2007 Vol. 32, No. 6
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2007, 32(6): 727-740.
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Abstract:
Hydrocarbon source rocks are characterized by the hydrocarbon discharge, and the alteration and variation in organic compositions and organic content due to the enhanced thermal maturation. These variations throw constraints on the application of the conventional inversion evaluation of hydrocarbon potential by assessing the residual organic matter left in source rocks. Geobiology, probing the interaction between the life system and the earth system, provides new principles in deciphering the whole dynamic processes related to the organic evolution history from living biomass to organic burial. Geobiological subdisciplines, including molecular geobiology, geomicrobiology, geoecology and biogeochemistry, offer new methodology and techniques to estimate the abundance and the composition of biomass, sedimentary organics and preserved organic matter, which are discussed here in detail. Geobiofacies, proposed herein, is terminologized to define the geobiological dynamic processes through the combination of biofacies with organic facies and sedimentary facies, and expressed by the biohabitat types, paleoproductivity and organic burial capacity. Geobiofacies is identified as a useful means to create the geobiological evaluation system, which in turn rectifies the conventional evaluation system for the marine source rocks.
Hydrocarbon source rocks are characterized by the hydrocarbon discharge, and the alteration and variation in organic compositions and organic content due to the enhanced thermal maturation. These variations throw constraints on the application of the conventional inversion evaluation of hydrocarbon potential by assessing the residual organic matter left in source rocks. Geobiology, probing the interaction between the life system and the earth system, provides new principles in deciphering the whole dynamic processes related to the organic evolution history from living biomass to organic burial. Geobiological subdisciplines, including molecular geobiology, geomicrobiology, geoecology and biogeochemistry, offer new methodology and techniques to estimate the abundance and the composition of biomass, sedimentary organics and preserved organic matter, which are discussed here in detail. Geobiofacies, proposed herein, is terminologized to define the geobiological dynamic processes through the combination of biofacies with organic facies and sedimentary facies, and expressed by the biohabitat types, paleoproductivity and organic burial capacity. Geobiofacies is identified as a useful means to create the geobiological evaluation system, which in turn rectifies the conventional evaluation system for the marine source rocks.
2007, 32(6): 741-747.
Abstract:
Geobiofacies is proposed to be an important aspect of geobiology. Geobiofacies shows the biological features indicative of the dynamic processes related to the interactions between the earth and organisms in the geological history. Geobiofacies covers the whole process of the formation, the sedimentation and the burial of both the biomass and organic matter, indicative of the whole dynamic variation of the organics. In this paper biological communities and biohabitat types reflecting the variation of the primary productivity are particularly discussed, which include fossil types, and the abundance, the diversity, the preserved state and the texture of fossil groups (e.g. dominant population, peculiar population, fossil assemblage or fossil community, relationship of fossils). The models of habitat types of carbonate ramp and platform, beach, tidal flat and barrier-lagoon system of clastic deposits, and the columnar, the sectional and the planar maps of biohabitat types are provided with some typical examples.
Geobiofacies is proposed to be an important aspect of geobiology. Geobiofacies shows the biological features indicative of the dynamic processes related to the interactions between the earth and organisms in the geological history. Geobiofacies covers the whole process of the formation, the sedimentation and the burial of both the biomass and organic matter, indicative of the whole dynamic variation of the organics. In this paper biological communities and biohabitat types reflecting the variation of the primary productivity are particularly discussed, which include fossil types, and the abundance, the diversity, the preserved state and the texture of fossil groups (e.g. dominant population, peculiar population, fossil assemblage or fossil community, relationship of fossils). The models of habitat types of carbonate ramp and platform, beach, tidal flat and barrier-lagoon system of clastic deposits, and the columnar, the sectional and the planar maps of biohabitat types are provided with some typical examples.
2007, 32(6): 748-754.
Abstract:
Understanding the dynamics of organic matter in marine water columns greatly favors the geobiologcal evaluation of hydrocarbon source rocks. Biolipids could make great contribution to petroleum due to their comparable chemical components and the slightly refractory characteristics of biolipids during the microbial/thermal degradation. A variety of environmental factors such as temperature, CO2 and salinity could affect the biochemical contents in microorganisms. As a result, microorganisms living in a changing environmental condition might have a different contribution to petroleum formation. Organic carbon flux shows a positive correlation with biological productivity only within a certain range of biomass volumes in a specific biohabitat. Furthermore, organic matter is degraded much more quickly in a water column with oxic conditions. Therefore the anoxic condition, along with the enhanced biological productivity, would be one of the significant factors in contributing the formation of high-quality hydrocarbon source rocks. The formation of biofilms favors the preservation of sedimentary organics by enhancing the deposition rate and decreasing the degradation rate of organics. Identification of biofilms in sedimentary rocks will thus greatly help to understand the depositional processes of organic matter finally preserved in hydrocarbon source rocks.
Understanding the dynamics of organic matter in marine water columns greatly favors the geobiologcal evaluation of hydrocarbon source rocks. Biolipids could make great contribution to petroleum due to their comparable chemical components and the slightly refractory characteristics of biolipids during the microbial/thermal degradation. A variety of environmental factors such as temperature, CO2 and salinity could affect the biochemical contents in microorganisms. As a result, microorganisms living in a changing environmental condition might have a different contribution to petroleum formation. Organic carbon flux shows a positive correlation with biological productivity only within a certain range of biomass volumes in a specific biohabitat. Furthermore, organic matter is degraded much more quickly in a water column with oxic conditions. Therefore the anoxic condition, along with the enhanced biological productivity, would be one of the significant factors in contributing the formation of high-quality hydrocarbon source rocks. The formation of biofilms favors the preservation of sedimentary organics by enhancing the deposition rate and decreasing the degradation rate of organics. Identification of biofilms in sedimentary rocks will thus greatly help to understand the depositional processes of organic matter finally preserved in hydrocarbon source rocks.
2007, 32(6): 755-758.
Abstract:
The assessment of paleo-productivity is of importance, not only for the understanding of paleoclimate and paleoceanography, but for studies on the genesis and the distribution of petroleum and natural gas. A lot of geochemical proxies have been developed to evaluate the past marine productivity. However, most of them are inclined to diagenetic alteration, with the application being limited in Quaternary sediments. Noticeably, dissolved iron deficiency will prevent phytoplankton growth in the modern oceans, and iron trapped in the lattice of carbonate minerals was not easily affected by the post-depositional processes. This raises the possibility of iron abundance as a proxy of paleo-productivity. Here we present an investigation on iron trapped in the marine carbonate in the Permian boundary between the Guadalupian and the Lopingian, Penglaitan, Laibin, Guangxi. Our data show that iron abundance could be used as a proxy of paleo-productivity, supported by its co-variation with δ13C of bulk carbonate which is proposed to relate to the productivity.
The assessment of paleo-productivity is of importance, not only for the understanding of paleoclimate and paleoceanography, but for studies on the genesis and the distribution of petroleum and natural gas. A lot of geochemical proxies have been developed to evaluate the past marine productivity. However, most of them are inclined to diagenetic alteration, with the application being limited in Quaternary sediments. Noticeably, dissolved iron deficiency will prevent phytoplankton growth in the modern oceans, and iron trapped in the lattice of carbonate minerals was not easily affected by the post-depositional processes. This raises the possibility of iron abundance as a proxy of paleo-productivity. Here we present an investigation on iron trapped in the marine carbonate in the Permian boundary between the Guadalupian and the Lopingian, Penglaitan, Laibin, Guangxi. Our data show that iron abundance could be used as a proxy of paleo-productivity, supported by its co-variation with δ13C of bulk carbonate which is proposed to relate to the productivity.
2007, 32(6): 759-766.
Abstract:
The paper presents the molybdenum isotope data, along with the trace element content, to investigate the geochemical behavior of authigenic Mo during long-term burial in sediments in continental margin settings of Yangtze craton, as well as their indication to the burial of original organic carbon. The burial rate of original organic carbon was estimated on the basis of the amount of sedimentary sulphur (TS content), whilst the carbon loss by aerobic degradation was estimated according to calculated Mn contents. On these points, the original organic carbon flux was calculated, exhibiting a large range of variation (0.17-0.67 mmol/m2/day). The strong correlation between sedimentary Mo isotope values and organic carbon burial rates previously proposed on the basis of the investigations on modern ocean sediments, was also used here to estimate the organic carbon burial rate. The data gained through this model showed that organic carbon burial rates have large variations, ranging from 0.43-2.87 mmol/m2/day. Although the two sets of data gained through different geochemical records in the Yangtze craton show a deviation of one order of magnitude, they do display a strong correlation. It is thus tempting to speculate that the Mo isotope signature of sediments may serve as a tracer for the accumulation rate of original organic carbon in the continental margin sediments.
The paper presents the molybdenum isotope data, along with the trace element content, to investigate the geochemical behavior of authigenic Mo during long-term burial in sediments in continental margin settings of Yangtze craton, as well as their indication to the burial of original organic carbon. The burial rate of original organic carbon was estimated on the basis of the amount of sedimentary sulphur (TS content), whilst the carbon loss by aerobic degradation was estimated according to calculated Mn contents. On these points, the original organic carbon flux was calculated, exhibiting a large range of variation (0.17-0.67 mmol/m2/day). The strong correlation between sedimentary Mo isotope values and organic carbon burial rates previously proposed on the basis of the investigations on modern ocean sediments, was also used here to estimate the organic carbon burial rate. The data gained through this model showed that organic carbon burial rates have large variations, ranging from 0.43-2.87 mmol/m2/day. Although the two sets of data gained through different geochemical records in the Yangtze craton show a deviation of one order of magnitude, they do display a strong correlation. It is thus tempting to speculate that the Mo isotope signature of sediments may serve as a tracer for the accumulation rate of original organic carbon in the continental margin sediments.
2007, 32(6): 767-773.
Abstract:
By combining the carbon cycle model with the records of carbonate and organic (kerogen) carbon isotope composition, this paper presents the calculation of the fraction of organic carbon burial (forg) of beds 23-40 at the GSSP of the Permian-Triassic boundary at Meishan, Zhejiang Province. The resulting calculation produces two episodes of forg maxima observed to occur at beds 23-24 and 27-29, which respectively corresponds to the two episodic anoxic events indicated by the flourish of green sulfur bacteria. Two episodic forg minima occurred at beds 25-26 and 32-34, generally coincident with the flourish of cyanobacteria (bed 26 and upper part of beds 29 to 34) as shown by the high value of 2-melthyhopanes. It appears that the forg is related to the redox conditions, with greater forg values observed at the reductive condition. The relationship between forg and the total organic carbon (TOC) content was complex. The forg value was low at some beds with high TOC content (such as bed 26), whilst high forg values observed at some beds with low TOC content (e. g. bed 27). This association infers the important contribution of primary productivity to the TOC content. The original organic burial could be thus calculated through the configuration of the function of the primary productivity and forg, which can be used to correct the residual TOC measured in modern time. This investigation indicates that compiling the organic-inorganic carbon isotopes with the carbon cycle model favors to understand the fraction of organic carbon burial, providing information for the reconstruction of the coupling among biota, environments and organic burial.
By combining the carbon cycle model with the records of carbonate and organic (kerogen) carbon isotope composition, this paper presents the calculation of the fraction of organic carbon burial (forg) of beds 23-40 at the GSSP of the Permian-Triassic boundary at Meishan, Zhejiang Province. The resulting calculation produces two episodes of forg maxima observed to occur at beds 23-24 and 27-29, which respectively corresponds to the two episodic anoxic events indicated by the flourish of green sulfur bacteria. Two episodic forg minima occurred at beds 25-26 and 32-34, generally coincident with the flourish of cyanobacteria (bed 26 and upper part of beds 29 to 34) as shown by the high value of 2-melthyhopanes. It appears that the forg is related to the redox conditions, with greater forg values observed at the reductive condition. The relationship between forg and the total organic carbon (TOC) content was complex. The forg value was low at some beds with high TOC content (such as bed 26), whilst high forg values observed at some beds with low TOC content (e. g. bed 27). This association infers the important contribution of primary productivity to the TOC content. The original organic burial could be thus calculated through the configuration of the function of the primary productivity and forg, which can be used to correct the residual TOC measured in modern time. This investigation indicates that compiling the organic-inorganic carbon isotopes with the carbon cycle model favors to understand the fraction of organic carbon burial, providing information for the reconstruction of the coupling among biota, environments and organic burial.
2007, 32(6): 774-780.
Abstract:
Dalong Formation of Late Permian in Hubei, Hunan, Guizhou and Guangxi in South China could be subdivided into seven sedimentary types according to the lithology, indicative of the great spatial variations of both the lithology and sedimentary facies. Dalong Formation deposited during the highest transgression of Late Permian, showing a strong linkage with the hydrocarbon source rocks, in particular those deposited under the anaerobic and suboxic conditions. Dalong Formation shows potential as hydrocarbon source rocks due to the deposition in the highest transgression sea level, the deep slope sedimentary environments and the low sedimentary rate.
Dalong Formation of Late Permian in Hubei, Hunan, Guizhou and Guangxi in South China could be subdivided into seven sedimentary types according to the lithology, indicative of the great spatial variations of both the lithology and sedimentary facies. Dalong Formation deposited during the highest transgression of Late Permian, showing a strong linkage with the hydrocarbon source rocks, in particular those deposited under the anaerobic and suboxic conditions. Dalong Formation shows potential as hydrocarbon source rocks due to the deposition in the highest transgression sea level, the deep slope sedimentary environments and the low sedimentary rate.
Potential Relationship between Extremophiles and Hydrocarbon Resources in Marine Extreme Environment
2007, 32(6): 781-788.
Abstract:
To understand the potential mechanism of marine extremophiles participating in the formation and the evolution of hydrocarbon resources in marine extreme environments, some typical kinds of extremophiles and their distributions in marine hydrothermal venting and cold venting are discussed and evaluated respectively. The potential relationship between extremophile activities and hydrocarbon resources in marine extreme environments are then discussed in details. It could be now preliminary concluded that archaea and bacteria are the two main kinds of extremophiles in marine extreme environments. The dominating microbial communities in hydrothermal venting are heterotrophic zymogens, sulphate reducers and methanogens, while the ANME-2 group (Methanosarcinales) surrounded by sulfate-reducing bacteria and ANME-1 group are dominated in cold venting. Marine extremophiles would be able to use CH4 and H2S to synthesize energy for metabolism and to support food chains for other unique macrobiota nearby, which together present a high abundance but a low diversity with distinct characteristics of horizontal and vertical distributions. Marine extremophiles might play an important role either directly or indirectly in the processes of hydrocarbon formation and later alteration, and could indicate the evolution of hydrocarbon resources in marine extreme environments. Our research thus has a great significance both in theoretical approach of potential hydrocarbon resources formed by marine extremophile activities and in practical exploration of the potential hydrocarbon source sedimentary layers formed in the earth history or the potential strata in South China.
To understand the potential mechanism of marine extremophiles participating in the formation and the evolution of hydrocarbon resources in marine extreme environments, some typical kinds of extremophiles and their distributions in marine hydrothermal venting and cold venting are discussed and evaluated respectively. The potential relationship between extremophile activities and hydrocarbon resources in marine extreme environments are then discussed in details. It could be now preliminary concluded that archaea and bacteria are the two main kinds of extremophiles in marine extreme environments. The dominating microbial communities in hydrothermal venting are heterotrophic zymogens, sulphate reducers and methanogens, while the ANME-2 group (Methanosarcinales) surrounded by sulfate-reducing bacteria and ANME-1 group are dominated in cold venting. Marine extremophiles would be able to use CH4 and H2S to synthesize energy for metabolism and to support food chains for other unique macrobiota nearby, which together present a high abundance but a low diversity with distinct characteristics of horizontal and vertical distributions. Marine extremophiles might play an important role either directly or indirectly in the processes of hydrocarbon formation and later alteration, and could indicate the evolution of hydrocarbon resources in marine extreme environments. Our research thus has a great significance both in theoretical approach of potential hydrocarbon resources formed by marine extremophile activities and in practical exploration of the potential hydrocarbon source sedimentary layers formed in the earth history or the potential strata in South China.
2007, 32(6): 789-796.
Abstract:
To decipher the origin of oxygen-deficient shelf deposits is significant for tracing the distributions of marine hydrocarbon source rocks and interpreting the evolution of depositional environments. The origin of the Middle Permian Chihsia Formation from South China remains a puzzle for long, with its evident oxygen-deficient features associated with diverse benthos. This paper introduce briefly the nature of oxygen-deficient shelf environment as well as subdivision of related paleoxygenation facies, and shows a typical Chihsian depositional sequence with ecological and geochemical features. Massive bioclastic limestone from the middle of the sequence was believed to be formed under the aerobic conditions, supporting by both ecological and geochemical features. However, a contradictory conclusion is observed for "laminated" limestones from the lower and upper parts of the sequence; the aerobic ecological signal was associated with the anoxic geochemical record. The association of contradictory signals in ecology and geochemistry was here interpreted as the instability of paleoxygenation condition in shelf environment, caused by enhanced paleoproductivity. Repetitive occurrence of oxygen-deficient condition might have been stemmed from paleo-Tethyan currents flowing across the South China.
To decipher the origin of oxygen-deficient shelf deposits is significant for tracing the distributions of marine hydrocarbon source rocks and interpreting the evolution of depositional environments. The origin of the Middle Permian Chihsia Formation from South China remains a puzzle for long, with its evident oxygen-deficient features associated with diverse benthos. This paper introduce briefly the nature of oxygen-deficient shelf environment as well as subdivision of related paleoxygenation facies, and shows a typical Chihsian depositional sequence with ecological and geochemical features. Massive bioclastic limestone from the middle of the sequence was believed to be formed under the aerobic conditions, supporting by both ecological and geochemical features. However, a contradictory conclusion is observed for "laminated" limestones from the lower and upper parts of the sequence; the aerobic ecological signal was associated with the anoxic geochemical record. The association of contradictory signals in ecology and geochemistry was here interpreted as the instability of paleoxygenation condition in shelf environment, caused by enhanced paleoproductivity. Repetitive occurrence of oxygen-deficient condition might have been stemmed from paleo-Tethyan currents flowing across the South China.
2007, 32(6): 797-802.
Abstract:
Calci-microbialite is a special carbonate buildup, formed due to the activities of different kinds of microbes. Most of microbialites were developed in Precambrian or at the great transitional stages in Phanerozoic. Widespread microbialites have been found both in North and South China. Abundant microfossils preserved in the microbialites show high-level productivity during deposition. The obviously lower value of Mo isotope shows that the seafloor is of anoxic condition during the long time of Precambrian. The sharp reduction of Th/U value as well as the wide spread pyrite in shallow carbonate platforms indicate that the ocean environment is anoxic after end-Permian mass extinction. High-level productivity and anoxic sedimentary environment favor the efficient preservation of organic matter and thus the formation of source rocks. Bitumen observed in many outcrops of Precambrian and Late Devonian microbialites further raises the possibility of calci-microbialite as a potential hydrocarbon source rock.
Calci-microbialite is a special carbonate buildup, formed due to the activities of different kinds of microbes. Most of microbialites were developed in Precambrian or at the great transitional stages in Phanerozoic. Widespread microbialites have been found both in North and South China. Abundant microfossils preserved in the microbialites show high-level productivity during deposition. The obviously lower value of Mo isotope shows that the seafloor is of anoxic condition during the long time of Precambrian. The sharp reduction of Th/U value as well as the wide spread pyrite in shallow carbonate platforms indicate that the ocean environment is anoxic after end-Permian mass extinction. High-level productivity and anoxic sedimentary environment favor the efficient preservation of organic matter and thus the formation of source rocks. Bitumen observed in many outcrops of Precambrian and Late Devonian microbialites further raises the possibility of calci-microbialite as a potential hydrocarbon source rock.
2007, 32(6): 803-810.
Abstract:
From Carboniferous to Early Permian, the glaciation of Gondwanaland played a crucial role in controlling changes of the sea level, the biohabitat types and the productivity, in particular in South China. Four global regression events identified during this period were attributed to the glaciation of Gondwanaland. In South China, the regression event occurred between the latest Tournasian and early Visean led to an obvious change of the biohabitat types. In the intraplatform basin, the biohabitat types shifted from Ⅴ1 to Ⅳ1. In the isolated carbonate platform, the biohabitat types shifted from Ⅲ1-Ⅲ2 to Ⅱ1-Ⅱ2. In the carbonate platform connected with the land, the biohabitat types shifted from Ⅱ1 and Ⅱ2 to terrestrial or marine-continental sedimentary environment. Analyses of the biohabit at types, paleocommunities and redox conditions, enable us to investigate the paleocommunity evolution, the productivity of marine organisms and the changes of organic burial conditions in the glacial and postglacial period, and thus to explore the control of Carboniferous glaciation on the generation and distribution of hydrocarbon source rocks.
From Carboniferous to Early Permian, the glaciation of Gondwanaland played a crucial role in controlling changes of the sea level, the biohabitat types and the productivity, in particular in South China. Four global regression events identified during this period were attributed to the glaciation of Gondwanaland. In South China, the regression event occurred between the latest Tournasian and early Visean led to an obvious change of the biohabitat types. In the intraplatform basin, the biohabitat types shifted from Ⅴ1 to Ⅳ1. In the isolated carbonate platform, the biohabitat types shifted from Ⅲ1-Ⅲ2 to Ⅱ1-Ⅱ2. In the carbonate platform connected with the land, the biohabitat types shifted from Ⅱ1 and Ⅱ2 to terrestrial or marine-continental sedimentary environment. Analyses of the biohabit at types, paleocommunities and redox conditions, enable us to investigate the paleocommunity evolution, the productivity of marine organisms and the changes of organic burial conditions in the glacial and postglacial period, and thus to explore the control of Carboniferous glaciation on the generation and distribution of hydrocarbon source rocks.
2007, 32(6): 811-818.
Abstract:
The carbonate platforms and the intraplatform troughs resulted from intracontinental rifting were simultaneously developed in Guangxi since the Emsian stage of Devonian. An isolated platform was found to occur at Liujing and Litang areas in central Guangxi since the Givetian stage. A fore-reef slope ecosystem characterized by tentaculites was identifiable in Mintang Fm. in Liujing during the Givetian stage. The occurrence of the reef ecosystem was inferred by the reef congregates in Mintang Fm. rather than the reef outcrop. The carbonate platform ecosystem in the Givetian Tangjiawan Fm. in Litang is composed of corals, stromatopoidea and brachiopoda. An ecosystem of a restricted and partly-open platform developed in Frasinan Gubi Fm., characterized by a small proportion of corals, brachiopoda and stromatopoidea. Frasnian Guilin Fm. in Litang is shown to be an ecosystem of a carbonate platform composed of corals, brachiopoda and stromatopoidea. An abrupt ecosystem change was observed to occur at the boundary between Frasnian and Famennian. Ecosystems of Rongxian Fm. in Liujing and Dongcun Fm. in Litang of Famennian are composed of algae reef and algae sheet. The ecosystem variation was proposed to result from the well known F-F faunal mass extinction. These ecosystem features are closely related to the forming and the evolution of the source rocks in this region.
The carbonate platforms and the intraplatform troughs resulted from intracontinental rifting were simultaneously developed in Guangxi since the Emsian stage of Devonian. An isolated platform was found to occur at Liujing and Litang areas in central Guangxi since the Givetian stage. A fore-reef slope ecosystem characterized by tentaculites was identifiable in Mintang Fm. in Liujing during the Givetian stage. The occurrence of the reef ecosystem was inferred by the reef congregates in Mintang Fm. rather than the reef outcrop. The carbonate platform ecosystem in the Givetian Tangjiawan Fm. in Litang is composed of corals, stromatopoidea and brachiopoda. An ecosystem of a restricted and partly-open platform developed in Frasinan Gubi Fm., characterized by a small proportion of corals, brachiopoda and stromatopoidea. Frasnian Guilin Fm. in Litang is shown to be an ecosystem of a carbonate platform composed of corals, brachiopoda and stromatopoidea. An abrupt ecosystem change was observed to occur at the boundary between Frasnian and Famennian. Ecosystems of Rongxian Fm. in Liujing and Dongcun Fm. in Litang of Famennian are composed of algae reef and algae sheet. The ecosystem variation was proposed to result from the well known F-F faunal mass extinction. These ecosystem features are closely related to the forming and the evolution of the source rocks in this region.
2007, 32(6): 819-827.
Abstract:
Review of lithostratigraphic criteria for subdividing the Wufeng (Ordovician) and Longmaxi (Silurian) formations reaffirms their integrity and subdivisions and provides new regional correlations between the formations and related stratigraphic successions and facies. Both the black shales and the related overlying flysch deposits and other successions at the Ordovician-Silurian transition in South China appear to have migrated northwestward in time and space, reflecting probable flexural control in a foreland basin that developed in response to subduction-type orogeny southeast of the Yangtze block. The black shales also contain K-bentonites from explosive, felsic-intermediate volcanism, the distribution of which also supports orogeny to the southeast. Finally, analysis of sequence stratigraphy, which shows that the initiation of transgressive system tracts (TST) and condensed section (cs) in the related 3rd-order sequences coincided with the two black-shale horizons respectively, indicates that the major controlling factors for the deposition of the Ordovician-Silurian black shales in South China are (1) northwestwardly migrating, foreland-basin subsidence caused by deformational loading related to episodic accretion of the Cathaysia block to the Yangtze block during this period, and (2) the anoxic, sediment-starved water column caused by rapid rise of the sea level during the two successive phases of the 3rd-order, eustatic, sea-level rise near the Ordovician-Silurian transition in South China. In the future exploration for source rocks from the area in various periods, consideration of likely flexural and eustatic causes for subsiding, deep, anoxic seas may be important in recognizing other source-rock intervals.
Review of lithostratigraphic criteria for subdividing the Wufeng (Ordovician) and Longmaxi (Silurian) formations reaffirms their integrity and subdivisions and provides new regional correlations between the formations and related stratigraphic successions and facies. Both the black shales and the related overlying flysch deposits and other successions at the Ordovician-Silurian transition in South China appear to have migrated northwestward in time and space, reflecting probable flexural control in a foreland basin that developed in response to subduction-type orogeny southeast of the Yangtze block. The black shales also contain K-bentonites from explosive, felsic-intermediate volcanism, the distribution of which also supports orogeny to the southeast. Finally, analysis of sequence stratigraphy, which shows that the initiation of transgressive system tracts (TST) and condensed section (cs) in the related 3rd-order sequences coincided with the two black-shale horizons respectively, indicates that the major controlling factors for the deposition of the Ordovician-Silurian black shales in South China are (1) northwestwardly migrating, foreland-basin subsidence caused by deformational loading related to episodic accretion of the Cathaysia block to the Yangtze block during this period, and (2) the anoxic, sediment-starved water column caused by rapid rise of the sea level during the two successive phases of the 3rd-order, eustatic, sea-level rise near the Ordovician-Silurian transition in South China. In the future exploration for source rocks from the area in various periods, consideration of likely flexural and eustatic causes for subsiding, deep, anoxic seas may be important in recognizing other source-rock intervals.
2007, 32(6): 828-844.
Abstract:
Macroscopic algae widely lived in the Doushantuo-period sea of the Ediacaran in the Yangtze region, South China. They have successfully laid down an essential basis in a new environment and an ecosystem chain for the evolution and the development of the metazoan. The Wenghui biota, which is found in the Ediacaran Doushantuo Formation in Northeast Guizhou, is composed mainly of macroalgae (18 species of 15 genera, one unnamed macroalgae, and macroalgal holdfast forms described herein), and contains macroscopic metazoan, putative metazoan and trace fossils. This biota not only contains some elements of the Miaohe biota from West Hubei, the Lantian flora from South Anhui, the Ediacara fauna in Australia, and the White Sea biota in Russia, but also has its own characters to distinguish other Ediacaran biota. The diverse macroalgae of the Wenghui biota clearly show organic and functional differentiations with some exhibiting a relationship with higher plants. The Wenghui biota lived in the euphotic zone of shallow sea with lower-energy and poor oxygen. Numerous macroalgae settled on the deposits by their holdfasts, and their semi-floating bodies erected on sea floors. They advanced the photosynthetic efficiency and increased oxygen in water. In addition, the macroalgae served as a primary producer in the paleoecosystem to provide an alternative food for the metazoan. Therefore, the appearance of macroscopic algae not only improved the environment and played an essential support for the growth and propagation of the metazoan which depended on oxygen, but also provided a new food source that served as a new basis in the ecosystem for metazoan's evolution and devel-opment. Moreover, prosperous macrobiota not only increased the productivity of organic matter but also favored the sedi- memation and the preservation of organic carbon.
Macroscopic algae widely lived in the Doushantuo-period sea of the Ediacaran in the Yangtze region, South China. They have successfully laid down an essential basis in a new environment and an ecosystem chain for the evolution and the development of the metazoan. The Wenghui biota, which is found in the Ediacaran Doushantuo Formation in Northeast Guizhou, is composed mainly of macroalgae (18 species of 15 genera, one unnamed macroalgae, and macroalgal holdfast forms described herein), and contains macroscopic metazoan, putative metazoan and trace fossils. This biota not only contains some elements of the Miaohe biota from West Hubei, the Lantian flora from South Anhui, the Ediacara fauna in Australia, and the White Sea biota in Russia, but also has its own characters to distinguish other Ediacaran biota. The diverse macroalgae of the Wenghui biota clearly show organic and functional differentiations with some exhibiting a relationship with higher plants. The Wenghui biota lived in the euphotic zone of shallow sea with lower-energy and poor oxygen. Numerous macroalgae settled on the deposits by their holdfasts, and their semi-floating bodies erected on sea floors. They advanced the photosynthetic efficiency and increased oxygen in water. In addition, the macroalgae served as a primary producer in the paleoecosystem to provide an alternative food for the metazoan. Therefore, the appearance of macroscopic algae not only improved the environment and played an essential support for the growth and propagation of the metazoan which depended on oxygen, but also provided a new food source that served as a new basis in the ecosystem for metazoan's evolution and devel-opment. Moreover, prosperous macrobiota not only increased the productivity of organic matter but also favored the sedi- memation and the preservation of organic carbon.
2007, 32(6): 845-852.
Abstract:
Nanhua System in Datangpo, Songtao County, Guizhou Province is proposed to be one of the ideal areas to investigate the ancient gas seep and the associated cold spring carbonates. The cold seep carbonates found here occur as two kinds of rocks. One is the dolomite hills or lens in the clastic rocks of Liangjiehe Formation, which is thought to be formed during the early Sturtian glacial stage, and thus to be the oldest cold spring carbonates. The other is represented by the rhodochrosite orebodies occurring as layers and lens in association with some dolomite lens in the black manganese-bearing rocks in the first member in Datangpo Formation. The carbon isotope composition of the cold spring carbonates in the area shows a negative anomaly, and the carbon isotope composition of the asphalt filled in the gas holes in rhodochrosite orebodies is measured to be -30.98‰. The negative carbon isotope values are associated with the positive anomaly of the sulfur isotope composition. The cold seep carbonate rocks identified in this region are comparable with modern counterparts in the characteristics and the distribution of the carbonates, the gas seep structures and the depositional organics in the rhodochrosite orebodies. The dolomite hills and the rhodochrosite orebodies are formed in the same ancient gas seep system during different periods, showing a close relationship with the hydrocarbon source rocks of Datangpo Formation.
Nanhua System in Datangpo, Songtao County, Guizhou Province is proposed to be one of the ideal areas to investigate the ancient gas seep and the associated cold spring carbonates. The cold seep carbonates found here occur as two kinds of rocks. One is the dolomite hills or lens in the clastic rocks of Liangjiehe Formation, which is thought to be formed during the early Sturtian glacial stage, and thus to be the oldest cold spring carbonates. The other is represented by the rhodochrosite orebodies occurring as layers and lens in association with some dolomite lens in the black manganese-bearing rocks in the first member in Datangpo Formation. The carbon isotope composition of the cold spring carbonates in the area shows a negative anomaly, and the carbon isotope composition of the asphalt filled in the gas holes in rhodochrosite orebodies is measured to be -30.98‰. The negative carbon isotope values are associated with the positive anomaly of the sulfur isotope composition. The cold seep carbonate rocks identified in this region are comparable with modern counterparts in the characteristics and the distribution of the carbonates, the gas seep structures and the depositional organics in the rhodochrosite orebodies. The dolomite hills and the rhodochrosite orebodies are formed in the same ancient gas seep system during different periods, showing a close relationship with the hydrocarbon source rocks of Datangpo Formation.
2007, 32(6): 853-860.
Abstract:
On the basis of the results of hydropyrolysis simulations for about 90 different types immature to mature source rocks selected from about 5 000 marine source rocks in China, and accompanied by the investigation on the natural thermal evolution profiles, it was suggested that, the content of total organic carbon (TOC) of excellent marine source rocks does not change obviously at immature stage Ro < 0.8%, or at high-post mature stage Ro > 1.3%, and the residual TOC content decreases gradually with the maturity increase during the immature to low mature stage. The maximum of the decrease of the residual TOC content for sapropel (Ⅰ), Ⅱ1 and Ⅱ2 type kerogen from excellent source rocks is usually 40%, 32% and 24%, and the TOC restitution coefficient is 1.68, 1.48, 1.32, respectively. Both the amplitude and the restitution coefficient of TOC decrease gradually with the decrease of TOC content for the source rocks with low organic matter abundance (0.3 % < TOC < 1.0%), and the TOC restitution coefficient is 1.20 when 0.3% < TOC < 0.5%, and close to 1.0 when TOC < 0.3%. The TOC content of solid bitumen and shale with high organic matter abundance (TOC > 30%) also required no restitution at high mature stage. The decrease of S1+S2 of marine source rocks (0.3% < TOC < 30%) and the increase of the hydrocarbon quantity generated and expulsed in laboratory simulation are consistent with the decrease of the residual TOC content observed in natural thermal evolution profiles.
On the basis of the results of hydropyrolysis simulations for about 90 different types immature to mature source rocks selected from about 5 000 marine source rocks in China, and accompanied by the investigation on the natural thermal evolution profiles, it was suggested that, the content of total organic carbon (TOC) of excellent marine source rocks does not change obviously at immature stage Ro < 0.8%, or at high-post mature stage Ro > 1.3%, and the residual TOC content decreases gradually with the maturity increase during the immature to low mature stage. The maximum of the decrease of the residual TOC content for sapropel (Ⅰ), Ⅱ1 and Ⅱ2 type kerogen from excellent source rocks is usually 40%, 32% and 24%, and the TOC restitution coefficient is 1.68, 1.48, 1.32, respectively. Both the amplitude and the restitution coefficient of TOC decrease gradually with the decrease of TOC content for the source rocks with low organic matter abundance (0.3 % < TOC < 1.0%), and the TOC restitution coefficient is 1.20 when 0.3% < TOC < 0.5%, and close to 1.0 when TOC < 0.3%. The TOC content of solid bitumen and shale with high organic matter abundance (TOC > 30%) also required no restitution at high mature stage. The decrease of S1+S2 of marine source rocks (0.3% < TOC < 30%) and the increase of the hydrocarbon quantity generated and expulsed in laboratory simulation are consistent with the decrease of the residual TOC content observed in natural thermal evolution profiles.
2007, 32(6): 861-867.
Abstract:
An effective evaluation system of high-quality marine source rocks plays an important role in hydrocarbon exploration and exploitation of marine strata. Remnant organic matter preserved in a sedimentary basin underwent two evolutionary stages from organic matter to source rocks, i.e. the formation of deposited organic matter and the formation of source rocks transferring from deposited organic matter to remnant organic matter. The latter includes the immature stage during which the deposited organic matter experienced biogeochemical processes to become burial organic matter, and the mature and the post-mature stages during which the burial organic matter underwent geothermal processes to become remnant organic matter and subsequently to be weathered. The inverse analysis of source rocks is defined as a method and technique to estimate the amount of burial organic matter and original potential of hydrocarbon generation of source rocks based on the analysis of remnant or weathered organic matter. The forward analysis of source rocks is defined as a method to estimate the amount of depositional organic matter or burial organic matter, and ulteriorly to estimate their original potential of hydrocarbon generation of source rocks by using the information of sedimentary environments, features of living biomass and physical chemistry in water medium through the geobiological analysis of geoecology, geomicrobiology, molecular geobiology and biogeochemistry. Quality and quantity of depositional organic matter can be calculated according to the productivity and preservation environments. Comparison between the forward and the inverse analysis can be validated and complemented by each other, which would reveal the features of source rocks and provide an effective method for the prediction and evaluation of good-quality marine source rocks.
An effective evaluation system of high-quality marine source rocks plays an important role in hydrocarbon exploration and exploitation of marine strata. Remnant organic matter preserved in a sedimentary basin underwent two evolutionary stages from organic matter to source rocks, i.e. the formation of deposited organic matter and the formation of source rocks transferring from deposited organic matter to remnant organic matter. The latter includes the immature stage during which the deposited organic matter experienced biogeochemical processes to become burial organic matter, and the mature and the post-mature stages during which the burial organic matter underwent geothermal processes to become remnant organic matter and subsequently to be weathered. The inverse analysis of source rocks is defined as a method and technique to estimate the amount of burial organic matter and original potential of hydrocarbon generation of source rocks based on the analysis of remnant or weathered organic matter. The forward analysis of source rocks is defined as a method to estimate the amount of depositional organic matter or burial organic matter, and ulteriorly to estimate their original potential of hydrocarbon generation of source rocks by using the information of sedimentary environments, features of living biomass and physical chemistry in water medium through the geobiological analysis of geoecology, geomicrobiology, molecular geobiology and biogeochemistry. Quality and quantity of depositional organic matter can be calculated according to the productivity and preservation environments. Comparison between the forward and the inverse analysis can be validated and complemented by each other, which would reveal the features of source rocks and provide an effective method for the prediction and evaluation of good-quality marine source rocks.
2007, 32(6): 868-870.
Abstract:
The studied region, located between Dongmen and Liuqiao at Fusui in Guangxi, mainly consists of Upper Permian and Lower Triassic strata. The Upper Permian is divided into three units; they include the Heshan Formation, Upper Permian reef limestones, and Dalong Formation. The Lower Triassic mudstone and limestone are known as the Luolou Formation. The Dalong Formation consists predominantly of radiolarian cherts and mudstones, and contains abundant late Changhsingian fossils, including radiolarians, foraminifera, ostracods, fungus spores, brachiopods, ammonoids and bivalves. These sedimentary and lithological features of the Dalong Formation are indicative of good source rocks. The reef limestones are characterized by plentiful fractures and pore spaces, showing a nature of good reservoirs. The fractures and pores are generally filled by asphalt in Dongpan, Balong and Baliao sections at Liuqiao in southwestern Guangxi. On these points, the Upper Permian reef limestones in Liuqiao area would be of significance for the future exploration of oil and natural gas.
The studied region, located between Dongmen and Liuqiao at Fusui in Guangxi, mainly consists of Upper Permian and Lower Triassic strata. The Upper Permian is divided into three units; they include the Heshan Formation, Upper Permian reef limestones, and Dalong Formation. The Lower Triassic mudstone and limestone are known as the Luolou Formation. The Dalong Formation consists predominantly of radiolarian cherts and mudstones, and contains abundant late Changhsingian fossils, including radiolarians, foraminifera, ostracods, fungus spores, brachiopods, ammonoids and bivalves. These sedimentary and lithological features of the Dalong Formation are indicative of good source rocks. The reef limestones are characterized by plentiful fractures and pore spaces, showing a nature of good reservoirs. The fractures and pores are generally filled by asphalt in Dongpan, Balong and Baliao sections at Liuqiao in southwestern Guangxi. On these points, the Upper Permian reef limestones in Liuqiao area would be of significance for the future exploration of oil and natural gas.
2007, 32(6): 871-878.
Abstract:
Reefs were believed to be favorable rocks for hydrocarbon accumulation. Great progress was made in the past decades through the investigation of reefs in China; they include the distributions, the controlling factors and the oil-bearing properties of the reefs. The identification and the prediction of underlying reefs were widely carried out by geophysical techniques, and were more and more successful due to the enhanced understandings of both the geophysical features of the reefs and the mechanisms of the reef formation. Some issues related to the reef systems, however, need to be investigated and strengthened in the future. The comprehensive investigation of a reef awaits an integrated study on sedimentology, ecology, in particular geobiology. The deepgoing description and the elevated understanding of reef distributions help to enhance the prediction of reef reservoirs, and to acknowledge their relation with the petroleum-generation system. Multiple techniques and integrated methodology would be widely explored and cross-checked to enhance the prediction probability of underlying reefs in association with oil and gas.
Reefs were believed to be favorable rocks for hydrocarbon accumulation. Great progress was made in the past decades through the investigation of reefs in China; they include the distributions, the controlling factors and the oil-bearing properties of the reefs. The identification and the prediction of underlying reefs were widely carried out by geophysical techniques, and were more and more successful due to the enhanced understandings of both the geophysical features of the reefs and the mechanisms of the reef formation. Some issues related to the reef systems, however, need to be investigated and strengthened in the future. The comprehensive investigation of a reef awaits an integrated study on sedimentology, ecology, in particular geobiology. The deepgoing description and the elevated understanding of reef distributions help to enhance the prediction of reef reservoirs, and to acknowledge their relation with the petroleum-generation system. Multiple techniques and integrated methodology would be widely explored and cross-checked to enhance the prediction probability of underlying reefs in association with oil and gas.
