Based on core and thin section data,the source rock samples from the Fengcheng Formation in the Mahu Sag of the Junggar Basin were analyzed in terms of zircon SIMS U-Pb geochronology,organic carbon isotopic compositio...Based on core and thin section data,the source rock samples from the Fengcheng Formation in the Mahu Sag of the Junggar Basin were analyzed in terms of zircon SIMS U-Pb geochronology,organic carbon isotopic composition,major and trace element contents,as well as petrology.Two zircon U-Pb ages of(306.0±5.2)Ma and(303.5±3.7)Ma were obtained from the first member of the Fengcheng Formation.Combined with carbon isotopic stratigraphy,it is inferred that the depositional age of the Fengcheng Formation is about 297-306 Ma,spanning the Carboniferous-Permian boundary and corresponding to the interglacial period between C4 and P1 glacial events.Multiple increases in Hg/TOC ratios and altered volcanic ash were found in the shale rocks of the Fengcheng Formation,indicating that multiple phases of volcanic activity occurred during its deposition.An interval with a high B/Ga ratio was found in the middle of the second member of the Fengcheng Formation,associated with the occurrence of evaporite minerals and reedmergnerite,indicating that the high salinity of the water mass was related to hydrothermal activity.Comprehensive analysis suggests that the warm and humid climate during the deposition of Fengcheng Formation is conducive to the growth of organic matter such as algae and bacteria in the lake,and accelerates the continental weathering,driving the input of nutrients.Volcanic activities supply a large amount of nutrients and stimulate primary productivity.The warm climate and high salinity are conducive to water stratification,leading to water anoxia that benefits organic matter preservation.The above factors interact and jointly control the enrichment of organic matter in the Fengcheng Formation of Mahu Sag.展开更多
Through microscopic analyses(e.g.,organic macerals,thin section observation,scanning electron microscope(SEM)imaging of fresh bedding planes via argon ion milling,and energy spectrum tests)combined with Rock-Eval anal...Through microscopic analyses(e.g.,organic macerals,thin section observation,scanning electron microscope(SEM)imaging of fresh bedding planes via argon ion milling,and energy spectrum tests)combined with Rock-Eval analyses,this study systematically investigated the organic matter and pyrites in the continental shales in the 3^(rd)submember of the Chang 7 Member(Chang 7^(3)submember)in the Yanchang Formation,Ordos Basin and determined their types and the formation and evolutionary characteristics.The results are as follows.The organic matter of the continental shales in the Chang 7^(3)submember is dominated by amorphous bituminites and migrabitumens,which have come into being since the early diagenetic stage and middle diagenetic stage A,respectively.The formation and transformation of organic matter is a prerequisite for the formation of pyrites.The Ordos Basin was a continental freshwater lacustrine basin and lacked sulphates in waters during the deposition of the Chang 7 Member.Therefore,the syndiagenetic stage did not witness the formation of large quantities of pyrites.Since the basin entered early diagenetic stage A,large quantities of sulfur ions were released as the primary organic matter got converted into bituminites and,accordingly,pyrites started to form.However,this stage featured poorer fluid and spatial conditions compared with the syndepositional stage due to withdraw of water,the partial formation of bituminites,and a certain degree of compaction.As a result,large quantities of pyrrhotite failed to transition into typical spherical framboidal pyrites but grew into euhedral monocrystal aggregates.In addition,pyrites are still visible in the migrabitumens in both microfractures and inorganic pores of mudstones and shales,indicating that the pyrite formation period can extend until the middle diagenetic stage A.展开更多
Large-scale exploration and development of shale gas in Lower Cambrian Niutitang Formation of South China has been carried out in recent years,but the result is not good,only except some drilling wells in Jingyan-Qian...Large-scale exploration and development of shale gas in Lower Cambrian Niutitang Formation of South China has been carried out in recent years,but the result is not good,only except some drilling wells in Jingyan-Qianwei area of Sichuan Basin and Yichang of Hubei obtain some commercial gas flows.In order to clarify reasons for failure of shale gas exploration and development in Niutitang Formation around Sichuan Basin and to provide reservoir geological parameters for subsequent efficient exploration and development,taking a case of shale gas reservoirs in Lower Cambrian Niutitang Formation in northeast Chongqing around Sichuan Basin,some experimental methods,such as analysis of organic carbon pyrolysis,determination of equivalent vitrinite reflectance,focused ion beam scanning electron microscope(FIB-SEM)and other are adopted in this study.The results show that the average TOC of shale samples in Niutitang Formation in northeast Chongqing is 3.1%,the equivalent vitrinite reflectance ranges from 3.0%to 4.0%,and the degree of thermal evolution reaches the post-matureemetamorphic stage.Due to excessively high degree of the thermal evolution,organic pores in shale samples are not developed in Lower Cambrian Niutitang Formation in the study area,instead,micro-nano pores dominated by intergranular pores and intragranular pores are developed in the shale.The degree of the thermal evolution controls the sustaining gas generation of kerogen and retained liquid hydrocarbons in the shale,it also controls the development of organic pores of the shale.The evolution of organic pores and hydrocarbon generation in the shale of Niutitang Formation in northeast Chongqing around Sichuan Basin do not match best with each other,that is,during development period of a large number of organic pores,thermal evolution degree of reservoirs is further enhanced because the strata are not uplifted in time,therefore,the quantity of organic pores is decreased sharply,the shale gas would be escaped due to the absence of organic pores as effective storage space after the shale gas generation.Therefore,the efficient exploration and development of the shale gas in Lower Cambrian Niutitang Formation in South China should be focused on the shale development area where the shale is characterized by moderate thermal evolution degree(2.0%<R_(o)<3.0%)and shallow buried depth,that is,the shale distribution area with paleo-uplift or paleo-buried hill.展开更多
基金Supported by the National Natural Science Foundation of China(41802177,42272188,42303056)PetroChina Prospective and Basic Technological Project(2022DJ0507)+1 种基金Research Fund of PetroChina Basic Scientific Research and Strategic Reserve Technology(2020D-5008-04)National Natural Science of Sichuan Province(23NSFSC546)。
文摘Based on core and thin section data,the source rock samples from the Fengcheng Formation in the Mahu Sag of the Junggar Basin were analyzed in terms of zircon SIMS U-Pb geochronology,organic carbon isotopic composition,major and trace element contents,as well as petrology.Two zircon U-Pb ages of(306.0±5.2)Ma and(303.5±3.7)Ma were obtained from the first member of the Fengcheng Formation.Combined with carbon isotopic stratigraphy,it is inferred that the depositional age of the Fengcheng Formation is about 297-306 Ma,spanning the Carboniferous-Permian boundary and corresponding to the interglacial period between C4 and P1 glacial events.Multiple increases in Hg/TOC ratios and altered volcanic ash were found in the shale rocks of the Fengcheng Formation,indicating that multiple phases of volcanic activity occurred during its deposition.An interval with a high B/Ga ratio was found in the middle of the second member of the Fengcheng Formation,associated with the occurrence of evaporite minerals and reedmergnerite,indicating that the high salinity of the water mass was related to hydrothermal activity.Comprehensive analysis suggests that the warm and humid climate during the deposition of Fengcheng Formation is conducive to the growth of organic matter such as algae and bacteria in the lake,and accelerates the continental weathering,driving the input of nutrients.Volcanic activities supply a large amount of nutrients and stimulate primary productivity.The warm climate and high salinity are conducive to water stratification,leading to water anoxia that benefits organic matter preservation.The above factors interact and jointly control the enrichment of organic matter in the Fengcheng Formation of Mahu Sag.
基金funded by the subproject of the National Science and Technology Major Project(No.2017ZX05036004).
文摘Through microscopic analyses(e.g.,organic macerals,thin section observation,scanning electron microscope(SEM)imaging of fresh bedding planes via argon ion milling,and energy spectrum tests)combined with Rock-Eval analyses,this study systematically investigated the organic matter and pyrites in the continental shales in the 3^(rd)submember of the Chang 7 Member(Chang 7^(3)submember)in the Yanchang Formation,Ordos Basin and determined their types and the formation and evolutionary characteristics.The results are as follows.The organic matter of the continental shales in the Chang 7^(3)submember is dominated by amorphous bituminites and migrabitumens,which have come into being since the early diagenetic stage and middle diagenetic stage A,respectively.The formation and transformation of organic matter is a prerequisite for the formation of pyrites.The Ordos Basin was a continental freshwater lacustrine basin and lacked sulphates in waters during the deposition of the Chang 7 Member.Therefore,the syndiagenetic stage did not witness the formation of large quantities of pyrites.Since the basin entered early diagenetic stage A,large quantities of sulfur ions were released as the primary organic matter got converted into bituminites and,accordingly,pyrites started to form.However,this stage featured poorer fluid and spatial conditions compared with the syndepositional stage due to withdraw of water,the partial formation of bituminites,and a certain degree of compaction.As a result,large quantities of pyrrhotite failed to transition into typical spherical framboidal pyrites but grew into euhedral monocrystal aggregates.In addition,pyrites are still visible in the migrabitumens in both microfractures and inorganic pores of mudstones and shales,indicating that the pyrite formation period can extend until the middle diagenetic stage A.
基金The work was supported by the Geological Information Comprehensive Research Project of China Geological Survey(No.121201015000150002)Basic Geological Survey Project of Shale Gas in South China of China Geological Survey(No.12120114046701).
文摘Large-scale exploration and development of shale gas in Lower Cambrian Niutitang Formation of South China has been carried out in recent years,but the result is not good,only except some drilling wells in Jingyan-Qianwei area of Sichuan Basin and Yichang of Hubei obtain some commercial gas flows.In order to clarify reasons for failure of shale gas exploration and development in Niutitang Formation around Sichuan Basin and to provide reservoir geological parameters for subsequent efficient exploration and development,taking a case of shale gas reservoirs in Lower Cambrian Niutitang Formation in northeast Chongqing around Sichuan Basin,some experimental methods,such as analysis of organic carbon pyrolysis,determination of equivalent vitrinite reflectance,focused ion beam scanning electron microscope(FIB-SEM)and other are adopted in this study.The results show that the average TOC of shale samples in Niutitang Formation in northeast Chongqing is 3.1%,the equivalent vitrinite reflectance ranges from 3.0%to 4.0%,and the degree of thermal evolution reaches the post-matureemetamorphic stage.Due to excessively high degree of the thermal evolution,organic pores in shale samples are not developed in Lower Cambrian Niutitang Formation in the study area,instead,micro-nano pores dominated by intergranular pores and intragranular pores are developed in the shale.The degree of the thermal evolution controls the sustaining gas generation of kerogen and retained liquid hydrocarbons in the shale,it also controls the development of organic pores of the shale.The evolution of organic pores and hydrocarbon generation in the shale of Niutitang Formation in northeast Chongqing around Sichuan Basin do not match best with each other,that is,during development period of a large number of organic pores,thermal evolution degree of reservoirs is further enhanced because the strata are not uplifted in time,therefore,the quantity of organic pores is decreased sharply,the shale gas would be escaped due to the absence of organic pores as effective storage space after the shale gas generation.Therefore,the efficient exploration and development of the shale gas in Lower Cambrian Niutitang Formation in South China should be focused on the shale development area where the shale is characterized by moderate thermal evolution degree(2.0%<R_(o)<3.0%)and shallow buried depth,that is,the shale distribution area with paleo-uplift or paleo-buried hill.
