Lithofacies and reservoir characteristics of saline lacustrine fine-grained sedimentary rocks in the northern Dongpu Sag, Bohai Bay Basin: Implications for shale oil exploration

Recent economic production of shale oil in the lacustrine basin has prompted the reevaluation of fine-grained sedimentary rocks (FGSRs) in the Dongpu Sag, Bohai Bay Basin. In this study, FGSRs in the Eocene Shahejie Formation were examined using petrological, geochemical, and quantitative pore size characterization analyses to clarify lithofacies classification, sedimentary environment, controls of reservoir quality, and shale oil potential. The classification of lithofacies was based on total organic carbon content, sedimentary structure, and mineral content. The laminae formation processes and distribution indicate that the FGSRs were deposited in semi-deep to deep anoxic lacustrine environments with humid to semiarid climates. Laminated and layered FGSRs primarily comprise macropores and mesopores, accounting for an average of 68.02% and 27.33%, respectively, whereas micropores account for 4.65%. The macropores show a positive relationship with the carbonate mineral content and primarily comprise interparticle pores between recrystallized calcite and parallel interlayer microfractures. The mesopores are controlled by the content of clay and quartz-feldspathic minerals, revealing that they are primarily interparticle pores between rigid and soft grains. Micropores are dominated by crystalline pores in clay mineral. A low sedimentation rate is available for organic matter (OM) enrichment. The lithofacies containing recrystallized calcite laminae are high-quality shale oil reservoirs. The OM abundance, hydrocarbon generation potential, reservoir quality, and brittleness collectively highlight that OM-rich and moderately laminated calcareous mudstone and calcareous fine-grained mixed sedimentary rocks should be the focus of shale oil exploration. The results have important implications for shale oil development of the similar saline lacustrine basin worldwide.