Geological Controlling Factors of Oil Content in the Paleogene Kongdian Formation, Bohai Bay Basin

A thorough understanding of the roles and controlling factors for shale oil enrichment is a prerequisite for subsequent exploration and development. The oil-bearing properties of the second member of the Kongdian Formation (Ek(2)) in the Cangdong Sag, Bohai Bay Basin was evaluated. Total oil is obtained by the combination of thermal evaporation and solvent extraction. The main controlling factors of the shale oil content are systematically analyzed by combining experimental data (organic geochemical and nanopetrophysical techniques) and theoretical calculation [petroleum expulsion efficiency (PEE) estimation]. The research results show that the oil-bearing properties are jointly controlled by five main factors, namely, organic matter richness, organic matter types, lithofacies, pore characteristics, and PEE. Among them, organic matter richness in the source rock determines the lower limit of shale oil content; organic matter type is important because shales with different organofacies are characterized by varying petroleum generation kinetics; laminated felsic shale provides more ideal storage spaces for the enrichment of shale oil, and the spatial characteristics of the shale pore define the upper limit of oil content; in addition, PEE, which is collectively influenced by the previous four factors, is negatively correlated to the amount of shale oil retained in the shale. This work illustrates the impact of a series of organic and inorganic factors on shale oil content from both macro- and micro perspectives, which has reference values for research areas with similar geological backgrounds.