Sedimentary evolution and shale oil potential of Shahejie Formation in Zhanhua sag, Bohai Bay Basin, China

ABSTRACT It is important to accurately classify shale lithofacies and identify their origin and hydrocarbon content for both shale oil and gas reservoirs. Oil-bearing lacustrine shales (average total organic carbon [TOC] content = 3.12 wt. %, average vitrinite reflectance = 0.81%) of the lower third member of the Eocene Shahejie Formation in the Zhanhua sag, Bohai Bay Basin contain eight lithofacies that are differentiated based on mineralogy, TOC content, grayscale, elemental composition, and Rock-Eval pyrolysis data. The results show that four lithofacies are most common: moderately organic-rich laminated calcareous shale, organic-rich laminated calcareous shale, and organic-rich calcareous/massive mixed shale. Paleoenvironment controls lithofacies evolution. Unit 1 (arid) is characterized by limited detrital/nutrient inflow and low water energy, coupled with transgression-induced high salinity suppressed freshwater bioproductivity, favoring moderately enriched laminated calcareous shale. Unit 2 (transition) shows increased productivity promoted organic-rich laminated calcareous shale. Unit 3 (humid) exhibits nutrient and detrital input (TOC undiluted) in the lake increases; however, the destruction of the quiet water environment leads to the gradual disappearance of laminae, shifting lithofacies to organic-rich massive calcareous and organic-rich massive mixed shales, moderately enriched laminated calcareous shale, organic-rich massive calcareous shale, and organic-rich massive mixed shale that have high shale oil potential. Two high-brittleness and high-oil sweet-spot intervals were identified (2988 ∼ 3012 m and 3108 ∼ 3124 m in well A): interval 2 is enriched in situ hydrocarbon via high TOC and large storage; interval 1, despite lower TOC, accumulated migrated hydrocarbons via micromigration and large reservoir space.