Various data were utilized to analyze the sedimentary facies,completely reconstruct the palaeogeographic maps,and evaluate the reservoir quality of the Middle-Upper Cambrian Xixiangchi Formation in the Sichuan Basin,i...Various data were utilized to analyze the sedimentary facies,completely reconstruct the palaeogeographic maps,and evaluate the reservoir quality of the Middle-Upper Cambrian Xixiangchi Formation in the Sichuan Basin,including evidence from field outcrops,drilling cores,microscopic thin sections,scanning electron microscopy(SEM),combined with experimental analysis data,such as petrophysical data(porosity(%)and permeability(mD)),and radioactive elements(uranium(^(238)U),thorium(^(232)Th),and potassium(^(40)K),and isotopic data(δ^(13)C)).In the Sichuan Basin,the Middle-Upper Cambrian Xixiangchi Formation was principally deposited in a restricted platform with a lithology predominantly composed of dolomite,with local occurrences of limestone and other rock types in small thicknesses.Graded beddings,cross beddings,horizontal beddings,storm depositions,and mud cracks locally developed in some samples.Four 3rd-order sequences(SQ1-SQ4)were identified within the Xixiangchi Formation in this study.Each sequence is subdivided into a highstand systems tract(HST)and a transgressive systems tract(TST).Reservoirs are principally developed in high-energy grain shoal deposits located in SQ2 and SQ3,with a minor occurrence in SQ1 and SQ4.The lithology of these shoal deposits is essentially composed of sandy dolomite and crystalline dolomite characterized by relatively low average porosity(2.61%)and permeability(1.0073 mD)values.The increase of these values in several studied samples might be related to seepage and connecting dissolution pores and vugs through fractures which have a constructive effect on porosity and permeability.The occurrence of karst caves through superimposed supergene karstification and bedding karstification highly improved the reservoir's physical properties in some areas.The reservoir space is dominated by intergranular dissolution pores,karst caves,and fractures,and their formation was controlled by sedimentation,diagenesis,penecontemporaneous dissolution,and tectonic fractures,which can also be influenced by palaeogeomorphology and sea level fluctuation.Favorable hydrocarbon exploration areas with relatively good source conditions,high positions in the paleostructure,which are conducive to the migration and accumulations of hydrocarbon were identified in three areas of the basin:(1)on the slope around the Central Sichuan Uplift;(2)in the northeastern part,dolomite reservoirs developed with good physical properties and large cumulative thicknesses;and(3)in the southeastern part,shoal facies reservoirs were developed in high and steep structural settings with anticlinal traps.The insights gained in this study fill a significant gap in geological complexities related to sedimentology,palaeogeography,and hydrocarbon reservoir prediction.展开更多
During the Ordovician-Silurian transition period,the Yangtze area experienced significant environmental changes,which were recorded by the widely distributed black shale of the Upper Ordovician Wufeng and Lower Siluri...During the Ordovician-Silurian transition period,the Yangtze area experienced significant environmental changes,which were recorded by the widely distributed black shale of the Upper Ordovician Wufeng and Lower Silurian Longmaxi formations.However,the petrological and geochemical analyses of the Upper Ordovician-Lower Silurian black shale in Well JY1(eastern Sichuan Basin)and Well YY2(western Hubei Province)drill cores show that there are some significant differences with respect to lithology,thickness,and geochemical characteristics despite the long-term similar variation trend in petrology and geochemistry.This may suggest some major environmental changes and their differential influence on black shale deposition in the Yangtze area.Based on the analyses of lithology,total organic carbon(TOC),Mo_(EF),and U_(EF),four long-term sedimentary cycles(one for the Wufeng Formation and three for the Longmaxi Formation)were identified in Well JY1.In Well YY2,only three long-term sedimentary cycles can be recognized(one for the Wufeng Formation and two for the Longmaxi Formation).Different from the gradual increase of the geochemical proxies(such as TOC,MoEF,and NiEF)followed by an abrupt decrease in the Wufeng Formation,these proxies of the Silurian sedimentary cycles in both wells exhibit an abrupt increase at the base followed by a gradual decrease,showing that controlling mechanisms for the Lower Silurian black shale deposition in the study areas were different from those of the Upper Ordovician black shale.However,the black shale of each Silurian cycle in Well JY1 is much thicker than that in Well YY2.Compared with the thick silty mudstone developed in Well JY1 during the Rhuddanian-Aeronian transition,there is a stratigraphic gap in Well YY2 except for a thin calcareous dolostone formed in a strongly restricted environment.These differences indicate that tectonism and glacial eustasy should be the key factors for black shale development,but their influences on the deposition of the Lower Silurian black shale varied between eastern Sichuan Basin and western Hubei province.Since the early Rhuddanian stage,continuous tectonic subsidence in eastern Sichuan Basin promoted the development of thick black shale,while a persistent tectonic uplift in western Hubei Province led to the formation of thin black shale.Meanwhile,the study areas experienced a significant sealevel drop related to intensified glaciation during the Rhuddanian-Aeronian transition,which is recorded by the low chemical index of alteration(CIA)values in both wells.This resulted in the formation of a stratigraphic gap in western Hubei Province and the deposition of thick silty mudstone in eastern Sichuan Basin.Moreover,abrupt increases in CIA,TOC,Mo_(EF),and Ni_(EF) observed in the lower part of the Aeronian black shale in the two wells suggest that eastern Sichuan Basin and western Hubei Province experienced a significant sealevel rise during the early Aeronian stage,which may be related to a global warming event.Subsequently,eastern Sichuan Basin and western Hubei Province both suffered a significant tectonic uplift related to the Kwangsian Movement leading to the termination of the Aeronian black shale formation.However,the thickness variations of the Aeronian black shale in the two study areas indicate that remarkable geomorphological differences persisted in these two areas until the middle Aeronian stage.Thus,this study not only reveals differences of the Upper Ordovician-Lower Silurian black shale in eastern Sichuan Basin and western Hubei Province,but also interprets the causes of these differences,which provide new data and perspectives for the understanding of global paleoclimate and sea-level changes during the Ordovician-Silurian transition.展开更多
基金supported by the National Natural Science Foundation of China(Grant number 42072171)
文摘Various data were utilized to analyze the sedimentary facies,completely reconstruct the palaeogeographic maps,and evaluate the reservoir quality of the Middle-Upper Cambrian Xixiangchi Formation in the Sichuan Basin,including evidence from field outcrops,drilling cores,microscopic thin sections,scanning electron microscopy(SEM),combined with experimental analysis data,such as petrophysical data(porosity(%)and permeability(mD)),and radioactive elements(uranium(^(238)U),thorium(^(232)Th),and potassium(^(40)K),and isotopic data(δ^(13)C)).In the Sichuan Basin,the Middle-Upper Cambrian Xixiangchi Formation was principally deposited in a restricted platform with a lithology predominantly composed of dolomite,with local occurrences of limestone and other rock types in small thicknesses.Graded beddings,cross beddings,horizontal beddings,storm depositions,and mud cracks locally developed in some samples.Four 3rd-order sequences(SQ1-SQ4)were identified within the Xixiangchi Formation in this study.Each sequence is subdivided into a highstand systems tract(HST)and a transgressive systems tract(TST).Reservoirs are principally developed in high-energy grain shoal deposits located in SQ2 and SQ3,with a minor occurrence in SQ1 and SQ4.The lithology of these shoal deposits is essentially composed of sandy dolomite and crystalline dolomite characterized by relatively low average porosity(2.61%)and permeability(1.0073 mD)values.The increase of these values in several studied samples might be related to seepage and connecting dissolution pores and vugs through fractures which have a constructive effect on porosity and permeability.The occurrence of karst caves through superimposed supergene karstification and bedding karstification highly improved the reservoir's physical properties in some areas.The reservoir space is dominated by intergranular dissolution pores,karst caves,and fractures,and their formation was controlled by sedimentation,diagenesis,penecontemporaneous dissolution,and tectonic fractures,which can also be influenced by palaeogeomorphology and sea level fluctuation.Favorable hydrocarbon exploration areas with relatively good source conditions,high positions in the paleostructure,which are conducive to the migration and accumulations of hydrocarbon were identified in three areas of the basin:(1)on the slope around the Central Sichuan Uplift;(2)in the northeastern part,dolomite reservoirs developed with good physical properties and large cumulative thicknesses;and(3)in the southeastern part,shoal facies reservoirs were developed in high and steep structural settings with anticlinal traps.The insights gained in this study fill a significant gap in geological complexities related to sedimentology,palaeogeography,and hydrocarbon reservoir prediction.
基金sponsored by the China National Petroleum Corporation Innovation Found(Grant No.2021DQ02-0101)the Open Foundation of Cooperative Innovation Center of Unconventional Oil and Gas,Yangtze University(Ministry of Education and Hubei Province)(Grant No.UOG 2022-08)+2 种基金the Open Foundation of the Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education,China University of Geosciences(Wuhan)(Grant No.TPR-2021-13)the Natural Gas Geology Key Laboratory of Sichuan Province(Grant No.2021trqdz02)the Hubei Provincial Natural Science Foundation of China(Grant No.2020CFB501).
文摘During the Ordovician-Silurian transition period,the Yangtze area experienced significant environmental changes,which were recorded by the widely distributed black shale of the Upper Ordovician Wufeng and Lower Silurian Longmaxi formations.However,the petrological and geochemical analyses of the Upper Ordovician-Lower Silurian black shale in Well JY1(eastern Sichuan Basin)and Well YY2(western Hubei Province)drill cores show that there are some significant differences with respect to lithology,thickness,and geochemical characteristics despite the long-term similar variation trend in petrology and geochemistry.This may suggest some major environmental changes and their differential influence on black shale deposition in the Yangtze area.Based on the analyses of lithology,total organic carbon(TOC),Mo_(EF),and U_(EF),four long-term sedimentary cycles(one for the Wufeng Formation and three for the Longmaxi Formation)were identified in Well JY1.In Well YY2,only three long-term sedimentary cycles can be recognized(one for the Wufeng Formation and two for the Longmaxi Formation).Different from the gradual increase of the geochemical proxies(such as TOC,MoEF,and NiEF)followed by an abrupt decrease in the Wufeng Formation,these proxies of the Silurian sedimentary cycles in both wells exhibit an abrupt increase at the base followed by a gradual decrease,showing that controlling mechanisms for the Lower Silurian black shale deposition in the study areas were different from those of the Upper Ordovician black shale.However,the black shale of each Silurian cycle in Well JY1 is much thicker than that in Well YY2.Compared with the thick silty mudstone developed in Well JY1 during the Rhuddanian-Aeronian transition,there is a stratigraphic gap in Well YY2 except for a thin calcareous dolostone formed in a strongly restricted environment.These differences indicate that tectonism and glacial eustasy should be the key factors for black shale development,but their influences on the deposition of the Lower Silurian black shale varied between eastern Sichuan Basin and western Hubei province.Since the early Rhuddanian stage,continuous tectonic subsidence in eastern Sichuan Basin promoted the development of thick black shale,while a persistent tectonic uplift in western Hubei Province led to the formation of thin black shale.Meanwhile,the study areas experienced a significant sealevel drop related to intensified glaciation during the Rhuddanian-Aeronian transition,which is recorded by the low chemical index of alteration(CIA)values in both wells.This resulted in the formation of a stratigraphic gap in western Hubei Province and the deposition of thick silty mudstone in eastern Sichuan Basin.Moreover,abrupt increases in CIA,TOC,Mo_(EF),and Ni_(EF) observed in the lower part of the Aeronian black shale in the two wells suggest that eastern Sichuan Basin and western Hubei Province experienced a significant sealevel rise during the early Aeronian stage,which may be related to a global warming event.Subsequently,eastern Sichuan Basin and western Hubei Province both suffered a significant tectonic uplift related to the Kwangsian Movement leading to the termination of the Aeronian black shale formation.However,the thickness variations of the Aeronian black shale in the two study areas indicate that remarkable geomorphological differences persisted in these two areas until the middle Aeronian stage.Thus,this study not only reveals differences of the Upper Ordovician-Lower Silurian black shale in eastern Sichuan Basin and western Hubei Province,but also interprets the causes of these differences,which provide new data and perspectives for the understanding of global paleoclimate and sea-level changes during the Ordovician-Silurian transition.
