Intensely overpressured compartments are present in the centre of the Yinggehai Basin, South China Sea. In this part of the basin, a diapiric area can be distinguished from a non-diapiric area; structures in the former area result from shale diapirism at depth, and from the movement of hydrothermal fluids at more shallow levels. In the diapiric zone, the top of the overpressured compartment is relatively shallow (1,500m to 2,500m deep), whereas it is more than 3,200m deep in the non-diapiric area. The top of the overpressured compartment in the diapiric zone has been raised to relatively shallow levels due to vertical fluid expulsion.

Hydrofracturing has occurred in the Neogene- Quaternary marine sedimentary succession, particularly in mud-rich intervals such as the Lower Member of the Pliocene Yinggehai Formation. Hydrofracturing may have allowed the rapid lateral and vertical migration of enormous volumes of pore fluids. The presence of additional steeply-dipping faults in the diapiric area may have led to vertical expulsion of fluids from overpressured compartments to normally-pressured areas, and caused the top of the overpressured compartment to be uplifted. Gaseous hydrocarbons are assumed to have migrated vertically through fractures around diapiric structures from overpressured to normally-pressured zones. We believe that this mechanism has had a significant effect on hydrocarbon transport and accumulation in the Yinggehai Basin.