Overpressure development and hydrofracturing in the Yinggehai Basin, SouthChina Sea

Intensely overpressured compartments are present in the centre of the Yingg ehai 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 r esult from shale diapirism at depth, and from the movement of hydrothermal fluids at more shallow levels. In the diapiric zone, the top of the overpre ssured compartment is relatively shallow (1,500m to 2,500m deep), whereas i t is more than 3,200m deep in the non-diapiric area. The top of the overpre ssured compartment in the diapiric zone has been raised to relatively shall ow levels due to vertical fluid expulsion. Hydrofracturing has occurred in the Neogene-Quaternary marine sedimentary s uccession, particularly in mud-rich intervals such as the Lower Member of t he Pliocene Yinggehai Formation. Hydrofracturing may have allowed the rapid lateral and vertical migration of enormous volumes of pore fluids. The pre sence of additional steeply-dipping faults in the diapiric area may have le d to vertical expulsion of fluids from overpressured compartments to normal ly-pressured areas, and caused the top of the overpressured compartment to be uplifted. Gaseous hydrocarbons are assumed to have migrated vertically t hrough fractures around diapiric structures from overpressured to normally- pressured zones. We believe that this mechanism has had a significant effec t on hydrocarbon transport and accumulation in the Yinggehai Basin.