DISTRIBUTION OF EVAPORITES AND OVER-PRESSURED RESERVOIRS IN THE SHAHEJIE FORMATION IN THE EASTERN WENLIU AREA, DONGPU DEPRESSION, BOHAI BAY BASIN

Two thick sequences of evaporites consisting of gypsum and halite occur in the Paleogene Shahejie Formation of the eastern Wenliu Area, Dongpu Depression, Bohai Bay Basin. On the plane view, the distribution pattern of the evaporites is different from region to region, mainly depending upon the frequently shifting of depocenters with time. Statistics of the measured pressures suggest that the formation pressure of the Shahejie Formation shows a "double layer structure" consisting of upper normal pressure and lower abnormal high pressure. Very high abnormal pressure mainly occurs in the middle and lower part of the 3rd member of the Shahejie Formation. The top surface of the over-pressured zone ranges from 2 900 to 3 100 m in depth and gradually rises up from the west to the east. Formation pressure is closely related to the tectonic structure and depocenter. The occurrence of abnormal pressure matches well with distribution of the evaporite rocks. It suggests that the evaporite sequences act as seals of both pressure and physical property. The double seal mechanism is the key element to the formation of the abnormal pressure in the Dongpu Depression. The evaporite sequences and overpressure have great significance to the origin of hydrocarbon reservoir and the oil and gas accumulation.