Influence of Overpressured Fluid Expulsion on Hydrocarbon Accumulation in Deep-Buried Miocene Reservoirs of the Yinggehai Basin, Northwestern South China Sea

The overpressure record of the Cenozoic rifting basins in China was recently renewed in the Miocene reservoirs of the Yinggehai Basin, which became an ideal natural laboratory for revealing the hydrocarbon accumulation mechanism under the ultrahigh overpressure setting. Using 3D seismic data, two new fluid expulsion structures were identified in the Yinggehai Basin: layer-bound faults and pipes. Layer-bound faults were primarily generated at similar to 10.5 Ma and subsequently served as vertical channels for hydrocarbon migration. Pipes can be divided into three episodes, generated at similar to 5.5, similar to 2.4 and < 1.9 Ma, caused by hydraulic fracturing during focused flow in highly overpressured reservoirs. Pipes generated simultaneously with hydrocarbon charging indicate that the aqueous pressure before hydrocarbon charging did not reach the threshold of hydraulic fracturing and could result in dynamic hydrocarbon accumulation. In contrast, pipes generated before and after hydrocarbon charging increased the unfilling and draining risks of the traps, respectively.