Controls on Oil Accumulation in the Northern Bozhong Depression, Offshore Bohai Bay Basin: Insights From Oil–Source Correlation, Petroleum Generation Kinetics, and Overpressure Systems

The northern Bozhong Depression in the offshore Bohai Bay Basin exhibits a pronounced contrast in petroleum accumulation: giant oilfields occur in the uplift zone, whereas only non‐commercial oil‐bearing structures have been discovered in the adjacent sag zone. The principal controls on this contrasting accumulation remain poorly constrained. These controls were investigated by integrating oil‐source correlation, petroleum generation kinetics, basin modelling, and overpressure characterization. Three oil families are identified based on biomarker signatures and thermal maturity. Family I oils in the uplift zone are biodegraded, as indicated by the presence of 25‐norhopanes, have the lowest thermal maturity, and are derived from both the third member of the Shahejie Formation (Es 3 ) and the first member of the Shahejie Formation (Es 1 ) source rocks. Family II oils in the sag zone are non‐biodegraded, display moderate thermal maturity, with the third member of the Dongying Formation (Ed 3 ) source rocks as their primary source and the Es 1 source rocks as the secondary contributor. Family III oils in the steep slope zone have the highest thermal maturity and are predominantly derived from the Es 3 source rocks. Kinetic analysis revealed distinct petroleum generation behaviours among the three source rock intervals. The shallowly buried Ed 3 source rocks are characterized by relatively high activation energy, resulting in insufficient overall hydrocarbon generation potential. The Es 1 source rocks, which have the lowest generation activation energy, had limited remaining oil generation potential by 5.3 Ma. In contrast, the deeply buried Es 3 source rocks exhibit sustained and substantial petroleum generation capacity. Overpressure analysis showed that the Ed 3 source rocks are characterized by pressure coefficients of up to 1.5, with hydrocarbon generation‐related fluid expansion likely contributing significantly to the overpressure development, suggesting inefficient hydrocarbon expulsion. Conversely, the Es 1 and the upper part of the Es 3 interval exhibit normal pressure to weak overpressure, indicating efficient expulsion. The prolific oil accumulation in the uplift zone is therefore attributed to efficient expulsion from the Es 3 and Es 1 source rocks, coupled with effective vertical migration along syn‐sedimentary boundary faults. In contrast, limited petroleum accumulation in the sag zone is attributed to the combined effects of inefficient expulsion from the overpressured Ed 3 source rocks, limited late‐stage generation potential of the Es 1 source rocks, and insufficient connectivity between post‐depositional faults and the deeply buried Es 3 kitchen. These findings suggest that future exploration in the sag zone should target areas where the Ed 3 source rocks have attained higher thermal maturity, accounting for their expulsion constraints.