Occurrence Mechanisms of Laminated-Type and Sandwich-Type Shale Oil in the Fengcheng Formation of Mahu Sag, Junggar Basin

The shale oil resources of the Permian Fengcheng Formation in the Mahu Sag are abundant, and the types of shale oil reservoirs therein are diverse. However, differences in the reservoir spaces, occurrence states, and source–reservoir configurations of different types of shale oil reservoirs restrict the in-depth understanding of the enrichment and accumulation mechanisms of shale oil in this formation. Thus, using multiple complementary experiments, this study systematically compared laminated-type and sandwich-type shale oil reservoirs (LTSOR and STSOR), in terms of hydrocarbon generation potential, physical characteristics, and oil-bearing properties. These two types of shale oil reservoirs had distinct differences in total organic carbon (TOC), porosity, oil saturation index, and S1, demonstrating differences in accumulation mechanisms. These two types of reservoirs were mainly dominated by inorganic intergranular pores and dissolution pores, and the storage capability of STSOR was much better than that of LTSOR. Specifically, STSOR mainly developed macropores, whereas LTSOR developed macropores and mesopores. The free oil content in the STSOR was found to be higher than that of the adsorbed oil, whereas the free and adsorbed oil contents in the LTSOR were similar. Furthermore, the shale oil occurrence in the STSOR was mainly affected by pore structure and mineral composition, whereas the shale oil occurrence in the LTSOR was affected by pore structure, mineral composition, TOC content, and sedimentary structure. The source–reservoir configuration relationship of STSOR was one where the upper and lower shale layers were source rocks, whereas the intercalated siltstone layers were reservoirs. The source–reservoir configuration of the LTSOR involved bright laminae as reservoirs and dark laminae as source rocks (i.e., integrated source and reservoir). Hydrocarbon accumulation models were established for the STSOR and LTSOR that involved source–reservoir configurations, occurrence states, and migration directions.