Coupling of abnormal overpressure and hydrocarbon charging: A case from the Huagang and Pinghu formations of Xihu Sag, East China Sea Shelf Basin

Abnormal overpressure in the Huagang and Pinghu formations of the Xihu Sag, East China Sea Shelf Basin was studied based on sonic logging data, organic geochemistry and fluid inclusion testing. The abnormal pressure mainly originated from the lower Huagang Formation, and featured a "step type" distribution in the Xihu Sag. The top interface of high pressure becomes older from the Pinghu tectonic zone to the central inversion structural belt and from the edge to the center of the sag. Conductive overpressure due to the injection of high pressure fluid developed in the study area, and corresponds to good oil and gas reservoirs, which showed that the development of abnormal pressure and hydrocarbon charging had consistency in space. The evolution process of residual formation pressure was simulated using the basin simulation software, indicating that Miocene (15-7 Ma) and Pliocene to Quaternary (3-0 Ma) were the two key periods of residual formation pressure. Hydrocarbon charging history showed that hydrocarbon charging took place twice in the middle-late Miocene and the early Pliocene, which indicated that the development of abnormal pressure and hydrocarbon charging had consistency in time. Therefore, the abnormal overpressure of the Huagang and Pinghu formations coupled in time and space with hydrocarbon charging.