ABSTRACT

This study analyzed crude oils from the lower part of the third member of the Eocene Shahejie Formation (Es3L) and three prospective source rocks from the Shulu sag, Bohai Bay basin, eastern China, using a variety of organic geochemical methods. Biomarker characteristics were used to interpret source rock organic matter input and depositional environment, and oil–source rock correlation. The biomarker data indicate that the crude oils originated from the Es3L source rock, which contains a mixture of plankton and land plant organic matters deposited in brackish–fresh water under reducing conditions. The oil in the Es3L is self-sourced instead of migrated from the overlying source rocks. The petroleum generation potential of the Es3L source rock was evaluated using organic geochemistry. Total organic carbon (TOC) values for approximately 100 samples are between 1.02 and 4.92 wt. %, and hydrogen indices range from 285 to 810 mg hydrocarbons/g TOC. The Es3L source rock contains mainly type II and III kerogen, and most of the samples are thermally mature. The data show that the Es3L source rock has good potential for liquid hydrocarbon generation. The Es3L rock also acts as the oil reservoir, having very low bulk porosity and permeability. Various types of storage space in the marlstone and carbonate rudstone in the Es3L of the Shulu sag include (1) fractures, (2) intergranular pores, (3) dissolution pores, (4) organic matter pores, (5) intragranular pores, and (6) seams around gravels. Pore size ranges from nanometers to millimeters. Because the oil was generated and stored in Es3L strata, which lack any obvious trap and seal and have low permeability, the unit represents a continuous petroleum accumulation.

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