Organic geochemical characteristics of shale in the lower sub-member of the third member of Paleogene Shahejie Formation (Es3L) in Zhanhua Sag, Bohai Bay Basin, eastern China: significance for the shale oil-bearing evaluation and sedimentary environment

Organic geochemical compositions of shale are the key parameters for evaluating the potential of lacustrine shale oil. Systematic organic geochemical analyses (e.g., total organic carbon (TOC), Rock–Eval pyrolysis, chloroform bitumen “A,” group component, kerogen maceral, and saturated hydrocarbon chromatography) have been carried out on the shale in the lower sub-member of the third member of Eocene Shahejie Formation (Es3L) in the Well L69 from the Zhanhua Sag of Bohai Bay Basin, in order to constrain its sedimentary environment and evaluate the potential of lacustrine shale oil. The shale in the Es3L has TOC content in the range from 0.52% to 9.32%, free hydrocarbon (S1) from 0.03 mg/g to 13.12 mg/g, thermally-cracked hydrocarbon (S2) from 1.19 mg/g to 78.59 mg/g, hydrocarbon generation potential (S1 + S2) from 1.77 mg/g to 82.65 mg/g, chloroform bitumen “A” from 0.2515% to 2.1422%, and total hydrocarbon (HC) from 1630 μg/g to 10,500 μg/g, respectively. The kerogen is mainly composed of type I and type II1 sapropel, whereas the parent material is dominated by lower aquatic organisms. The original total organic carbon (TOCo), oil saturation index (OSI), and total oil (S’) vary from 1.23% to 11.33%, 3 mg oil/g TOC to 258 mg oil/g TOC, and 21.98 bbl oil/ac-ft to 980.38 bbl oil/ac-ft, respectively. The aforementioned organic geochemical compositions indicate that shale in the Es3L is characterized by high organic matter abundance, high paleoproductivity, and good organic matter type, with early maturation to peak maturation. The characteristics of saturated hydrocarbon chromatography indicate that the lake evolved from strong reducing brackish-saline water to weak reducing-weak oxidizing fresh water in the sedimentary period of Es3L in the Zhanhua Sag, providing favorable conditions for high accumulation and preservation of original organic matter. Comprehensive evaluation from these organic geochemical compositions suggests that the 3061–2983 m interval in the Well L69 has high oil content and good potential to further explore the shale oil. This provides good implications for the optimization of favorable intervals for shale oil exploration in the Bohai Bay Basin.