a series of small-scale, isolated depressions until J2d7, then evolved into a larger, partially unified depression. These findings are highly significant for gaining an understanding of the evolution of the Qaidam Basin and the temporal-spatial changes of the basin⁃mountain system at the northern margin of the Tibetan Plateau. It is also important for guiding oil and gas exploration in the Qaidam Basin.
Objective Jurassic strata are well exposed in the northeastern Qaidam Basin. However, the formation and scale of the Jurassic prototype-basin in this area has been obscured by intense tectonism, and currently remains unclear. Different views have been expressed regarding the original sedimentary relationships of the residual Jurassic depression. Methods An integrated analysis of the heavy-mineral assemblages comprising whole-rock geochemistry and U-Pb chronology of detrital zircon was conducted to determine the provenance of the Middle Jurassic in this area. Further, the evolution of Jurassic prototype basin in the study area combined with its paleo-tectonic setting and stratigraphic and sedimentary facies research were discussed. [Results and Conclusions] （1） During the Middle Jurassic, five distinct depositional areas existed in the study area, here designated as areas A, B, C, D and E. （2） The primary source of sediments in the study area was intermediate-to-acidic igneous rock, partially mixed with sedimentary rock. （3） The tectonic setting of the provenance area is dominated by continental island arcs and active continental margin. It is considered that the provenance area was affected by two periods of magmatic events （200⁃300 and 400⁃500 Ma）. （4） A comparison of the potential source areas and the depositional areas led to the following conclusions： Area A was mainly sourced from the North Qaidam UHP belt near the Xitie Mountain, with minor supplementary material from the Oulongbuluke Block. Area B was derived from the South Qilian Mountain in the northeast of the study area. Areas C and D, which have relatively low ZTR, are believed to be mainly sourced from near-source paleo-uplift. Area E was mainly sourced from the Ela Mountain in the east of the study area. （5） During the Middle Jurassic, the study area developed as a series of small-scale, isolated depressions until J2d7, then evolved into a larger, partially unified depression. These findings are highly significant for gaining an understanding of the evolution of the Qaidam Basin and the temporal-spatial changes of the basin⁃mountain system at the northern margin of the Tibetan Plateau. It is also important for guiding oil and gas exploration in the Qaidam Basin.
2024, 42(2): 486-501.
doi: 10.14027/j.issn.1000-0550.2022.064
Abstract:
Objective The Shanxi Graben System has become an important area of Late Cenozoic deformation research in eastern China for its unique tectonic location. However, due to the limitations of the outcrop profile, complete sedimentary environment evolution of the basins in the graben system is rarely reported, which hinders our understanding of its evolution process. Methods The depositional environments and provenance were analyzed using the detailed sedimentary record from the ZK01 borehole in the Taiyuan Basin. [Results and Conclusions] The analysis of the sedimentary environment shows that the Taiyuan Basin underwent a fluvial⁃delta/lake⁃river sedimentary environment evolution process. Two mega transgressions generated basin-wide lakes, one at ca. 5.8⁃4.4 and the other at ca. 2.2⁃1.6 Ma. Provenance analysis shows that the provenance was mainly theJurassic quartz sandstones of the Taihang Mountain before 5.8 Ma. After 5.8 Ma, the provenance changed to the Triassic feldspathic sandstones, Meanwhile, the debris of metamorphic and volcanic rocks from the northern part of the Lvliang Mountain began to enter the basin, indicating the development of Fen River in the Taiyuan Basin. Asynchronous processes between paleoclimatic variations and transgressions illustrate the lakes’ spreading processes as mainly influenced by tectonic subsidence, being a structural-geomorphic response to the eastward spreading of the Qinghai-Tibetan Plateau.
Objective The Shanxi Graben System has become an important area of Late Cenozoic deformation research in eastern China for its unique tectonic location. However, due to the limitations of the outcrop profile, complete sedimentary environment evolution of the basins in the graben system is rarely reported, which hinders our understanding of its evolution process. Methods The depositional environments and provenance were analyzed using the detailed sedimentary record from the ZK01 borehole in the Taiyuan Basin. [Results and Conclusions] The analysis of the sedimentary environment shows that the Taiyuan Basin underwent a fluvial⁃delta/lake⁃river sedimentary environment evolution process. Two mega transgressions generated basin-wide lakes, one at ca. 5.8⁃4.4 and the other at ca. 2.2⁃1.6 Ma. Provenance analysis shows that the provenance was mainly theJurassic quartz sandstones of the Taihang Mountain before 5.8 Ma. After 5.8 Ma, the provenance changed to the Triassic feldspathic sandstones, Meanwhile, the debris of metamorphic and volcanic rocks from the northern part of the Lvliang Mountain began to enter the basin, indicating the development of Fen River in the Taiyuan Basin. Asynchronous processes between paleoclimatic variations and transgressions illustrate the lakes’ spreading processes as mainly influenced by tectonic subsidence, being a structural-geomorphic response to the eastward spreading of the Qinghai-Tibetan Plateau.
2024, 42(2): 502-520.
doi: 10.14027/j.issn.1000-0550.2023.009
Abstract:
Objective The study of the sedimentology, geochemistry and zircon U-Pb dating of sand-gravel sediments is highly significant for determining stratigraphic division, river system evolution, mountain tectonic uplift history and regional tectono-magmatic events. However, little is understood regarding the Hulun Buir Bayan Tara sand-gravel profile, which has restricted its stratigraphic division and in-depth understanding of the regional surface process. Methods For this reason, analyses of the sedimentology, heavy mineral, geochemical （macronutrients, trace and rare earth elements） and zircon U-Pb chronology were performed on this section to determine the chemical weathering characteristics and sedimentary cycles, as well as the material sources, and to explore the stratigraphic delineation attribution as well as the tectonic context and indications of regional magmatic-tectonic events. This has provided a new basis for discussion of areas of sedimentary environment and stratigraphic division and correlation. Results The following results were obtained. （1） The gravels are poorly sorted and rounded, and have undergone a non-weak degree of weathering, with quartzaceous material （61.2%） and tuff （34.9%） dominating the gravel composition, followed by terrigenous clastics （2.6%） and rhyolite （1.3%）. The heavy mineral composition is clearly dominated by white titanium （32.54%） and ilmenite （33.01%）, followed by magnetite （12.11%）, zircon-tourmaline-rutile （ZTR） （6.53%）, hematite （5.47%） and anatase （1.34%）, with less than 1% chlorite, rutile and garnet. There are very few, if any, unstable minerals in the sediments, indicating that they have undergone strong chemical weathering. Combined with geochemical characteristics and geomorphological evidence, the comprehensive analysis shows that the maturity of the sediment in the Bayan Tara sand-gravel profile is low, and is the product of the first weathering cycle, with moderate chemical weathering after accumulation. Comparison with the age spectrum of detrital zircons in the northern section of the Greater Khingan Mountains indicates that this is the provenance area. （2） Combined with macro- and micro-perspectives （heavy minerals and geochemical indicators, etc.）, it is considered that the sediments of the Bayan Tala sand-gravel profile are alluvial-proluvial deposits formed in a warm, dry oxidizing environment. Combined with the new research findings, various methods （sedimentology, mineral petrology and geochronology） were integrated and analyzed and compared with the standard sections of the Baitushan Formation in Longjiang county and Ping'an town at the eastern foot of the Greater Khingan Mountains. This indicated that the stratigraphic units are part of the Baitushan Group. （3） The discriminant diagram of the tectonic background indicates a passive continental margin, which is consistent with the fact that the Greater Khingan Mountains are located within the tectonic plate and their structure is relatively stable. Peak U-Pb ages of clastic zircons at 100-200 Ma and 250-350 Ma record multi-stage magmatic tectonic events triggered by the closure of the Paleo-Asian Ocean and subduction and retrogression of the Paleo-Pacific Ocean in the northern segment of the Greater Khingan Mountains. Conclusions The study provides new evidence for division and further categorization of Quaternary strata and for reconstructing regional surface processes in the Greater Khingan Mountains during the early Pleistocene.
Objective The study of the sedimentology, geochemistry and zircon U-Pb dating of sand-gravel sediments is highly significant for determining stratigraphic division, river system evolution, mountain tectonic uplift history and regional tectono-magmatic events. However, little is understood regarding the Hulun Buir Bayan Tara sand-gravel profile, which has restricted its stratigraphic division and in-depth understanding of the regional surface process. Methods For this reason, analyses of the sedimentology, heavy mineral, geochemical （macronutrients, trace and rare earth elements） and zircon U-Pb chronology were performed on this section to determine the chemical weathering characteristics and sedimentary cycles, as well as the material sources, and to explore the stratigraphic delineation attribution as well as the tectonic context and indications of regional magmatic-tectonic events. This has provided a new basis for discussion of areas of sedimentary environment and stratigraphic division and correlation. Results The following results were obtained. （1） The gravels are poorly sorted and rounded, and have undergone a non-weak degree of weathering, with quartzaceous material （61.2%） and tuff （34.9%） dominating the gravel composition, followed by terrigenous clastics （2.6%） and rhyolite （1.3%）. The heavy mineral composition is clearly dominated by white titanium （32.54%） and ilmenite （33.01%）, followed by magnetite （12.11%）, zircon-tourmaline-rutile （ZTR） （6.53%）, hematite （5.47%） and anatase （1.34%）, with less than 1% chlorite, rutile and garnet. There are very few, if any, unstable minerals in the sediments, indicating that they have undergone strong chemical weathering. Combined with geochemical characteristics and geomorphological evidence, the comprehensive analysis shows that the maturity of the sediment in the Bayan Tara sand-gravel profile is low, and is the product of the first weathering cycle, with moderate chemical weathering after accumulation. Comparison with the age spectrum of detrital zircons in the northern section of the Greater Khingan Mountains indicates that this is the provenance area. （2） Combined with macro- and micro-perspectives （heavy minerals and geochemical indicators, etc.）, it is considered that the sediments of the Bayan Tala sand-gravel profile are alluvial-proluvial deposits formed in a warm, dry oxidizing environment. Combined with the new research findings, various methods （sedimentology, mineral petrology and geochronology） were integrated and analyzed and compared with the standard sections of the Baitushan Formation in Longjiang county and Ping'an town at the eastern foot of the Greater Khingan Mountains. This indicated that the stratigraphic units are part of the Baitushan Group. （3） The discriminant diagram of the tectonic background indicates a passive continental margin, which is consistent with the fact that the Greater Khingan Mountains are located within the tectonic plate and their structure is relatively stable. Peak U-Pb ages of clastic zircons at 100-200 Ma and 250-350 Ma record multi-stage magmatic tectonic events triggered by the closure of the Paleo-Asian Ocean and subduction and retrogression of the Paleo-Pacific Ocean in the northern segment of the Greater Khingan Mountains. Conclusions The study provides new evidence for division and further categorization of Quaternary strata and for reconstructing regional surface processes in the Greater Khingan Mountains during the early Pleistocene.
2024, 42(2): 521-533.
doi: 10.14027/j.issn.1000-0550.2022.085
Abstract:
Objective The Upper Pleistocene eolian loess accumulation in islands of the East China Sea is an important expansion for loess in China in the eastern Yangtze River delta and its extended sea area. The better study of the eolian loess provenance and its evolution in this region is particularly important for understanding regional paleoenvironmental changes. Methods Based on the analyses of optically stimulated luminescence （OSL） dating and magnetic susceptibility, parametric end-member analysis was used to clarify the grain size data for Shengshan Island loess and its paleoenvironmental effect. [ Results and Conclusions ] Three end-members （EM1-EM3） were identified： EM1 is the clay component transported by the westerly circulation； EM2 is the fine silt component transported over long distances, suspended in the form of floating dust that was deposited when the wind force weakened； and EM3 is the coarse silt component carried by winter winds for short distances and deposited near the land surface. It was inferred from the analysis that EM1 and EM2 were the main sources of dust in the loess profile on Shengshan Island. Combined with the optically stimulated luminescence （OSL） results, the variations of frequency susceptibility, median particle size, Rb/Sr values and the fluctuations of the end-members in the loess profile revealed that the Shengshan Island loess accumulated in marine isotope stage MIS2 experienced paleoclimatic fluctuations from warm and humid to cold and dry, and then to warm and humid in MIS3. The end-member properties also responded to the paleoenvironmental changes during the last glacial maximum and deglaciation period of the MIS2 stage. Climatic fluctuations recorded by the loess deposits also correspond to the trends of deep-sea oxygen isotopes and ancient sea-level changes. Thus the environmental evolution in the shelf areas of the East China Sea were closely related to global paleoclimatic variations.
Objective The Upper Pleistocene eolian loess accumulation in islands of the East China Sea is an important expansion for loess in China in the eastern Yangtze River delta and its extended sea area. The better study of the eolian loess provenance and its evolution in this region is particularly important for understanding regional paleoenvironmental changes. Methods Based on the analyses of optically stimulated luminescence （OSL） dating and magnetic susceptibility, parametric end-member analysis was used to clarify the grain size data for Shengshan Island loess and its paleoenvironmental effect. [ Results and Conclusions ] Three end-members （EM1-EM3） were identified： EM1 is the clay component transported by the westerly circulation； EM2 is the fine silt component transported over long distances, suspended in the form of floating dust that was deposited when the wind force weakened； and EM3 is the coarse silt component carried by winter winds for short distances and deposited near the land surface. It was inferred from the analysis that EM1 and EM2 were the main sources of dust in the loess profile on Shengshan Island. Combined with the optically stimulated luminescence （OSL） results, the variations of frequency susceptibility, median particle size, Rb/Sr values and the fluctuations of the end-members in the loess profile revealed that the Shengshan Island loess accumulated in marine isotope stage MIS2 experienced paleoclimatic fluctuations from warm and humid to cold and dry, and then to warm and humid in MIS3. The end-member properties also responded to the paleoenvironmental changes during the last glacial maximum and deglaciation period of the MIS2 stage. Climatic fluctuations recorded by the loess deposits also correspond to the trends of deep-sea oxygen isotopes and ancient sea-level changes. Thus the environmental evolution in the shelf areas of the East China Sea were closely related to global paleoclimatic variations.
2024, 42(2): 534-544.
doi: 10.14027/j.issn.1000-0550.2022.097
Abstract:
Objective Red earth aggradation in the Quaternary has a unique reticulate structure and is widely distributed in the middle and lower reaches of the Yangtze River, and is a record of the response to Quaternary environmental changes. Methods Prior to this study, most attention has been paid to the physico-chemical properties of the reticulate red earth but less on the morphology of the white veins. A comprehensive analysis of the white veins in combination with their physico-chemical properties is conducive to a deeper understanding of the environmental significance. This study analyzed these aspects of a reticulate red earth section in Jiujiang, Jiangxi, based on an understanding of the possible effects and the reticulation mechanism and emphasizing the environmental significance of the morphological properties of the white veins. [ Results and Conclusions ] （1） the shape of the white veins varies systematically in the section, from relatively fine in the upper part and coarsening in the lower part； （2） the extent of correspondence between the weathering index and the morphological index of the white veins shows that, on the whole, their shape corresponds to a climate change trend from warm and humid to cold and dry, but the indication has certain limitations.
Objective Red earth aggradation in the Quaternary has a unique reticulate structure and is widely distributed in the middle and lower reaches of the Yangtze River, and is a record of the response to Quaternary environmental changes. Methods Prior to this study, most attention has been paid to the physico-chemical properties of the reticulate red earth but less on the morphology of the white veins. A comprehensive analysis of the white veins in combination with their physico-chemical properties is conducive to a deeper understanding of the environmental significance. This study analyzed these aspects of a reticulate red earth section in Jiujiang, Jiangxi, based on an understanding of the possible effects and the reticulation mechanism and emphasizing the environmental significance of the morphological properties of the white veins. [ Results and Conclusions ] （1） the shape of the white veins varies systematically in the section, from relatively fine in the upper part and coarsening in the lower part； （2） the extent of correspondence between the weathering index and the morphological index of the white veins shows that, on the whole, their shape corresponds to a climate change trend from warm and humid to cold and dry, but the indication has certain limitations.
2024, 42(2): 545-557.
doi: 10.14027/j.issn.1000-0550.2022.063
Abstract:
Objective Since the Paleogene, with the uplift of the Qinghai-Tibetan Plateau and the large-scale thrust and strike-slip of the strata, many inland basins were formed in the northern part of the Qinghai-Tibetan Plateau. Because of their particular location, the evolution of paleoclimate and paleo-vegetation in the basins is very significant for clarifying the climatic evolution in the Paleogene in Eurasia. Methods In this study, the evolution history of paleoclimate and paleo-vegetation in the Nangqian Basin in central and eastern Qinghai-Tibetan Plateau （51.8-37.5 Ma） was reconstructed from biomarkers and total organic carbon （TOC）. [Results and Conclusions] The evolution history of Paleogene climate index in Nangqian Basin is in three stages, each with different changes of sedimentary facies. Stage I （51.8-46.4 Ma）： The sedimentary facies are mainly fluvial and short-term lacustrine, with lower CPI and TOC values and higher Paq values. ACL and nC27/nC31 show an obvious change trend in this period, which may have been affected by the mixing of terrestrial sediments brought by rivers. Most of the main peak carbon was nC22 and nC23； the vegetation type was predominantly woody plants, and the climate was relatively humid. Stage II （46.4-42.7 Ma）： The Paq value and nC27/nC31 ratio gradually decreased, ACL gradually increased, the carbon number of the main peak gradually changed from low to medium or high, the vegetation gradually changed from woody to herbaceous plants, and the climate gradually became drought alternating with wet conditions. Stage III （42.7-37.5 Ma）： Paq, nC27/nC31 and ACL changed little, the carbon number of the main peak increased significantly, n-alkanes with high carbon numbers increased significantly, and drought conditions increased. TOC also significantly increased, reflecting increasing total vegetation productivity, which may have been a response to the warm Middle Eocene Climatic Optimum （MECO）. In addition, from comparisons with previous reports of climate change in the study area and adjacent basins, it is considered that the climate change in the Nangqian Basin during 51.8-37.5 Ma was mainly affected by global climate change and the retreat of the Tethys Sea. The uplift of the Qinghai-Tibetan Plateau and its higher elevation greatly influenced water vapor in the Nangqian Basin, whereas monsoons have little influence on water vapor.
Objective Since the Paleogene, with the uplift of the Qinghai-Tibetan Plateau and the large-scale thrust and strike-slip of the strata, many inland basins were formed in the northern part of the Qinghai-Tibetan Plateau. Because of their particular location, the evolution of paleoclimate and paleo-vegetation in the basins is very significant for clarifying the climatic evolution in the Paleogene in Eurasia. Methods In this study, the evolution history of paleoclimate and paleo-vegetation in the Nangqian Basin in central and eastern Qinghai-Tibetan Plateau （51.8-37.5 Ma） was reconstructed from biomarkers and total organic carbon （TOC）. [Results and Conclusions] The evolution history of Paleogene climate index in Nangqian Basin is in three stages, each with different changes of sedimentary facies. Stage I （51.8-46.4 Ma）： The sedimentary facies are mainly fluvial and short-term lacustrine, with lower CPI and TOC values and higher Paq values. ACL and nC27/nC31 show an obvious change trend in this period, which may have been affected by the mixing of terrestrial sediments brought by rivers. Most of the main peak carbon was nC22 and nC23； the vegetation type was predominantly woody plants, and the climate was relatively humid. Stage II （46.4-42.7 Ma）： The Paq value and nC27/nC31 ratio gradually decreased, ACL gradually increased, the carbon number of the main peak gradually changed from low to medium or high, the vegetation gradually changed from woody to herbaceous plants, and the climate gradually became drought alternating with wet conditions. Stage III （42.7-37.5 Ma）： Paq, nC27/nC31 and ACL changed little, the carbon number of the main peak increased significantly, n-alkanes with high carbon numbers increased significantly, and drought conditions increased. TOC also significantly increased, reflecting increasing total vegetation productivity, which may have been a response to the warm Middle Eocene Climatic Optimum （MECO）. In addition, from comparisons with previous reports of climate change in the study area and adjacent basins, it is considered that the climate change in the Nangqian Basin during 51.8-37.5 Ma was mainly affected by global climate change and the retreat of the Tethys Sea. The uplift of the Qinghai-Tibetan Plateau and its higher elevation greatly influenced water vapor in the Nangqian Basin, whereas monsoons have little influence on water vapor.
2024, 42(2): 558-574.
doi: 10.14027/j.issn.1000-0550.2022.058
Abstract:
Objective River deltas are sensitive areas of land-sea interaction with intense human activities, and continue to be the focus of international research. Methods Based on grain-size analysis of surface sediments in the Minjiang River delta and surrounding waters, this study discusses the evolution of the contemporary sedimentary environment in the area. Results It is shown that the dominant to-and-fro tidal current within the estuary gradually changes into a rotating seaward flow, and that the current speed decreases from the subaqueous delta to the offshore area. The sediments in the study area are mainly sand, silt and sandy silt, with mean grain-sizes of 1.57 Φ, 6.7 Φ and 5.62 Φ. From the estuary to the adjacent coastal shelf area, coarse-particle content decreases and fine-particle content gradually increases. In addition, grain size trend analysis indicates that the sediment delivered by the Minjiang River is transported towards the E, SE and NE when it enters the estuary. After the fine sediment passes through the subaqueous delta, it tends to be transported offshore towards the SE and NE. A multivariate statistical analysis of sediment composition, grain size parameters and water depth integrating the geographical location, sediment source and hydrodynamic conditions indicate that the contemporary sedimentary environment in the study area is divided into four sub-sedimentary districts. The study reveals the effect of different influences on the sedimentary environment and demonstrates its response to human activities. Conclusions The results of this study provide advice for estuary resource development and ecological environment protection.
Objective River deltas are sensitive areas of land-sea interaction with intense human activities, and continue to be the focus of international research. Methods Based on grain-size analysis of surface sediments in the Minjiang River delta and surrounding waters, this study discusses the evolution of the contemporary sedimentary environment in the area. Results It is shown that the dominant to-and-fro tidal current within the estuary gradually changes into a rotating seaward flow, and that the current speed decreases from the subaqueous delta to the offshore area. The sediments in the study area are mainly sand, silt and sandy silt, with mean grain-sizes of 1.57 Φ, 6.7 Φ and 5.62 Φ. From the estuary to the adjacent coastal shelf area, coarse-particle content decreases and fine-particle content gradually increases. In addition, grain size trend analysis indicates that the sediment delivered by the Minjiang River is transported towards the E, SE and NE when it enters the estuary. After the fine sediment passes through the subaqueous delta, it tends to be transported offshore towards the SE and NE. A multivariate statistical analysis of sediment composition, grain size parameters and water depth integrating the geographical location, sediment source and hydrodynamic conditions indicate that the contemporary sedimentary environment in the study area is divided into four sub-sedimentary districts. The study reveals the effect of different influences on the sedimentary environment and demonstrates its response to human activities. Conclusions The results of this study provide advice for estuary resource development and ecological environment protection.
2024, 42(2): 575-592.
doi: 10.14027/j.issn.1000-0550.2022.043
Abstract:
Objective The Lower Permian Qixia Stage in southwest Sichuan Basin has shown significant exploration potential in recent years, but the views on the sedimentary filling pattern and paleogeographic framework in this area are not unified, which seriously restricts the further oil and gas exploration and well location deployment in the basin. Methods This study of the Lower Permian Liangshan Formation and Qixia Formation in southwestern Sichuan Basin was based on the comprehensive use of field profiles, drilling cores and logging data, and analysis of rock types and sedimentary facies. The application of the principles and methods of marine carbonate stratigraphical sequencing led to the identification of third-order sequence interfaces in the Qixia Stage. Single-factor analysis and multi-factor comprehensive mapping were adopted, taking the three-level sequence as the mapping unit. Combining these with the dominant phase, maps of the SQ0 + SQ1 and SQ2 sequence paleogeographic lithofacies were drawn for the Qixia Stage in the study area. Results Four third-order sequence interfaces were identified in the Qixia Stage, from bottom to top： lower interface of Liangshan Formation （SB I）, inner interface of Qixia Formation Group 1 section （SB II）, interface of Qixia Formation Groups 1 and 2 （SB II） and interface of the Qixia Formation and Maokou Formation （SB I）. These occurred in three tertiary sequences, each consisting of a transgressive domain and a high-order domain. Comparative analysis of the sequential stratigraphic framework suggested that the SQ0 sequence of the lower Qixia Stage corresponds roughly to the Liangshan Formation. In the lower Qixia Formation Group 1 section, SQ0 is evident only in the paleogeomorphic lowland before the deposition of Qixia Stage in the study area, with a filling sequence overlapping a paleogeomorphic highland. Sequence SQ1 in the middle region and SQ2 sequence in the upper region are found throughout the area. The sequence in the southwestern Sichuan Basin consists of a marine carbonate platform bordering the Kang-Dian ancient land in the west, with obvious differentiation of paleogeomorphology and sedimentary facies. From west to east, the Kang-Dian land and its eastern margin consisted of a tidal flat, intraplatform depression, intraplatform mound-shoal, and an open semi-confined platform. In addition, an intraplatform mound-shoal beach developed along the intraplatform slope break zone, tending to surround the intraplatform depression. Conclusions Analysis of the influence of the slope break zone on carbonate reservoir formation highlighted that the SQ2 sequence is the most favorable reservoir facies belt in the study area, with favorable exploration areas around the intraplatform mound-shoal facies belt. A new understanding of the filling pattern and paleogeographic lithofacies characteristics of the Qixia Stage, as determined from the sequence stratigraphy, provides a new framework for future reservoir studies and the prediction of favorable exploration areas in the Qixia Formation.
Objective The Lower Permian Qixia Stage in southwest Sichuan Basin has shown significant exploration potential in recent years, but the views on the sedimentary filling pattern and paleogeographic framework in this area are not unified, which seriously restricts the further oil and gas exploration and well location deployment in the basin. Methods This study of the Lower Permian Liangshan Formation and Qixia Formation in southwestern Sichuan Basin was based on the comprehensive use of field profiles, drilling cores and logging data, and analysis of rock types and sedimentary facies. The application of the principles and methods of marine carbonate stratigraphical sequencing led to the identification of third-order sequence interfaces in the Qixia Stage. Single-factor analysis and multi-factor comprehensive mapping were adopted, taking the three-level sequence as the mapping unit. Combining these with the dominant phase, maps of the SQ0 + SQ1 and SQ2 sequence paleogeographic lithofacies were drawn for the Qixia Stage in the study area. Results Four third-order sequence interfaces were identified in the Qixia Stage, from bottom to top： lower interface of Liangshan Formation （SB I）, inner interface of Qixia Formation Group 1 section （SB II）, interface of Qixia Formation Groups 1 and 2 （SB II） and interface of the Qixia Formation and Maokou Formation （SB I）. These occurred in three tertiary sequences, each consisting of a transgressive domain and a high-order domain. Comparative analysis of the sequential stratigraphic framework suggested that the SQ0 sequence of the lower Qixia Stage corresponds roughly to the Liangshan Formation. In the lower Qixia Formation Group 1 section, SQ0 is evident only in the paleogeomorphic lowland before the deposition of Qixia Stage in the study area, with a filling sequence overlapping a paleogeomorphic highland. Sequence SQ1 in the middle region and SQ2 sequence in the upper region are found throughout the area. The sequence in the southwestern Sichuan Basin consists of a marine carbonate platform bordering the Kang-Dian ancient land in the west, with obvious differentiation of paleogeomorphology and sedimentary facies. From west to east, the Kang-Dian land and its eastern margin consisted of a tidal flat, intraplatform depression, intraplatform mound-shoal, and an open semi-confined platform. In addition, an intraplatform mound-shoal beach developed along the intraplatform slope break zone, tending to surround the intraplatform depression. Conclusions Analysis of the influence of the slope break zone on carbonate reservoir formation highlighted that the SQ2 sequence is the most favorable reservoir facies belt in the study area, with favorable exploration areas around the intraplatform mound-shoal facies belt. A new understanding of the filling pattern and paleogeographic lithofacies characteristics of the Qixia Stage, as determined from the sequence stratigraphy, provides a new framework for future reservoir studies and the prediction of favorable exploration areas in the Qixia Formation.
2024, 42(2): 593-607.
doi: 10.14027/j.issn.1000-0550.2022.059
Abstract:
Objective The restricted environment reservoir of Cretaceous Mishrif Formation in M oilfield in the Middle East is complicated and the understanding of reservoir heterogeneity is insufficient, which restricts the effective development of this type of reservoir. Therefore, reservoir characteristics, distribution and main controlling factors of heterogeneity in restricted environment were studied. Methods Based on the data of core, cast thin section, physical properties and mercury intrusion experiment. Through mathematical statistics and well correlation, the petrophysical of different reservoirs are clarified and the spatial distribution is described. The main controlling factors of reservoir characteristics in restricted environment are explained, and the origin models of different reservoirs are established. Results Three types of reservoir were found in the restricted environment, designated as reservoir types 1, 2 and 3. Type 1 reservoirs were developed in high-energy depositional environments （tidal channels, intra-platform shoals etc.） and underwent quasi-contemporaneous dissolution. These reservoirs comprise mainly grainstone with medium-to-high porosity and medium-to-high permeability. The reserve space is mainly composed of intergranular pores and dissolution pores. It is the preferred target type for reservoir development, as it is present on a huge scale with stable distribution. Type 2 reservoirs were mainly developed in high-energy depositional environments such as lagoons, and experienced dissolution in the quasi-contemporaneous period. These reservoirs include grain-bearing limestone, wackestone and packstone with mainly medium-to-high porosity and medium-to-low permeability. No primary intergranular pores are developed, and the reserve space consists of matrix-host micropores, mddic pores and intercrystalline pores. This type of diagenetic reservoirs are present on the largest scale, but they are in the form of thin layers with highly frequent interlayers, and development would be difficult. Type 3 reservoirs were formed in intra-platform shoals and experienced quasi-contemporaneous dolomitization. They are mostly dolomite with medium-to-high porosity and medium-to-low permeability. Reservoir type 3 are small in scale and are only locally developed in the upper part of the Mishrif Formation. Conclusions The study concluded that the genesis of reservoir type 1 was mainly controlled by sedimentation, which controlled the original structural components of the rock and in turn controlled the type and intensity of diagenesis in the quasi-contemporaneous period. Type 2 reservoirs exhibit weakly sedimentary hydrodynamic conditions and no primary intergranular pores are developed. Their reservoir spaces are mainly the result of constructive diagenesis such as selective dissolution. Type 3 reservoirs have undergone greater diagenetic re-formation and severe damage to the original structural components. The content of local residual biological debris indicates that type 3 reservoirs were formed in a medium-to-high energy sedimentary environment, with mixed dolomitization.
Objective The restricted environment reservoir of Cretaceous Mishrif Formation in M oilfield in the Middle East is complicated and the understanding of reservoir heterogeneity is insufficient, which restricts the effective development of this type of reservoir. Therefore, reservoir characteristics, distribution and main controlling factors of heterogeneity in restricted environment were studied. Methods Based on the data of core, cast thin section, physical properties and mercury intrusion experiment. Through mathematical statistics and well correlation, the petrophysical of different reservoirs are clarified and the spatial distribution is described. The main controlling factors of reservoir characteristics in restricted environment are explained, and the origin models of different reservoirs are established. Results Three types of reservoir were found in the restricted environment, designated as reservoir types 1, 2 and 3. Type 1 reservoirs were developed in high-energy depositional environments （tidal channels, intra-platform shoals etc.） and underwent quasi-contemporaneous dissolution. These reservoirs comprise mainly grainstone with medium-to-high porosity and medium-to-high permeability. The reserve space is mainly composed of intergranular pores and dissolution pores. It is the preferred target type for reservoir development, as it is present on a huge scale with stable distribution. Type 2 reservoirs were mainly developed in high-energy depositional environments such as lagoons, and experienced dissolution in the quasi-contemporaneous period. These reservoirs include grain-bearing limestone, wackestone and packstone with mainly medium-to-high porosity and medium-to-low permeability. No primary intergranular pores are developed, and the reserve space consists of matrix-host micropores, mddic pores and intercrystalline pores. This type of diagenetic reservoirs are present on the largest scale, but they are in the form of thin layers with highly frequent interlayers, and development would be difficult. Type 3 reservoirs were formed in intra-platform shoals and experienced quasi-contemporaneous dolomitization. They are mostly dolomite with medium-to-high porosity and medium-to-low permeability. Reservoir type 3 are small in scale and are only locally developed in the upper part of the Mishrif Formation. Conclusions The study concluded that the genesis of reservoir type 1 was mainly controlled by sedimentation, which controlled the original structural components of the rock and in turn controlled the type and intensity of diagenesis in the quasi-contemporaneous period. Type 2 reservoirs exhibit weakly sedimentary hydrodynamic conditions and no primary intergranular pores are developed. Their reservoir spaces are mainly the result of constructive diagenesis such as selective dissolution. Type 3 reservoirs have undergone greater diagenetic re-formation and severe damage to the original structural components. The content of local residual biological debris indicates that type 3 reservoirs were formed in a medium-to-high energy sedimentary environment, with mixed dolomitization.
2024, 42(2): 608-618.
doi: 10.14027/j.issn.1000-0550.2022.082
Abstract:
Objective A key point in the evolutionary history of reefs is the widespread occurrence of associated archaeocyath-calcimicrobial communities in the Early Cambrian. The developmental environments and conditions for these reef-building communities deserve more attention. Methods We present a case study of the Daheba section in Mian county, located in the Hanzhong-Micangshan area along the northwestern margin of the South China Block. In this region, we conducted comprehensive petrographic, sedimentological, and geochemical analysis of sedimentary successions to investigate reef development in turbid environments. Results A detailed petrographic study indicates that the Daheba reef was formed in a near-shore, high-energy condition. Calcified microfossils, archaeocyaths and archaeocyath-calcimicrobial communities dominated reef construction from the early to late phases of reef growth. Small amounts of terrigenous particles were found within the inner reef framework, increasing from the base to the top. Conclusions This indicates that the reef was tolerant of a small input of terrigenous silts and fine sands during growth. These terrigenous materials material may have provided a source of nutrients and facilitated reef growth； however, an excess of siliciclastics would have abraded reef-building organisms and blocked their feeding passages, perhaps even buried the reef architecture completely. This study has clarified the growth process and environment of an Early Cambrian archaeocyathan-calcimicrobial reef in China, and may provide a reference for further study of Early Cambrian reef systems in paleoenvironmental reconstruction, sedimentology and petroleum geology.
Objective A key point in the evolutionary history of reefs is the widespread occurrence of associated archaeocyath-calcimicrobial communities in the Early Cambrian. The developmental environments and conditions for these reef-building communities deserve more attention. Methods We present a case study of the Daheba section in Mian county, located in the Hanzhong-Micangshan area along the northwestern margin of the South China Block. In this region, we conducted comprehensive petrographic, sedimentological, and geochemical analysis of sedimentary successions to investigate reef development in turbid environments. Results A detailed petrographic study indicates that the Daheba reef was formed in a near-shore, high-energy condition. Calcified microfossils, archaeocyaths and archaeocyath-calcimicrobial communities dominated reef construction from the early to late phases of reef growth. Small amounts of terrigenous particles were found within the inner reef framework, increasing from the base to the top. Conclusions This indicates that the reef was tolerant of a small input of terrigenous silts and fine sands during growth. These terrigenous materials material may have provided a source of nutrients and facilitated reef growth； however, an excess of siliciclastics would have abraded reef-building organisms and blocked their feeding passages, perhaps even buried the reef architecture completely. This study has clarified the growth process and environment of an Early Cambrian archaeocyathan-calcimicrobial reef in China, and may provide a reference for further study of Early Cambrian reef systems in paleoenvironmental reconstruction, sedimentology and petroleum geology.
2024, 42(2): 619-631.
doi: 10.14027/j.issn.1000-0550.2022.076
Abstract:
Objective Exploration practice of the Qaidam Basin show that the remaining petroleum in the Qaidam Basin is mainly concentrated in the western area, where relatively little exploration has been carried out due to the complex landform and the presence of typically low-permeability unconventional reservoirs. Accordingly, to increase petroleum exploration in the western Qaidam Basin. Methods A comprehensive study of the data from sedimentary structure observation, rock slices, X-ray diffraction whole-rock analyses and seismic data, among other techniques, was carried out on an interval of core from the Pliocene Xiayoushashan Formation in well Xiandong 1. [Results and Conclusions] The rock was found to comprise gray-black mudstone and shale interbedded with thin-to-medium massive fine-grained greywacke. The top and bottom of the sand bodies are in abrupt contact with the mudstone. The lower boundary of the sand bodies is obviously an uneven erosional surface, and the mudstone and shale beneath the erosion surface exhibits deformed bedding. The sand bodies are sandy debris flow deposits formed by re-transport. The dark-gray to gray black calcareous mudstone and shale contains interbeds of calcirudite bearing shell-terrigenous debris and siltstone, and a silty mass with obviously deformed bedding. These laminae are clearly seen and have an obvious deformation direction caused by clastic slump. The Pliocene Xiayoushashan Formation in well Xiandong 1 is mainly the dark gray-gray black mudstone and shale interbedded with siltstone bearing dolomite, calcite and clay. The mudstone and shale contains horizontal rhythmic bedding. The siltstone has developed slightly dipping cross-beds with more clay laminae, climbing-ripple lamination and coarsely interlayered bedding, which are mainly from semi-deep lake deposits interbedded with weaker bottom flow deposits derived from sediment gravity flows. In addition, the deeper-water areas of the Altun Mountain front appears to have developed slump-sediment gravity flow sedimentary reservoirs similar to well Xiandong 1 at a different period of basin development, but further research into this aspect is needed.
Objective Exploration practice of the Qaidam Basin show that the remaining petroleum in the Qaidam Basin is mainly concentrated in the western area, where relatively little exploration has been carried out due to the complex landform and the presence of typically low-permeability unconventional reservoirs. Accordingly, to increase petroleum exploration in the western Qaidam Basin. Methods A comprehensive study of the data from sedimentary structure observation, rock slices, X-ray diffraction whole-rock analyses and seismic data, among other techniques, was carried out on an interval of core from the Pliocene Xiayoushashan Formation in well Xiandong 1. [Results and Conclusions] The rock was found to comprise gray-black mudstone and shale interbedded with thin-to-medium massive fine-grained greywacke. The top and bottom of the sand bodies are in abrupt contact with the mudstone. The lower boundary of the sand bodies is obviously an uneven erosional surface, and the mudstone and shale beneath the erosion surface exhibits deformed bedding. The sand bodies are sandy debris flow deposits formed by re-transport. The dark-gray to gray black calcareous mudstone and shale contains interbeds of calcirudite bearing shell-terrigenous debris and siltstone, and a silty mass with obviously deformed bedding. These laminae are clearly seen and have an obvious deformation direction caused by clastic slump. The Pliocene Xiayoushashan Formation in well Xiandong 1 is mainly the dark gray-gray black mudstone and shale interbedded with siltstone bearing dolomite, calcite and clay. The mudstone and shale contains horizontal rhythmic bedding. The siltstone has developed slightly dipping cross-beds with more clay laminae, climbing-ripple lamination and coarsely interlayered bedding, which are mainly from semi-deep lake deposits interbedded with weaker bottom flow deposits derived from sediment gravity flows. In addition, the deeper-water areas of the Altun Mountain front appears to have developed slump-sediment gravity flow sedimentary reservoirs similar to well Xiandong 1 at a different period of basin development, but further research into this aspect is needed.
2024, 42(2): 632-642.
doi: 10.14027/j.issn.1000-0550.2022.062
Abstract:
Significance Dolomite is a common carbonate rock and servers as an important reservoir for oil and gas. In the Sichuan Basin marine carbonate successions, for example, approximately 96% of the proved gas reserves are in dolomite reservoirs. Thus, studies of the dolomitization process, and how pores originate in dolomite, have made significant progress. However, how dolomitization affects the development of pores is still argued. Methods By summarizing the main theoretical viewpoints of pore origin, combined with the results of various replacement experimental simulations, this study explains the formation and evolution of pores during dolomitization and suggests lines of future research on pore origin. [ Progress ] The pores in dolomite are either inherited from its precursor limestone, or they have resulted from the dissolution of calcite during or after dolomitization. Dolomitization might increase, maintain or reduce reservoir porosity, depending on the environment, residence time and nature of the dolomitizing fluids. [ Conclusions and Prospects ] Replacement observations in experimental simulations imply that replacement is a micro-process of dissolution, migration and precipitation, accompanied by the formation of new pores that act as a channel for fluid and ion exchange to maintain the reaction. The molar volume of minerals does not influence the formation of pores. This is determined by relative solubility： that is, if the dissolution of reactants is greater than the precipitation of products, pore volume increases. The whole replacement process is accompanied by the formation of new pores, but they are mainly micropores, and their contribution to the effective porosity of a reservoir remains to be further evaluated.
Significance Dolomite is a common carbonate rock and servers as an important reservoir for oil and gas. In the Sichuan Basin marine carbonate successions, for example, approximately 96% of the proved gas reserves are in dolomite reservoirs. Thus, studies of the dolomitization process, and how pores originate in dolomite, have made significant progress. However, how dolomitization affects the development of pores is still argued. Methods By summarizing the main theoretical viewpoints of pore origin, combined with the results of various replacement experimental simulations, this study explains the formation and evolution of pores during dolomitization and suggests lines of future research on pore origin. [ Progress ] The pores in dolomite are either inherited from its precursor limestone, or they have resulted from the dissolution of calcite during or after dolomitization. Dolomitization might increase, maintain or reduce reservoir porosity, depending on the environment, residence time and nature of the dolomitizing fluids. [ Conclusions and Prospects ] Replacement observations in experimental simulations imply that replacement is a micro-process of dissolution, migration and precipitation, accompanied by the formation of new pores that act as a channel for fluid and ion exchange to maintain the reaction. The molar volume of minerals does not influence the formation of pores. This is determined by relative solubility： that is, if the dissolution of reactants is greater than the precipitation of products, pore volume increases. The whole replacement process is accompanied by the formation of new pores, but they are mainly micropores, and their contribution to the effective porosity of a reservoir remains to be further evaluated.
2024, 42(2): 643-660.
doi: 10.14027/j.issn.1000-0550.2022.057
Abstract:
Objective Fan deltas are widely developed in the margins and interior of sedimentary basins, with a large scale, diverse types, and idea source-reservoir-caprock association, and have huge exploration potential for oil and gas resources. However, its fluid type transition and controlling factors, sedimentary process and characteristics of the fan deltas are still unclear. Methods Taking the Xiligou fan delta in Qinghai Province as the research object, based on the fine description of outcrop and trench profiles, the characteristics of grain size, structural characteristics of sediments in the Xiligou fan delta during different periods were studied by comprehensive use of 14C dating and grain size analysis. [ Results and Conclusions ] Through the fine description of the field outcrop trough section, 9 types of lithofacies and 7 vertical combination sequences are divided, and 3 types of fluid types including debris flow, sheet flood and traction flow are identified. According to AMS14C dating, the Xiligou Lake delta is divided into three stages, SI, SII, and SIII by calibrating the characteristics of the sedimentary interface, sediment grain size, and structure. The SI stage is a coarse-grained sedimentary stage of clastic flow, and the SII stage is the deposition of sheet-flood reformation. Stage SIII is the period of traction flow reformation and deposition. Based on the analysis of sedimentary particle size-sensitive components, three lobe stages were identified in the delta plane of Xiligou Lake. A comprehensive analysis of the Xiligou delta slopes and the changes in the sporopollenin concentration in Qinghai Lake since 15 ka shows that the fuid type conversion of the Xiligou fan delta is controlled by the changes in the sedimentary slope and climate. The debris flow channel and sheet flooding sediments are the main architectural units. The lower plain of the fan delta was dominated by sheet flooding, and braided channels, braided bar, and flood-diffusion sediments were the main architectural units. The front of the fan delta was dominated by traction flow, and the main architectural units were subaqueous distributary channels, mouth bars, and sheet sands.
Objective Fan deltas are widely developed in the margins and interior of sedimentary basins, with a large scale, diverse types, and idea source-reservoir-caprock association, and have huge exploration potential for oil and gas resources. However, its fluid type transition and controlling factors, sedimentary process and characteristics of the fan deltas are still unclear. Methods Taking the Xiligou fan delta in Qinghai Province as the research object, based on the fine description of outcrop and trench profiles, the characteristics of grain size, structural characteristics of sediments in the Xiligou fan delta during different periods were studied by comprehensive use of 14C dating and grain size analysis. [ Results and Conclusions ] Through the fine description of the field outcrop trough section, 9 types of lithofacies and 7 vertical combination sequences are divided, and 3 types of fluid types including debris flow, sheet flood and traction flow are identified. According to AMS14C dating, the Xiligou Lake delta is divided into three stages, SI, SII, and SIII by calibrating the characteristics of the sedimentary interface, sediment grain size, and structure. The SI stage is a coarse-grained sedimentary stage of clastic flow, and the SII stage is the deposition of sheet-flood reformation. Stage SIII is the period of traction flow reformation and deposition. Based on the analysis of sedimentary particle size-sensitive components, three lobe stages were identified in the delta plane of Xiligou Lake. A comprehensive analysis of the Xiligou delta slopes and the changes in the sporopollenin concentration in Qinghai Lake since 15 ka shows that the fuid type conversion of the Xiligou fan delta is controlled by the changes in the sedimentary slope and climate. The debris flow channel and sheet flooding sediments are the main architectural units. The lower plain of the fan delta was dominated by sheet flooding, and braided channels, braided bar, and flood-diffusion sediments were the main architectural units. The front of the fan delta was dominated by traction flow, and the main architectural units were subaqueous distributary channels, mouth bars, and sheet sands.
2024, 42(2): 661-674.
doi: 10.14027/j.issn.1000-0550.2022.075
Abstract:
Objective As a typical structure of carbonate rocks, the formation of stylolites is of great significance for the enrichment of organic matter and fluid migration within the carbonate rock itself. However, there is currently no in-depth research on the source of organic matter and the mechanism of fluid transport in stylolites. Methods This study collected carbonate core samples containing stylolites from the Dachigan structural belt in the eastern Sichuan Basin. The stylolites and matrix were respectively subjected to organic carbon and pyrolysis （Rock-Eval）, common thin section observation, thin section observation under fluorescent light, whole rock X-ray diffraction, carbon and oxygen isotopes and scanning electron microscopy （SEM） and other testing items. [ Results and Conclusions ] The organic matter occurred in the stylolites is mainly in the form of three types including soluble organic matter, pyrobitumen, and bitumen, indicating that the organic matter in the stylolites is mainly the secondary migrated organic matter. Secondary dolomite and pyrite are well developed in the stylolites that the intercrystalline pore spaces and fractures are retained, thereby to provide good conditions for the accumulation and migration of oil and gas. The strong cementation on the contact between the stylolites and matrix and the formation of secondary minerals nearby may be the results of fractionation when the hydrocarbon fluid migrates from the matrix to stylolites. Through analysis, it is believed that the strong cementation at the contact interface between the suture and the matrix, as well as the formation of nearby secondary minerals, are the reasons for the fractionation effect of hydrocarbon fluids when migrating from the matrix to the suture. Through this study, it was found that during the development of sutures, the organic matter in the sutures will crack and form natural gas during the high evolution stage. When mixed with natural gas of other origins, it will affect the identification of natural gas sources； When the suture line serves as a migration channel for crude oil, the fractionation effect on crude oil can also cause certain difficulties in identifying the source of crude oil. Therefore, appropriate biomarker comparison parameters should be selected to analyze the oil source relationship, and the spatial variation pattern of crude oil properties should also be considered.
Objective As a typical structure of carbonate rocks, the formation of stylolites is of great significance for the enrichment of organic matter and fluid migration within the carbonate rock itself. However, there is currently no in-depth research on the source of organic matter and the mechanism of fluid transport in stylolites. Methods This study collected carbonate core samples containing stylolites from the Dachigan structural belt in the eastern Sichuan Basin. The stylolites and matrix were respectively subjected to organic carbon and pyrolysis （Rock-Eval）, common thin section observation, thin section observation under fluorescent light, whole rock X-ray diffraction, carbon and oxygen isotopes and scanning electron microscopy （SEM） and other testing items. [ Results and Conclusions ] The organic matter occurred in the stylolites is mainly in the form of three types including soluble organic matter, pyrobitumen, and bitumen, indicating that the organic matter in the stylolites is mainly the secondary migrated organic matter. Secondary dolomite and pyrite are well developed in the stylolites that the intercrystalline pore spaces and fractures are retained, thereby to provide good conditions for the accumulation and migration of oil and gas. The strong cementation on the contact between the stylolites and matrix and the formation of secondary minerals nearby may be the results of fractionation when the hydrocarbon fluid migrates from the matrix to stylolites. Through analysis, it is believed that the strong cementation at the contact interface between the suture and the matrix, as well as the formation of nearby secondary minerals, are the reasons for the fractionation effect of hydrocarbon fluids when migrating from the matrix to the suture. Through this study, it was found that during the development of sutures, the organic matter in the sutures will crack and form natural gas during the high evolution stage. When mixed with natural gas of other origins, it will affect the identification of natural gas sources； When the suture line serves as a migration channel for crude oil, the fractionation effect on crude oil can also cause certain difficulties in identifying the source of crude oil. Therefore, appropriate biomarker comparison parameters should be selected to analyze the oil source relationship, and the spatial variation pattern of crude oil properties should also be considered.
2024, 42(2): 675-687.
doi: 10.14027/j.issn.1000-0550.2022.067
Abstract:
Objective The physical properties and geochemical characteristics of oil found in the Northern slope, Bongor Basin, Chad, are clearly different. This paper aims to determine their differences and the influences on their properties and provide practical guidance for the assessment and prediction of oil quality in future oil exploration in this region. Methods The physical properties and molecular marker compounds of 41 crude oil samples from the northern slope belt were analyzed by oil family separation, gas chromatography, gas chromatography-mass spectrometry. [ Results and Conclusions ]It was found that the degree of biodegradation is responsible for the differences. Non-, slightly- or heavily degraded oils were classified according to the relative content and distribution pattern of normal alkanes and acyclic isoprenoids, as well as the baseline characteristics of gas chromatograms. Burial depth is evidently the most important factor constraining the degree of biodegradation. No biodegraded oil was found deeper than 1 300 m； the most heavily biodegraded oil occurs shallower than 800 m. In reservoirs between these depths, the degree of biodegradation is associated with distance from main faults, thickness of local caprock and type of trap.
Objective The physical properties and geochemical characteristics of oil found in the Northern slope, Bongor Basin, Chad, are clearly different. This paper aims to determine their differences and the influences on their properties and provide practical guidance for the assessment and prediction of oil quality in future oil exploration in this region. Methods The physical properties and molecular marker compounds of 41 crude oil samples from the northern slope belt were analyzed by oil family separation, gas chromatography, gas chromatography-mass spectrometry. [ Results and Conclusions ]It was found that the degree of biodegradation is responsible for the differences. Non-, slightly- or heavily degraded oils were classified according to the relative content and distribution pattern of normal alkanes and acyclic isoprenoids, as well as the baseline characteristics of gas chromatograms. Burial depth is evidently the most important factor constraining the degree of biodegradation. No biodegraded oil was found deeper than 1 300 m； the most heavily biodegraded oil occurs shallower than 800 m. In reservoirs between these depths, the degree of biodegradation is associated with distance from main faults, thickness of local caprock and type of trap.
2024, 42(2): 688-700.
doi: 10.14027/j.issn.1000-0550.2022.070
Abstract:
Objective This study aims to conduct an in-depth investigation into the large set of rhythmically-characteristic lacustrine fine-grained sedimentary rocks rich in carbonate materials and organic matter developed in the Dongying Sag area of the Bohai Bay Basin in eastern China, exploring their genetic mechanisms as well as characteristics of paleoclimate and paleoenvironment changes. Methods Based on core and thin-section observations, we classified the lithofacies according to sedimentary structural features and mineral component content. Considering the differences in sedimentary environment and deposition rate, the target interval was divided into four units. The Multi-Taper Method （MTM） was applied to perform segmented spectral analysis on the natural gamma （GR） logging curve from the upper Fourth member of the Shahejie Formation （Es4 u 3 l
Objective This study aims to conduct an in-depth investigation into the large set of rhythmically-characteristic lacustrine fine-grained sedimentary rocks rich in carbonate materials and organic matter developed in the Dongying Sag area of the Bohai Bay Basin in eastern China, exploring their genetic mechanisms as well as characteristics of paleoclimate and paleoenvironment changes. Methods Based on core and thin-section observations, we classified the lithofacies according to sedimentary structural features and mineral component content. Considering the differences in sedimentary environment and deposition rate, the target interval was divided into four units. The Multi-Taper Method （MTM） was applied to perform segmented spectral analysis on the natural gamma （GR） logging curve from the upper Fourth member of the Shahejie Formation （Es
2024, 42(2): 701-722.
doi: 10.14027/j.issn.1000-0550.2022.096
Abstract:
Objective The development of the “sweet spot” in lacustrine fine-grained sedimentary shale reservoirs is closely related to the enrichment of organic matter （OM）. However, few studies on the causes of OM enrichment and the establishment of enrichment models of shale in faulted saline lacustrine basins have been reported. A typical continental rifted saline lake basin was developed for the upper sub-segment of the upper 4th member of the Shahejie Formation （Es4scs） in the Dongying Sag. To further understand the OM enrichment in fine-grained sedimentary shale during this period, Methods This study used thin slice identification, scanning electron microscopy, X-ray diffraction, sporopollenin identification, biomarker chromatography-mass spectrometry and other techniques. The controlling factors of organic matter enrichment were explored in detail by using each index, and the organic matter enrichment model suitable for each stage of environmental evolution of the study area was established. Results （1） OM occurs at the microscopic level as strips, infills and dispersions. The OM source was mainly lower aquatic organisms such as algae, with evidence of small amounts of terrestrial higher plant material in the middle and upper strata. （2） The sedimentary environment evolved in three stages： A shallow-water semi-arid low-oxygen environment, a deepwater semi-humid reducing environment, and a sub-deepwater semi-arid low-oxygen environment. （3） OM enrichment was mainly influenced by climate, salinity, redox condition, productivity and deposition rate. Water depth, terrestrial input, volcanism and hydrothermal activity were secondary influences. Conclusions A model of OM enrichment in fine-grained sedimentary strata of the continental rifted saline lake basin was established, which may be highly significant for the exploration and development of continental shale oil and gas in China. To summarize： high OM enrichment in the studied interval was dominated by very productive conditions, a reducing environment for preservation, suitable salinity and suitable deposition rate.
Objective The development of the “sweet spot” in lacustrine fine-grained sedimentary shale reservoirs is closely related to the enrichment of organic matter （OM）. However, few studies on the causes of OM enrichment and the establishment of enrichment models of shale in faulted saline lacustrine basins have been reported. A typical continental rifted saline lake basin was developed for the upper sub-segment of the upper 4th member of the Shahejie Formation （Es4scs） in the Dongying Sag. To further understand the OM enrichment in fine-grained sedimentary shale during this period, Methods This study used thin slice identification, scanning electron microscopy, X-ray diffraction, sporopollenin identification, biomarker chromatography-mass spectrometry and other techniques. The controlling factors of organic matter enrichment were explored in detail by using each index, and the organic matter enrichment model suitable for each stage of environmental evolution of the study area was established. Results （1） OM occurs at the microscopic level as strips, infills and dispersions. The OM source was mainly lower aquatic organisms such as algae, with evidence of small amounts of terrestrial higher plant material in the middle and upper strata. （2） The sedimentary environment evolved in three stages： A shallow-water semi-arid low-oxygen environment, a deepwater semi-humid reducing environment, and a sub-deepwater semi-arid low-oxygen environment. （3） OM enrichment was mainly influenced by climate, salinity, redox condition, productivity and deposition rate. Water depth, terrestrial input, volcanism and hydrothermal activity were secondary influences. Conclusions A model of OM enrichment in fine-grained sedimentary strata of the continental rifted saline lake basin was established, which may be highly significant for the exploration and development of continental shale oil and gas in China. To summarize： high OM enrichment in the studied interval was dominated by very productive conditions, a reducing environment for preservation, suitable salinity and suitable deposition rate.
