Oil accumulation process of the Kongdian reservoir in the deep subsag zone of the Cangdong Sag, Bohai Bay Basin, China

Abstract The hydrocarbon system of the second member of the Kongdian Formation (E k 2 ) in the deep subsag zone of the Cangdong Sag, Bohai Bay Basin, has recently witnessed exploration success. However, the oil accumulation process of the E k 2 reservoir in this deep subsag zone remains poorly understood. This lack of knowledge restricts our comprehension of the hydrocarbon system in the deep subsag zone and further hydrocarbon exploration efforts. In this study, the oil accumulation process of the Kongdian reservoir in the deep subsag zone of the Cangdong Sag was investigated by examining the origin of oil, oil generation history, and timing of oil charge. The E s 3 source rocks were deposited in a freshwater, anoxic to sub-oxic environment with a mixed input of aquatic organisms and terrigenous organic matter. In contrast, the E k 2 source rocks were deposited in a hypersaline, anoxic to sub-oxic environment with a substantial input of green algae. Analyses revealed that the oil in the deep subsag zone of the Cangdong Sag originated from the E k 2 source rocks. The dominant stage of intense oil generation for the E k 2 source rocks took place from 48 to 25 Ma. This process was evidently controlled by the rapid subsidence and temperature increase of the E k 2 Formation. During this dominant oil generation stage, the maximum oil generation rate of the E k 2 source rocks reached approximately 76 mg/g * TOC * my. The major episode of oil charge in the E k 2 reservoir occurred around 45–40 Ma, indicating an early oil accumulation event in the deep subsag zone of the Cangdong Sag. Owing to the underdeveloped fracture system, the oil generated from the E k 2 source rocks in the deep subsag zone of the Cangdong Sag could charge into the adjacent E k 2 reservoir. This proximity avoided the dispersal of oil and the readjustment of oil distribution caused by long-distance migration, thereby facilitating the formation of large lithologic reservoirs.