2020 Vol. 38, No. 1
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2020, 38(1): 1-29.
doi: 10.14027/j.issn.1000-0550.2019.116
Abstract:
The theoretical framework of Unconventional Petroleum Geology has been gradually established along with the rapid progresses in exploration and development of unconventional petroleum resources. It is now imperative for innovative new insights into the unconventional petroleum sedimentology. Herein, the concept and scientific connotations of “Unconventional Petroleum Sedimentology” are proposed and briefly introduced. The research progresses are summarized for the sedimentology of typical unconventional petroleum resources in China, such as Wufeng-Longmaxi Shale gas in Sichuan Basin and tight oil and shale oil of Yanchang Formation in Ordos Basin. Further research themes and challenges of Unconventional Petroleum Sedimentology are discussed. Sedimentary enrichment of unconventional petroleum resources can be closely related with some critical environmental changes, which would be the result of coupling sedimentology from several geological events such as global or regional tectonic activities, sea (lake) level changes, volcanic eruptions, climate changes, anoxic bottom water, biotic mass extinctions and radiations, and gravity currents. For better understanding of Unconventional Petroleum Sedimentology in future, the Earth Systems Science view and “unconventional” insights should be applied to its research by analyzing geological events in details, which can play an important role in the discovery of new unconventional petroleum resources.
The theoretical framework of Unconventional Petroleum Geology has been gradually established along with the rapid progresses in exploration and development of unconventional petroleum resources. It is now imperative for innovative new insights into the unconventional petroleum sedimentology. Herein, the concept and scientific connotations of “Unconventional Petroleum Sedimentology” are proposed and briefly introduced. The research progresses are summarized for the sedimentology of typical unconventional petroleum resources in China, such as Wufeng-Longmaxi Shale gas in Sichuan Basin and tight oil and shale oil of Yanchang Formation in Ordos Basin. Further research themes and challenges of Unconventional Petroleum Sedimentology are discussed. Sedimentary enrichment of unconventional petroleum resources can be closely related with some critical environmental changes, which would be the result of coupling sedimentology from several geological events such as global or regional tectonic activities, sea (lake) level changes, volcanic eruptions, climate changes, anoxic bottom water, biotic mass extinctions and radiations, and gravity currents. For better understanding of Unconventional Petroleum Sedimentology in future, the Earth Systems Science view and “unconventional” insights should be applied to its research by analyzing geological events in details, which can play an important role in the discovery of new unconventional petroleum resources.
2020, 38(1): 32-45.
doi: 10.14027/j.issn.1000-0550.2019.022
Abstract:
Numerous crack structures are found in the intertidal facies sandstone of the Mesoproterozoic Changzhougou Formation in Jiashan, Xingcheng area, Liaoning Province. The structures comprise 0.8-3.0 cm spindle-shaped short cracks, and extended cracks over 7 cm long. They are mainly isolated, some partially connected. By comparing them with other similar surface crack structures and the desiccation cracks developed in modern microbial mats, they are explained as shrinkage cracks in microbial mats ('mat-cracks'). Unconnected spindle-shaped mat-cracks indicate incomplete mat shrinkage; mat-crack polygons form when shrinkage is complete. The presence of MISS indicates that the microbial ecosystem was already affecting the paleocoastal geological processes during the early transgression stage of the North China Craton in the Mesoproterozoic. This information is crucial for reconstructing the biosedimentary mechanisms and paleoenvironment of the aulacogenic Yanshan Basin during its early evolution.
Numerous crack structures are found in the intertidal facies sandstone of the Mesoproterozoic Changzhougou Formation in Jiashan, Xingcheng area, Liaoning Province. The structures comprise 0.8-3.0 cm spindle-shaped short cracks, and extended cracks over 7 cm long. They are mainly isolated, some partially connected. By comparing them with other similar surface crack structures and the desiccation cracks developed in modern microbial mats, they are explained as shrinkage cracks in microbial mats ('mat-cracks'). Unconnected spindle-shaped mat-cracks indicate incomplete mat shrinkage; mat-crack polygons form when shrinkage is complete. The presence of MISS indicates that the microbial ecosystem was already affecting the paleocoastal geological processes during the early transgression stage of the North China Craton in the Mesoproterozoic. This information is crucial for reconstructing the biosedimentary mechanisms and paleoenvironment of the aulacogenic Yanshan Basin during its early evolution.
2020, 38(1): 46-54.
doi: 10.14027/j.issn.1000-0550.2019.072
Abstract:
The Precambrian biosphere was dominated by microorganisms. The study of biosedimentary structures caused by such microorganisms has always been paid great attention by geologists; however, previous studies have tended to be mainly focused on carbonate strata, and in recent years showed a particular interest in Microbially Induced Sedimentary Structures (MISS), which are mainly developed in clastic strata. The Yunmengshan Formation in the Lushan area consists mainly of a set of Mesoproterozoic clastic strata. At the base of the Ruyang Group, the formation lies at the southern margin of the North China Platform. MISS are especially abundant in the bedding surface, and display a large range of morphologies. Various types of MISS distribution indicate the paleoenvironment in detail. The extensive development of MISS in the Yunmengshan Formation reveals a flourishing microbial community at that time, which provides strong evidence for reconstructing the ancient environment.
The Precambrian biosphere was dominated by microorganisms. The study of biosedimentary structures caused by such microorganisms has always been paid great attention by geologists; however, previous studies have tended to be mainly focused on carbonate strata, and in recent years showed a particular interest in Microbially Induced Sedimentary Structures (MISS), which are mainly developed in clastic strata. The Yunmengshan Formation in the Lushan area consists mainly of a set of Mesoproterozoic clastic strata. At the base of the Ruyang Group, the formation lies at the southern margin of the North China Platform. MISS are especially abundant in the bedding surface, and display a large range of morphologies. Various types of MISS distribution indicate the paleoenvironment in detail. The extensive development of MISS in the Yunmengshan Formation reveals a flourishing microbial community at that time, which provides strong evidence for reconstructing the ancient environment.
2020, 38(1): 55-63.
doi: 10.14027/j.issn.1000-0550.2019.024
Abstract:
Except for a few dozen meters of ribbon mudstone in the lower part, the Xiaoerbulake section of the Tarim Basin, NW China, most of the Lower Xiaoerbulake Formation deposits contain microbial carbonates; however, few researchers have paid attention on this. Detailed observations of field outcrops and thin sections in this study has identified and classified these microbial carbonates, and restored the local depositional paleoenvironment. Six lithofacies are found in the Lower Xiaoerbulake Formation deposit, including coarse-laminated and fenestral thrombolites in the intertidal zone, and thick-bedded and massive thrombolites restricted to the intermediate-to-shallow subtidal region. Various types of microbial carbonates are present in different locations, depending on ancient sea-level fluctuations, topography and microbial communities.
Except for a few dozen meters of ribbon mudstone in the lower part, the Xiaoerbulake section of the Tarim Basin, NW China, most of the Lower Xiaoerbulake Formation deposits contain microbial carbonates; however, few researchers have paid attention on this. Detailed observations of field outcrops and thin sections in this study has identified and classified these microbial carbonates, and restored the local depositional paleoenvironment. Six lithofacies are found in the Lower Xiaoerbulake Formation deposit, including coarse-laminated and fenestral thrombolites in the intertidal zone, and thick-bedded and massive thrombolites restricted to the intermediate-to-shallow subtidal region. Various types of microbial carbonates are present in different locations, depending on ancient sea-level fluctuations, topography and microbial communities.
2020, 38(1): 64-75.
doi: 10.14027/j.issn.1000-0550.2019.071
Abstract:
The Dengjiazhuang section at Feicheng city, Shandong province, is characterized by a continuous Cambrian stratigraphic succession comprising seven third-order sequences. Microbial carbonates in the Dengjiazhuang section were developed in the HST and FRST, represented by thrombolite in the Xuzhuang Formation, oncolite in the Zhangxia Formation, dendrite in the Gushan Formation, stromatolite in the Changshan Formation, and leiolite in the Fengshan Formation, which is a typical example for studying a resuscitate period of microbial carbonates in the Early Paleozoic. Besides, a large number of well-preserved calcified microorganism fossils such as Epiphyton, Girvanella and Renalcis spp. are found in the microbial carbonates, which indicates that there are complex microbial metabolic activities led by cyanobacteria in the microbial mat during the formation of carbonates. Furthermore, the sedimentary characteristics of the microbial carbonates reflect the diversity of sedimentary environments. Leiolite and thrombolite grew respectively in the lower and upper part of the subtidal zone; columnar stromatolites were developed generally in the high-energy intertidal zone; oncolites indicate oolitic beach facies with shallow water and high energy. Therefore, the diversity of microbial carbonates at the Dengjiazhuang section in the Cambrian provides an important rock record for the study of earth microorganisms, paleoenvironment and paleoclimate in the Cambrian.
The Dengjiazhuang section at Feicheng city, Shandong province, is characterized by a continuous Cambrian stratigraphic succession comprising seven third-order sequences. Microbial carbonates in the Dengjiazhuang section were developed in the HST and FRST, represented by thrombolite in the Xuzhuang Formation, oncolite in the Zhangxia Formation, dendrite in the Gushan Formation, stromatolite in the Changshan Formation, and leiolite in the Fengshan Formation, which is a typical example for studying a resuscitate period of microbial carbonates in the Early Paleozoic. Besides, a large number of well-preserved calcified microorganism fossils such as Epiphyton, Girvanella and Renalcis spp. are found in the microbial carbonates, which indicates that there are complex microbial metabolic activities led by cyanobacteria in the microbial mat during the formation of carbonates. Furthermore, the sedimentary characteristics of the microbial carbonates reflect the diversity of sedimentary environments. Leiolite and thrombolite grew respectively in the lower and upper part of the subtidal zone; columnar stromatolites were developed generally in the high-energy intertidal zone; oncolites indicate oolitic beach facies with shallow water and high energy. Therefore, the diversity of microbial carbonates at the Dengjiazhuang section in the Cambrian provides an important rock record for the study of earth microorganisms, paleoenvironment and paleoclimate in the Cambrian.
2020, 38(1): 76-90.
doi: 10.14027/j.issn.1000-0550.2019.025
Abstract:
Leiolite is characterized by relatively structureless, aphanitic, mesostructure, lacking lamination or clots, and is classified as typical microbial rocks together with stromatolites, thrombolites and dendrites. However, because of the lack of leiolite research reports in either modern or old records since it was named in 1995, the report of a leiolite example has great research value. The Cambrian Changshan Formation at the Qijiayu section constitutes the lower part of the Furongian series. The formation is characterized by a generally shallowing-upward succession of sedimentary facies ranging from mudstone of shelf marls to micrite of shallow ramp facies, and therefore comprises a third-order carbonate depositional sequence of the drowning-unconformity type in the north of the Taihang Mountain in Laiyuan city, Heibei province. A single bed of the micrite of shallow ramp facies in the upper part of the Changshan Formation contains many dome-shaped carbonate structures distributed similar to a string of beads. These structures are described as a leiolitic bioherm, and is evidence of a forced regression. Various kinds of calcified microorganisms such as Epiphyton, Girvanella and Renalcis developed within the leiolitic bioherm. These are evidence of the first episode of a cyanobacteria calcification event in the Phanerozoic, and describe a leiolitic bioherm formed from microbial mats dominated by cyanobacteria. Furthermore, the partial concentrations of benthic ooids in the bioherm show complex microbial sedimentation during its formation. Although the bioherm basic components are micrite and microspar, the presence of calcified microorganisms and benthic ooids offers a reference for studying the complex microbial activity signals in the microbial mats, which are dominated by calcified microorganisms. It can also be regarded as an instance for further research on the mechanism of bioherm formation and the first episode of cyanobacteria calcification event during the Phanerozoic.
Leiolite is characterized by relatively structureless, aphanitic, mesostructure, lacking lamination or clots, and is classified as typical microbial rocks together with stromatolites, thrombolites and dendrites. However, because of the lack of leiolite research reports in either modern or old records since it was named in 1995, the report of a leiolite example has great research value. The Cambrian Changshan Formation at the Qijiayu section constitutes the lower part of the Furongian series. The formation is characterized by a generally shallowing-upward succession of sedimentary facies ranging from mudstone of shelf marls to micrite of shallow ramp facies, and therefore comprises a third-order carbonate depositional sequence of the drowning-unconformity type in the north of the Taihang Mountain in Laiyuan city, Heibei province. A single bed of the micrite of shallow ramp facies in the upper part of the Changshan Formation contains many dome-shaped carbonate structures distributed similar to a string of beads. These structures are described as a leiolitic bioherm, and is evidence of a forced regression. Various kinds of calcified microorganisms such as Epiphyton, Girvanella and Renalcis developed within the leiolitic bioherm. These are evidence of the first episode of a cyanobacteria calcification event in the Phanerozoic, and describe a leiolitic bioherm formed from microbial mats dominated by cyanobacteria. Furthermore, the partial concentrations of benthic ooids in the bioherm show complex microbial sedimentation during its formation. Although the bioherm basic components are micrite and microspar, the presence of calcified microorganisms and benthic ooids offers a reference for studying the complex microbial activity signals in the microbial mats, which are dominated by calcified microorganisms. It can also be regarded as an instance for further research on the mechanism of bioherm formation and the first episode of cyanobacteria calcification event during the Phanerozoic.
2020, 38(1): 91-103.
doi: 10.14027/j.issn.1000-0550.2019.042
Abstract:
A large number of bioturbated dolomites/bioturbated limestones were developed in the Cambrian and Ordovician in North China. Two representative stratal sections, Dengfeng (Cambrian) and Xingxian (Ordovician), were selected for study. Rocks are the cause of bioturbation. The trace fossils in the Cambrian series2 Zhushadong Formation in the Dengfeng Henan are relatively simple, mainly Thalassinoides along with a small amount of Planolites. Abundant fossil traces are present in submember 55 of the Middle Ordovician Majiagou Formation in the Xingxian section. Seven trace fossil ichnogenera were identified: Arenicolites, Cylindricum, Helminthopsis, Lorenzinia, Planolites, Teichichnus and Thalassinoides, of which Thalassinoides is dominant. The Lower Cambrian Zhushadong Formation from the Dengfeng section mainly occurs in the carbonate tidal flat and in the 55 submember of the Middle Ordovician Majiagou Formation from the Xingxian deposit in the carbonate platform. Field geological investigation and laboratory analysis by polarizing microscope and scanning electron microscope established the macro-and microstructure of the bioturbation in the surrounding rocks. Dolomite was predominant in the burrows, and calcite was the main material in the matrix. The burrows are seen in the outcrops as patches of different colors. The abundant three-dimensional burrowing system of Thalassinoides in both sections benefited the transport of dolomitization fluids and created favorable conditions for reflux osmosis, thus dolomitizing the burrow infill and forming a large number of bioturbated dolomite plaques.
A large number of bioturbated dolomites/bioturbated limestones were developed in the Cambrian and Ordovician in North China. Two representative stratal sections, Dengfeng (Cambrian) and Xingxian (Ordovician), were selected for study. Rocks are the cause of bioturbation. The trace fossils in the Cambrian series2 Zhushadong Formation in the Dengfeng Henan are relatively simple, mainly Thalassinoides along with a small amount of Planolites. Abundant fossil traces are present in submember 55 of the Middle Ordovician Majiagou Formation in the Xingxian section. Seven trace fossil ichnogenera were identified: Arenicolites, Cylindricum, Helminthopsis, Lorenzinia, Planolites, Teichichnus and Thalassinoides, of which Thalassinoides is dominant. The Lower Cambrian Zhushadong Formation from the Dengfeng section mainly occurs in the carbonate tidal flat and in the 55 submember of the Middle Ordovician Majiagou Formation from the Xingxian deposit in the carbonate platform. Field geological investigation and laboratory analysis by polarizing microscope and scanning electron microscope established the macro-and microstructure of the bioturbation in the surrounding rocks. Dolomite was predominant in the burrows, and calcite was the main material in the matrix. The burrows are seen in the outcrops as patches of different colors. The abundant three-dimensional burrowing system of Thalassinoides in both sections benefited the transport of dolomitization fluids and created favorable conditions for reflux osmosis, thus dolomitizing the burrow infill and forming a large number of bioturbated dolomite plaques.
2020, 38(1): 104-112.
doi: 10.14027/j.issn.1000-0550.2019.020
Abstract:
Following the mass extinction at the end of the Permian, a set of calcareous microbialites was widely developed in South China. These mainly consist of clotted limestone, which may represent bio-sedimentary formation in a specific environment after the mass extinction. The microstructures of the clotted limestone were studied in detail by microscopic observation, cathodoluminescence photography, backscattered imaging and probe element analysis. The distribution of fossil elements and mineral composition in the clotted limestone were observed and the findings compared with the diagenesis process of modern clotted limestone. The sedimentary and diagenetic process of the end-Permian clotted limestone are summarized, including early growth stage, sedimentary stage, early diagenetic stage and late diagenetic stage. The clear sedimentary and diagenetic processes of clotted limestone lays a foundation for the analysis of microbial sedimentary formation during this critical period.
Following the mass extinction at the end of the Permian, a set of calcareous microbialites was widely developed in South China. These mainly consist of clotted limestone, which may represent bio-sedimentary formation in a specific environment after the mass extinction. The microstructures of the clotted limestone were studied in detail by microscopic observation, cathodoluminescence photography, backscattered imaging and probe element analysis. The distribution of fossil elements and mineral composition in the clotted limestone were observed and the findings compared with the diagenesis process of modern clotted limestone. The sedimentary and diagenetic process of the end-Permian clotted limestone are summarized, including early growth stage, sedimentary stage, early diagenetic stage and late diagenetic stage. The clear sedimentary and diagenetic processes of clotted limestone lays a foundation for the analysis of microbial sedimentary formation during this critical period.
Structures of Siliceous Hot Spring Deposits and Mechanism of Microbial Mineralization and Diagenesis
2020, 38(1): 113-123.
doi: 10.14027/j.issn.1000-0550.2019.023
Abstract:
Hot springs are examples of continuous deposition in extreme environments, thus exploring the mineralization of hot spring microbes helps in understanding the Earth's ancient environment. Modern siliceous sinter is formed by both biological and non-biological interaction: the main mineral is Opal-A, with spicule, column and cluster etc. sedimentary structures. Petrographic studies of modern siliceous sinter are a good foundation for the study of the microbial mineralization process. In addition, it is significant for such studies to know more about the microbial mineralization process and the dissolution and re-deposition processes of opal diagenesis. At present, the study of microbial mineralization processes is flourishing. Specifically, microbially induced mineralization and microbially controlled mineralization are the two major mineralization processes, but there are many microbial mineralization mechanisms yet to be discovered. Therefore, examining mineralization by hot spring microbes is highly significant for strengthening our understanding of the formation and transformation of hot spring deposits.
Hot springs are examples of continuous deposition in extreme environments, thus exploring the mineralization of hot spring microbes helps in understanding the Earth's ancient environment. Modern siliceous sinter is formed by both biological and non-biological interaction: the main mineral is Opal-A, with spicule, column and cluster etc. sedimentary structures. Petrographic studies of modern siliceous sinter are a good foundation for the study of the microbial mineralization process. In addition, it is significant for such studies to know more about the microbial mineralization process and the dissolution and re-deposition processes of opal diagenesis. At present, the study of microbial mineralization processes is flourishing. Specifically, microbially induced mineralization and microbially controlled mineralization are the two major mineralization processes, but there are many microbial mineralization mechanisms yet to be discovered. Therefore, examining mineralization by hot spring microbes is highly significant for strengthening our understanding of the formation and transformation of hot spring deposits.
2020, 38(1): 124-137.
doi: 10.14027/j.issn.1000-0550.2019.073
Abstract:
Authigenic pyrite is the main mineral specie of reduced sulfur in marine sediments. Its formation process is related to organic mineralization and affects the global C-S-Fe biogeochemical cycle. Sulfur isotope fractionation of authigenic pyrite is mainly controlled by microbial sulfate reduction, but recent studies have indicated that the local depositional environment also affects the composition of pyrite sulfur isotopes, especially in shallow depositional environments. In an unsteady shallow environment, physical reworking and bioturbation lead to reoxidation of sulfides formed in the sulfate reduction zone, which in turn affects the sulfur isotopes of pyrite. The sedimentation process in a shallow depositional environment is readily affected by paleoclimate and sea-level changes, which cause drastic fluctuations in sedimentation rate as well as instable input of, for instance, organic matter and active iron. This in turn affects the openness of the diagenetic system and ultimately affects the isotopic value of pyritic sulfur. In addition, any change in sedimentation rate also affects the movement of the sulfate-methane transition zone, resulting in the conversion of organic matter and anaerobic oxidation methane sulfate reduction, producing different sulfur isotope signals. The study of the sulfur isotopes of authigenic pyrite in the mud area of the inner shelf of the East China Sea provides a good example for depositional control on the formation of authigenic pyrite and its sulfur isotope composition. The sedimentary process of this area has been well studied, and its sediments have been shown to be enriched in authigenic pyrite and biogas (CH4). Therefore, it is an ideal site for studying the sulfur cycle in a marginal sea, and is expected to provide a new perspective on the global C-S-Fe biogeochemical cycle.
Authigenic pyrite is the main mineral specie of reduced sulfur in marine sediments. Its formation process is related to organic mineralization and affects the global C-S-Fe biogeochemical cycle. Sulfur isotope fractionation of authigenic pyrite is mainly controlled by microbial sulfate reduction, but recent studies have indicated that the local depositional environment also affects the composition of pyrite sulfur isotopes, especially in shallow depositional environments. In an unsteady shallow environment, physical reworking and bioturbation lead to reoxidation of sulfides formed in the sulfate reduction zone, which in turn affects the sulfur isotopes of pyrite. The sedimentation process in a shallow depositional environment is readily affected by paleoclimate and sea-level changes, which cause drastic fluctuations in sedimentation rate as well as instable input of, for instance, organic matter and active iron. This in turn affects the openness of the diagenetic system and ultimately affects the isotopic value of pyritic sulfur. In addition, any change in sedimentation rate also affects the movement of the sulfate-methane transition zone, resulting in the conversion of organic matter and anaerobic oxidation methane sulfate reduction, producing different sulfur isotope signals. The study of the sulfur isotopes of authigenic pyrite in the mud area of the inner shelf of the East China Sea provides a good example for depositional control on the formation of authigenic pyrite and its sulfur isotope composition. The sedimentary process of this area has been well studied, and its sediments have been shown to be enriched in authigenic pyrite and biogas (CH4). Therefore, it is an ideal site for studying the sulfur cycle in a marginal sea, and is expected to provide a new perspective on the global C-S-Fe biogeochemical cycle.
2020, 38(1): 138-149.
doi: 10.14027/j.issn.1000-0550.2019.021
Abstract:
In the low-oxygen atmospheric conditions during the Neoproterozoic, continental sulfate was a major oxidant in anoxic deep ocean water. Bacterial sulfate reduction (BSR) caused significant isotopic fractionation between seawater sulfates and sedimentary pyrite, which reveals the paleo-ocean redox conditions. Sedimentary pyrite occurs in a variety of forms in sedimentary rocks, each indicating its depositional process and microenvironment. For example, most framboidal pyrite was deposited from the euxinic bottom water or in surface sediments overlain by oxic or suboxic bottom water. Large-grained single-crystal pyrite mainly formed in pore-water during early diagenesis, and pyrite veins were formed by late hydrothermal activity; neither of these indicate paleo-ocean redox conditions. Identifying sedimentary pyrite morphologies and determining their formation processes are fundamental for determining the sulfur isotopes of the ancient seawater. In this study, sedimentary pyrite morphological types and formation processes are briefly summarized, along with morphological observations and sulfur isotope analyses for pyrite in fresh drill-core samples of Ediacaran Lantian Formation (deep-water facies) in the southern Anhui Province, South China. The morphologies and contents of the pyrite were found to vary in different lithological samples. Euhedral crystal and framboidal pyrites preserved in Lantian Formation black shales have better morphology than in carbonates, indicating that less crystal alteration took place during late diagenesis in the black shales. The sulfur isotope content in large-grained pyrite (δ34SL-pyr) is generally greater than in bulk rock samples (δ34ST-pyr), with differences up to 48.16‰. The study suggests that analysis of sedimentary pyrite with different morphologies is needed to obtain reliable sulfur isotope values indicative of seawater redox conditions. High-resolution analysis requires secondary-ion mass spectrometry (SIMS) measurements on individual pyrite crystals and framboids.
In the low-oxygen atmospheric conditions during the Neoproterozoic, continental sulfate was a major oxidant in anoxic deep ocean water. Bacterial sulfate reduction (BSR) caused significant isotopic fractionation between seawater sulfates and sedimentary pyrite, which reveals the paleo-ocean redox conditions. Sedimentary pyrite occurs in a variety of forms in sedimentary rocks, each indicating its depositional process and microenvironment. For example, most framboidal pyrite was deposited from the euxinic bottom water or in surface sediments overlain by oxic or suboxic bottom water. Large-grained single-crystal pyrite mainly formed in pore-water during early diagenesis, and pyrite veins were formed by late hydrothermal activity; neither of these indicate paleo-ocean redox conditions. Identifying sedimentary pyrite morphologies and determining their formation processes are fundamental for determining the sulfur isotopes of the ancient seawater. In this study, sedimentary pyrite morphological types and formation processes are briefly summarized, along with morphological observations and sulfur isotope analyses for pyrite in fresh drill-core samples of Ediacaran Lantian Formation (deep-water facies) in the southern Anhui Province, South China. The morphologies and contents of the pyrite were found to vary in different lithological samples. Euhedral crystal and framboidal pyrites preserved in Lantian Formation black shales have better morphology than in carbonates, indicating that less crystal alteration took place during late diagenesis in the black shales. The sulfur isotope content in large-grained pyrite (δ34SL-pyr) is generally greater than in bulk rock samples (δ34ST-pyr), with differences up to 48.16‰. The study suggests that analysis of sedimentary pyrite with different morphologies is needed to obtain reliable sulfur isotope values indicative of seawater redox conditions. High-resolution analysis requires secondary-ion mass spectrometry (SIMS) measurements on individual pyrite crystals and framboids.
2020, 38(1): 150-165.
doi: 10.14027/j.issn.1000-0550.2019.099
Abstract:
Pyrite framboids are defined as microscopic spheroidal to sub-spheroidal clusters of equidimensional and equimorphic pyrite microcrystals. The size distribution of pyrite framboids has been widely used as a promising tool for reconstructing the redox conditions of geological periods. Summarizing the formation mechanism, paleo-oceanographic implications, discrimination standard, and experimental methods using pyrite framboids might help to understand why they play such an important role. The formation mechanism of pyrite framboids differs in oxic and euxinic conditions. Their size, abundance and distribution range are all important proxies in inferring the oxygen levels of the water column. The microcrystal geometries and morphological characteristics are closely related to the redox condition of the paleo-ocean in the deposition process. Furthermore, although one or two unknown problems may yet be present in the method, their potential effect may be offset by using additional effective methods (e.g., sedimentary microfacies analysis, paleocommunity reconstruction, and specific geochemical indices) to analyze the redox condition and to recognize unusual settings. The validity of the oxidized pyrite framboid in reconstructing the redox conditions of paleo-ocean has been evaluated. The results show that the oxidation of pyrite has little effect on redox interpretations based on pyrite framboid distributions.
Pyrite framboids are defined as microscopic spheroidal to sub-spheroidal clusters of equidimensional and equimorphic pyrite microcrystals. The size distribution of pyrite framboids has been widely used as a promising tool for reconstructing the redox conditions of geological periods. Summarizing the formation mechanism, paleo-oceanographic implications, discrimination standard, and experimental methods using pyrite framboids might help to understand why they play such an important role. The formation mechanism of pyrite framboids differs in oxic and euxinic conditions. Their size, abundance and distribution range are all important proxies in inferring the oxygen levels of the water column. The microcrystal geometries and morphological characteristics are closely related to the redox condition of the paleo-ocean in the deposition process. Furthermore, although one or two unknown problems may yet be present in the method, their potential effect may be offset by using additional effective methods (e.g., sedimentary microfacies analysis, paleocommunity reconstruction, and specific geochemical indices) to analyze the redox condition and to recognize unusual settings. The validity of the oxidized pyrite framboid in reconstructing the redox conditions of paleo-ocean has been evaluated. The results show that the oxidation of pyrite has little effect on redox interpretations based on pyrite framboid distributions.
2020, 38(1): 166-181.
doi: 10.14027/j.issn.1000-0550.2019.015
Abstract:
Mixed siliciclastic-carbonate sediments (MSS) have been studied in recent years with regard to the reconstruction of palaeoclimate and paleoenvironment. Here, we provide an example of MSS that was widely developed in shallow-water settings of the Upper Yangtze region during the Canglangpuian Stage of the Early Cambrian. It is characterized by the concurrent development of siliciclastics (predominantly quartz grains and lithic fragments) and carbonates (mainly ooids and cements) from platform interior to shelf settings, all with different compositions. The siliciclastic-dominant MSS in the uppermost Guojiaba and basal Yanwangbian formations exhibit thin mudstone layers interbedded with calcareous siltstone, and alternating calcareous mudstone and calcareous siltstone beds, implying a relatively low-energy sedimentation environment. By contrast, the carbonate-dominated MSS developed in high-energy conditions early in the Canglangpu stage (corresponding to Xiannüdong Formation strata). It exhibits features typical of (1) alternating limestone and calcareous mudstone, (2) successions of calcareous lithic sandstone and sandy oolite, and (3) shallowing-upward sequences composed of calcareous fine sandstone (lower), sandy oolite (middle) and pure oolite (upper), suggesting a mixed siliciclastic-carbonate system ranging from upper slope to platform margin. With the regression of the Canglangpuian Stage, the mixed siliciclastic-carbonate system of the Xiannüdong Formation in the northern Sichuan Basin was close to the provenance of siliciclastic sediments; it is probable that the source was deltaic and riverine inputs from the neighboring Hannan Massif, transported to the platform interior and margin by longshore currents and waves. The distributions of siliciclastic components are likely to have been constrained by conditions of increased water energy and biological processes on platform rims where carbonate factories developed, featuring ooid shoals, microbial mounds and archaeocyatha mounds. In addition, a lower supply of terrigenous material and a warm-to-arid climate may also have contributed to the construction of carbonate-dominant MSS systems in shallow-water settings, creating conditions favorable to facies mixing and in-situ mixing. In addition, events such as storms and turbid currents may have played essential roles in the formation of MSS in slope-to-shelf facies, where shallow-water sediments were episodically transported into siliciclastic-dominant depositional environments to form "punctuated mixing". A case study of the characteristics and spatial distributions of MSS in the Early Cambrian would be of benefit to the understanding of temporal paleogeography, paleoenvironmental evolution, and reservoir implications in the Upper Yangtze region.
Mixed siliciclastic-carbonate sediments (MSS) have been studied in recent years with regard to the reconstruction of palaeoclimate and paleoenvironment. Here, we provide an example of MSS that was widely developed in shallow-water settings of the Upper Yangtze region during the Canglangpuian Stage of the Early Cambrian. It is characterized by the concurrent development of siliciclastics (predominantly quartz grains and lithic fragments) and carbonates (mainly ooids and cements) from platform interior to shelf settings, all with different compositions. The siliciclastic-dominant MSS in the uppermost Guojiaba and basal Yanwangbian formations exhibit thin mudstone layers interbedded with calcareous siltstone, and alternating calcareous mudstone and calcareous siltstone beds, implying a relatively low-energy sedimentation environment. By contrast, the carbonate-dominated MSS developed in high-energy conditions early in the Canglangpu stage (corresponding to Xiannüdong Formation strata). It exhibits features typical of (1) alternating limestone and calcareous mudstone, (2) successions of calcareous lithic sandstone and sandy oolite, and (3) shallowing-upward sequences composed of calcareous fine sandstone (lower), sandy oolite (middle) and pure oolite (upper), suggesting a mixed siliciclastic-carbonate system ranging from upper slope to platform margin. With the regression of the Canglangpuian Stage, the mixed siliciclastic-carbonate system of the Xiannüdong Formation in the northern Sichuan Basin was close to the provenance of siliciclastic sediments; it is probable that the source was deltaic and riverine inputs from the neighboring Hannan Massif, transported to the platform interior and margin by longshore currents and waves. The distributions of siliciclastic components are likely to have been constrained by conditions of increased water energy and biological processes on platform rims where carbonate factories developed, featuring ooid shoals, microbial mounds and archaeocyatha mounds. In addition, a lower supply of terrigenous material and a warm-to-arid climate may also have contributed to the construction of carbonate-dominant MSS systems in shallow-water settings, creating conditions favorable to facies mixing and in-situ mixing. In addition, events such as storms and turbid currents may have played essential roles in the formation of MSS in slope-to-shelf facies, where shallow-water sediments were episodically transported into siliciclastic-dominant depositional environments to form "punctuated mixing". A case study of the characteristics and spatial distributions of MSS in the Early Cambrian would be of benefit to the understanding of temporal paleogeography, paleoenvironmental evolution, and reservoir implications in the Upper Yangtze region.
2020, 38(1): 182-195.
doi: 10.14027/j.issn.1000-0550.2019.003
Abstract:
As an important transition period of the global geological environment, geological responses during the Early Sinian to climate catastrophes and significant sea-level rise are found in the geological record all over the world. This paper is the first report of the effect of storm deposits in the lower part of the Sinian Doushantuo Formation in South China. Detailed observation and measurement of drill cores from the eastern Yangtze Gorges area show that storm deposits mainly developed in the first member and the lower part of the second member of the Sinian Doushantuo Formation (mainly dolostone and black mudstone, respectively). Sedimentary structures recognized include erosion surfaces, stirred layers, lag deposits, graded bedding, sunken cross-bedding, wave bedding, ripple marks, and horizontal bedding. Observed combinations of vertical features and sedimentary structural differences indicate five storm sequences at times of different sedimentary background (shallow-water environment in tidal flats, or deep-water environment in the continental shelf). The deposits from storms during the first member of the Doushantuo Formation indicate a mainly shallow-water environment; in the second member, storm deposits formed mainly in deep water, with the storm action strengthening upward. Moreover, black pebbly mudstone with graded bedding was recognized under or near the storm wave base. These observations indicate that the storm deposits in the lower part of the Doushantuo Formation contain a considerable amount of information (paleogeographical, paleoclimatic, paleogeomorphological and stratigraphical), and is of great significance for recovering Early Sinian geological environmental changes in South China.
As an important transition period of the global geological environment, geological responses during the Early Sinian to climate catastrophes and significant sea-level rise are found in the geological record all over the world. This paper is the first report of the effect of storm deposits in the lower part of the Sinian Doushantuo Formation in South China. Detailed observation and measurement of drill cores from the eastern Yangtze Gorges area show that storm deposits mainly developed in the first member and the lower part of the second member of the Sinian Doushantuo Formation (mainly dolostone and black mudstone, respectively). Sedimentary structures recognized include erosion surfaces, stirred layers, lag deposits, graded bedding, sunken cross-bedding, wave bedding, ripple marks, and horizontal bedding. Observed combinations of vertical features and sedimentary structural differences indicate five storm sequences at times of different sedimentary background (shallow-water environment in tidal flats, or deep-water environment in the continental shelf). The deposits from storms during the first member of the Doushantuo Formation indicate a mainly shallow-water environment; in the second member, storm deposits formed mainly in deep water, with the storm action strengthening upward. Moreover, black pebbly mudstone with graded bedding was recognized under or near the storm wave base. These observations indicate that the storm deposits in the lower part of the Doushantuo Formation contain a considerable amount of information (paleogeographical, paleoclimatic, paleogeomorphological and stratigraphical), and is of great significance for recovering Early Sinian geological environmental changes in South China.
2020, 38(1): 196-204.
doi: 10.14027/j.issn.1000-0550.2020.2019.001
Abstract:
The ophiolite in Malipo, which is located at the SW margin of the Youjiang Basin, is considered to be a remnant paleo-Tethys Ocean. The study of the Babu Ocean is an important part of a comprehensive understanding of the tectonic evolution of the Youjiang Basin and the southwestern margin of south China. The Babu Ocean consisted primarily of deepwater sediments, including cherts, in that area. Major and rare earth elements in the cherts from the Middle-Lower Permian Tapi Formation in Babu and Xiniubao were analyzed in this study. The values of Al/(Al+Fe+Mn) (0.43-0.70), Al2O3/TiO2 (25.5-44.8) and Eu/Eu* (0.81-1.02) indicate that the cherts are of non-hydrothermal origin, within acid pyroclastics, and show low Y/Ho ratios (28.84-31.23 and 26.41-28.91, respectively) and high Ce/Ce* values (0.67-1.29 and 0.77-1.18, respectively), indicating deposition at the continental margin. From comparisons with the geochemistry of cherts in other areas, we conclude that an active continental margin may have developed in the SW Babu Ocean as the ocean crust subducted towards the southwest.
The ophiolite in Malipo, which is located at the SW margin of the Youjiang Basin, is considered to be a remnant paleo-Tethys Ocean. The study of the Babu Ocean is an important part of a comprehensive understanding of the tectonic evolution of the Youjiang Basin and the southwestern margin of south China. The Babu Ocean consisted primarily of deepwater sediments, including cherts, in that area. Major and rare earth elements in the cherts from the Middle-Lower Permian Tapi Formation in Babu and Xiniubao were analyzed in this study. The values of Al/(Al+Fe+Mn) (0.43-0.70), Al2O3/TiO2 (25.5-44.8) and Eu/Eu* (0.81-1.02) indicate that the cherts are of non-hydrothermal origin, within acid pyroclastics, and show low Y/Ho ratios (28.84-31.23 and 26.41-28.91, respectively) and high Ce/Ce* values (0.67-1.29 and 0.77-1.18, respectively), indicating deposition at the continental margin. From comparisons with the geochemistry of cherts in other areas, we conclude that an active continental margin may have developed in the SW Babu Ocean as the ocean crust subducted towards the southwest.
2020, 38(1): 205-217.
doi: 10.14027/j.issn.1000-0550.2019.011
Abstract:
The identification of single sand bodies in meandering river deposits is significant and challenging. For decades, Miall's theory of architecture elements has greatly enhanced the description of single sand bodies, but the variety of facies types and the characteristics of facies combinations in the main architecture elements interfere with reservoir description and sand thickness when using well logs. Based on previous studies, this paper simplifies Bridge's sedimentation theory for describing reservoirs, and briefly examines the fluid flow properties and sedimentary sequences at different locations in a selected point bar and its associated river channel. It outlines how to use well-log combinations to identify six types of sand body superposition and apply them in oilfield practice. The study found that: (1) Different well logs in plan view of a single layer reflect the complex hydrodynamics of the sedimentation process. Bridge's theory states that the maximum flow velocity occurs near the bar head and the minimum occurs away from the bar tail, so the sediment sequences in well logs are generally box-like at the bar head and bell-like at the bar tail. This describes in plan view the stretches of river in fluvial reservoirs. (2) To obtain an accurate description of superimposed types of sand bodies, different parts of the point bar and different splicing modes may be identified from changes in well-log sections. (3) The simplified Bridge theory agrees closely with outcrop surveys, and completely preserves the strata in meandering river reservoirs.
The identification of single sand bodies in meandering river deposits is significant and challenging. For decades, Miall's theory of architecture elements has greatly enhanced the description of single sand bodies, but the variety of facies types and the characteristics of facies combinations in the main architecture elements interfere with reservoir description and sand thickness when using well logs. Based on previous studies, this paper simplifies Bridge's sedimentation theory for describing reservoirs, and briefly examines the fluid flow properties and sedimentary sequences at different locations in a selected point bar and its associated river channel. It outlines how to use well-log combinations to identify six types of sand body superposition and apply them in oilfield practice. The study found that: (1) Different well logs in plan view of a single layer reflect the complex hydrodynamics of the sedimentation process. Bridge's theory states that the maximum flow velocity occurs near the bar head and the minimum occurs away from the bar tail, so the sediment sequences in well logs are generally box-like at the bar head and bell-like at the bar tail. This describes in plan view the stretches of river in fluvial reservoirs. (2) To obtain an accurate description of superimposed types of sand bodies, different parts of the point bar and different splicing modes may be identified from changes in well-log sections. (3) The simplified Bridge theory agrees closely with outcrop surveys, and completely preserves the strata in meandering river reservoirs.
2020, 38(1): 218-230.
doi: 10.14027/j.issn.1000-0550.2019.026
Abstract:
The Ordovician Yanxi Formation in Hunan, especially in central and southern Hunan, is a newly discovered shale gas exploration horizon in China. It has large lithological variation, high organic carbon content and high maturity. Outcrop data and drilling data analysis indicate that the Yanxi Formation is mainly composed of siliceous rock and carbonaceous shale overlain by sandy shale and sandstone. The lithology, sedimentary structure and vertical rock assemblage characteristics show that the Yanxi Formation contains various sedimentary facies types, including deep-water basin facies, deep-water shelf facies, shallow-water shelf facies, and turbidite fan facies. It was found that, in plan, deep-water basin facies are distributed in southern Hunan, deep-water shelf and shallow-water shelf facies in central Hunan, and turbidite fan facies in northern and eastern parts of central and western Hunan. The facies distribution determines the direction for shale gas exploration in the Yanxi Formation. The TOC content of siliceous rocks and carbonaceous shales in the deep-water basin of southern Hunan is low, whereas the thick carbonaceous shales of the shelf facies in central Hunan have a high TOC, and are the most favorable for exploration in the Yanxi Formation. In particular, the carbonaceous shale in the third lithology section has the highest abundance of organic matter and a high brittle mineral content, and has the greatest shale gas exploration potential.
The Ordovician Yanxi Formation in Hunan, especially in central and southern Hunan, is a newly discovered shale gas exploration horizon in China. It has large lithological variation, high organic carbon content and high maturity. Outcrop data and drilling data analysis indicate that the Yanxi Formation is mainly composed of siliceous rock and carbonaceous shale overlain by sandy shale and sandstone. The lithology, sedimentary structure and vertical rock assemblage characteristics show that the Yanxi Formation contains various sedimentary facies types, including deep-water basin facies, deep-water shelf facies, shallow-water shelf facies, and turbidite fan facies. It was found that, in plan, deep-water basin facies are distributed in southern Hunan, deep-water shelf and shallow-water shelf facies in central Hunan, and turbidite fan facies in northern and eastern parts of central and western Hunan. The facies distribution determines the direction for shale gas exploration in the Yanxi Formation. The TOC content of siliceous rocks and carbonaceous shales in the deep-water basin of southern Hunan is low, whereas the thick carbonaceous shales of the shelf facies in central Hunan have a high TOC, and are the most favorable for exploration in the Yanxi Formation. In particular, the carbonaceous shale in the third lithology section has the highest abundance of organic matter and a high brittle mineral content, and has the greatest shale gas exploration potential.
2020, 38(1): 231-243.
doi: 10.14027/j.issn.1000-0550.2019.012
Abstract:
Tight sandstone reservoirs occur widely throughout the Triassic Yanchang Formation in the Ordos Basin. The Chang 8 oil-bearing layer is a potential exploration and development horizon in the area. Complex destructive diagenesis further compacted the reservoir, such that present-day porosity differs considerably from what is usual for the accumulation period. Therefore, determining its critical physical properties during knowledge of the charging periods helps to dynamically evaluate the quality of the reservoirs and their oil and gas properties. Firstly, by combining grain-size analysis, sandstone sheet data and comprehensive statistics of the oil grade, the lower limits of the lithology in the area were determined from spot-oil traces in the fine sandstone. Then, the log of the Chang 8 oil-bearing sandstone and oil production tests was analyzed by conventional physical property testing, mercury injection analysis, nuclear magnetic resonance (NMR) testing, two-phase permeability analysis, overburden pressure and stress sensitivity analysis, etc. Based on statistical principles and fluid seepage mechanisms of ultra-low-permeability reservoirs, several methods applicable to tight reservoirs were selected to comprehensively determine the lower limits of the physical properties of effective reservoirs in the Chang 8 oil-bearing layer. Finally, the initial porosity was reconstructed by geohistory modeling from the relationships between porosity and time, temperature, depth and pressure. The measured porosity and the critical physical properties of the reservoirs are reported in terms of a time-depth index. The preferred NMR method of comprehensively determining the lower physical limits, along with displacement pressure testing and oil-bearing occurrence methods appropriate to tight reservoirs, gave a porosity of 7.17% and seepage of 0.1×10–3 µm2. The geohistory model endpoint constraints were the original (pre-compaction) and measured porosities. The time-depth index showed that the critical porosities during the two periods of hydrocarbon charging were about 27.0% and 17.0%, indicating that the reservoirs were significantly compacted after hydrocarbon charging.
Tight sandstone reservoirs occur widely throughout the Triassic Yanchang Formation in the Ordos Basin. The Chang 8 oil-bearing layer is a potential exploration and development horizon in the area. Complex destructive diagenesis further compacted the reservoir, such that present-day porosity differs considerably from what is usual for the accumulation period. Therefore, determining its critical physical properties during knowledge of the charging periods helps to dynamically evaluate the quality of the reservoirs and their oil and gas properties. Firstly, by combining grain-size analysis, sandstone sheet data and comprehensive statistics of the oil grade, the lower limits of the lithology in the area were determined from spot-oil traces in the fine sandstone. Then, the log of the Chang 8 oil-bearing sandstone and oil production tests was analyzed by conventional physical property testing, mercury injection analysis, nuclear magnetic resonance (NMR) testing, two-phase permeability analysis, overburden pressure and stress sensitivity analysis, etc. Based on statistical principles and fluid seepage mechanisms of ultra-low-permeability reservoirs, several methods applicable to tight reservoirs were selected to comprehensively determine the lower limits of the physical properties of effective reservoirs in the Chang 8 oil-bearing layer. Finally, the initial porosity was reconstructed by geohistory modeling from the relationships between porosity and time, temperature, depth and pressure. The measured porosity and the critical physical properties of the reservoirs are reported in terms of a time-depth index. The preferred NMR method of comprehensively determining the lower physical limits, along with displacement pressure testing and oil-bearing occurrence methods appropriate to tight reservoirs, gave a porosity of 7.17% and seepage of 0.1×10–3 µm2. The geohistory model endpoint constraints were the original (pre-compaction) and measured porosities. The time-depth index showed that the critical porosities during the two periods of hydrocarbon charging were about 27.0% and 17.0%, indicating that the reservoirs were significantly compacted after hydrocarbon charging.
