Geochemical characteristics of lacustrine shale and enrichment mechanism of organic matter in Zhanhua Sag, Bohai Bay Basin

The lacustrine shale in the lower part of the third member of Shahejie Formation (Es₃) in well L69 in the Zhanhua Sag of Bohai Bay Basin was studied. The geochemical characteristics of lacustrine shale in different system tracts were analyzed in a framework of sequence stratigraphy. The evolution process of sedimentary environment and the enrichment mechanism of organic matter were clarified, which provided theoretical support for the identification of favorable targets of unconventional shale oil. The combination of logging and lithology reveals that the sequence of the lower Es³ can be divided into transgression, early highstand and late highstand system tracts from bottom to top. Productivity and preservation conditions are the controlling factors of organic matter enrichment. The higher productivity and better preservation conditions together determine that the early highstand system tract is the most favorable organic matter accumulation interval, which provides a good material basis for a good shale oil development target.