Occurrence state of lacustrine shale oil in the second member of the Paleogene Kongdian Formation, Cangdong Sag, Bohai Bay Basin

Understanding the occurrence state of shale oil is crucial for the effective development of shale oil resources. Although the second member of the Kongdian Formation (Ek₂) is a key interval for lacustrine shale oil production in the Cangdong Sag, Bohai Bay Basin, the occurrence state and controlling factors of shale oil in this formation remain poorly understood. This study established a multi-step programmed pyrolysis, combined with a light hydrocarbon recovery scheme, to quantitatively characterize the shale oil in different occurrence states. An integrated approach utilizing Rock-Eval pyrolysis, pyrolysis-gas chromatography, and crude oil gas chromatography was employed. Factors influencing the shale oil occurrence state were analyzed from petrology and organic geochemistry perspectives. The study revealed significant variations of shale oil occurrence states within the Ek₂, attributed to differences in sedimentary organic matter, mineral compositions, sedimentary structures, and thermal maturity. Felsic laminae are the primary reservoir space for oil in laminated shales, and the frequent interbedding of felsic and organic-rich laminae facilitates the retention of free oil. The contents of free and adsorbed oil are primarily influenced by organic matter content and shale storage capacity, both of which exhibit distinct occurrence patterns. Based on the shale reservoir quality classification using the pyrolysis values of S_1-1 + S_1-2 and (S_1-1 + S_1-2) × 100/TOC, the Ek₂ shale demonstrates significant exploitation potential, with the first-level reservoirs comprising 66%, second-level reservoirs 11%, and third-level reservoirs 23%. These findings provide new insights into the geological accumulation and production of shale oil.