Lamina-scale diagenetic mass transfer in lacustrine organicrich shales and impacts on shale oil reservoir formation
ABSTRACT
Lacustrine organic-rich shales have recently become importantpetroleum exploration targets. Adequate reservoir characterizationis vital for determining the potential for shale oil explorationand development. Fluid–rock interaction and diagenetic masstransfer in organic-rich shales are essential processes during shaleoil reservoir formation. Based on detailed petrographic investigation,in situ element and isotope measurement, and organicgeochemistry analysis, diagenetic mass transfers and relatedorganic–inorganic interactions were investigated using a suite oforganic-rich shales from the Triassic Yanchang Formation of theOrdos Basin. Organic-rich shales consist of silt-sized felsic laminaeand organic-rich laminae. Silt-sized felsic laminae are dominatedmainly by K-feldspar, whereas illite is the most abundantmineral in organic-rich laminae. Authigenic quartz and euhedralpyrite are the major diagenetic minerals in organic-rich laminae,whereas K-feldspar dissolution occurs extensively in silt-sizedfelsic laminae. Smectite-to-illite conversion has played a significantrole in the diagenetic alteration of organic-rich shales. Thisreaction not only induced overpressure to generate microfracturesfor authigenic quartz growth but it also provided therequired silica source for authigenic quartz precipitation. Petrographicand geochemical evidence indicates that organic acidsgenerated in organic-rich laminae have migrated to silt-sized felsiclaminae, and K+ and aqueous SiO2 yielded from K-feldspardissolution in silt-sized felsic laminae have been transported toorganic-rich laminae. Based on organic–inorganic interactionsrelated to authigenic quartz formation, we conclude that thelamina-scale open diagenetic system allows mass transfer to occur at the microscale within shales. The lamina-scale diageneticmass transfer and material redistribution may contributesignificantly to effective pore space formation in shales.
ABSTRACT
Lacustrine organic-rich shales have recently become importantpetroleum exploration targets. Adequate reservoir characterizationis vital for determining the potential for shale oil explorationand development. Fluid–rock interaction and diagenetic masstransfer in organic-rich shales are essential processes during shaleoil reservoir formation. Based on detailed petrographic investigation,in situ element and isotope measurement, and organicgeochemistry analysis, diagenetic mass transfers and relatedorganic–inorganic interactions were investigated using a suite oforganic-rich shales from the Triassic Yanchang Formation of theOrdos Basin. Organic-rich shales consist of silt-sized felsic laminaeand organic-rich laminae. Silt-sized felsic laminae are dominatedmainly by K-feldspar, whereas illite is the most abundantmineral in organic-rich laminae. Authigenic quartz and euhedralpyrite are the major diagenetic minerals in organic-rich laminae,whereas K-feldspar dissolution occurs extensively in silt-sizedfelsic laminae. Smectite-to-illite conversion has played a significantrole in the diagenetic alteration of organic-rich shales. Thisreaction not only induced overpressure to generate microfracturesfor authigenic quartz growth but it also provided therequired silica source for authigenic quartz precipitation. Petrographicand geochemical evidence indicates that organic acidsgenerated in organic-rich laminae have migrated to silt-sized felsiclaminae, and K+ and aqueous SiO2 yielded from K-feldspardissolution in silt-sized felsic laminae have been transported toorganic-rich laminae. Based on organic–inorganic interactionsrelated to authigenic quartz formation, we conclude that thelamina-scale open diagenetic system allows mass transfer to occur at the microscale within shales. The lamina-scale diageneticmass transfer and material redistribution may contributesignificantly to effective pore space formation in shales.
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