Lamina combination characteristics and differential shale oil enrichment mechanisms of continental organic-rich shale: A case study of Triassic Yanchang Formation Chang 73 sub-member, Ordos Basin, NW China

The lamina (combination) types, reservoir characteristics and shale oil occurrence states of organic-rich shale in the Triassic Yanchang Formation Chang 7₃ sub-member in the Ordos Basin were systematically investigated to reveal the main controlling factors of shale oil occurrence under different lamina combinations. The differential enrichment mechanisms and patterns of shale oil were discussed using the shale oil micro-migration characterization and evaluation methods from the perspectives of relay hydrocarbon supply, stepwise migration, and multi-stage differentiation. The results are obtained in five aspects. First, Chang 7₃ shale mainly develops five types of lamina combination, i.e. non-laminated shale, sandy laminated shale, tuffaceous laminated shale, mixed laminated shale, and organic-rich laminated shale. Second, shales with different lamina combinations are obviously different in the reservoir space. Specifically, shales with sandy laminae and tuffaceous laminae have a large number of intergranular pores, dissolution pores and hydrocarbon generation-induced fractures. The multi-scale pore and fracture system constitutes the main place for liquid hydrocarbon occurrence. Third, the occurrence and distribution of shale oil in shale with different lamina combinations are jointly controlled by organic matter abundance, reservoir property, thermal evolution degree, mineral composition and laminae scale. The micro-nano-scale pore-fracture networks within shales containing rigid laminae, particularly sandy and tuffaceous laminations, primarily contain free-state light hydrocarbon components. In contrast, adsorption-phase heavy hydrocarbon components predominantly occupy surfaces of organic matter assemblages, clay mineral matrices, and framework mineral particulates. Fourth, there is obvious shale oil micro-migration between shales with different lamina combinations in Chang 7₃. Generally, such micro-migration is stepwise in a sequence of organic-rich laminated shale → tuffaceous laminated shale → mixed laminated shale → sandy lamiated shale → non-laminated shale. Fifth, the relay hydrocarbon supply of organic matter under the control of the spatial superposition of shales with various laminae, the stepwise migration via multi-scale pore and fracture network, and the multi-differentiation in shales with different lamina combinations under the control of organic-inorganic interactions fundamentally decide the differences of shale oil components between shales with different lamina combinations.