Lamina-scale diagenetic mass transfer in lacustrine organic-rich shales and impacts on shale oil reservoir formation

Abstract

Lacustrine organic-rich shales have recently become important petroleum exploration targets. Adequate reservoir characterization is vital for determining the potential for shale oil exploration and development. Fluid–rock interaction and diagenetic mass transfer in organic-rich shales are essential processes during shale oil reservoir formation. Based on detailed petrographic investigation, in situ element and isotope measurement, and organic geochemistry analysis, diagenetic mass transfers and related organic–inorganic interactions were investigated using a suite of organic-rich shales from the Triassic Yanchang Formation of the Ordos Basin. Organic-rich shales consist of silt-sized felsic laminae and organic-rich laminae. Silt-sized felsic laminae are dominated mainly by K-feldspar, whereas illite is the most abundant mineral in organic-rich laminae. Authigenic quartz and euhedral pyrite are the major diagenetic minerals in organic-rich laminae, whereas K-feldspar dissolution occurs extensively in silt-sized felsic laminae. Smectite-to-illite conversion has played a significant role in the diagenetic alteration of organic-rich shales. This reaction not only induced overpressure to generate microfractures for authigenic quartz growth but it also provided the required silica source for authigenic quartz precipitation. Petrographic and geochemical evidence indicates that organic acids generated in organic-rich laminae have migrated to silt-sized felsic laminae, and K+ and aqueous SiO2 yielded from K-feldspar dissolution in silt-sized felsic laminae have been transported to organic-rich laminae. Based on organic–inorganic interactions related to authigenic quartz formation, we conclude that the lamina-scale open diagenetic system allows mass transfer to occur at the microscale within shales. The lamina-scale diagenetic mass transfer and material redistribution may contribute significantly to effective pore space formation in shales.