Mechanism of hydrocarbon accumulation in Oujiang Sag, the East China Sea Shelf Basin

Using seismic, logging and geochemical data combined with basin modeling, this paper discussed tectonic evolution, paleotemperature and overpressure evolution in Oujiang Sag. The results show that tectonic evolution stage of the Oujiang Depression experiences a rift, depression and regional deposition, which controls the development and distribution of depositional systems. It deposits three sets of source rocks, Yue Guifeng Formation, Lingfeng Formation and Mingyuefeng Formation, in which the Yueguifeng Formation is the main source rocks. Paleotemperature field of the study area has experienced the evolution of burial warming-uplift cooling-burial warming, and the paleo-temperature of west subsag is higher than the east subsag. Because of this thermal evolution difference, the source rocks maturity in the west subsag is higher than that in the east subsag. Abnormal high pressure experiences a supercharge-decompression-supercharge process in Oujiang Sag. Abnormal high pressure can provide a driving force for oil and gas migration. The abnormal high pressure decompression process occurs in Late Eocene, which is the importance time of oil and gas migration. The coupling of the tectonic evolution, the accumulation of the dynamic pressure and release, heat flow and hydrocarbon controls the formation of oil-gas reservoir in Oujiang Sag.