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OVERPRESSURE SYSTEM AND HYDROCARBON ACCUMULATION IN THE YINGGEHAI BASIN

Zhang Qiming et al. (China Offshore Oil Nanhai West Corporation, Guangdong)  
The Yinggehai basin is a Cenozoic sedimentary basin developed on the passive continental margin of the northern part of the South China Sea. It is a young, fast subsiding Tertiary marine basin with thick sediments, where abnormal high temperatures and high pressures (HTHP) have been encountered and mud diapirs were well developed. Presented in this paper are isopach maps of the tops of abnormal overpressure and charts of the transformation of pressure coefficients of horizons of the whole basin, which were derived by using improved trend lines and formation pressures converted from seismic velocity. It was discovered through mapping that the abnormal overpressure system in Yinggehai basin is in fact a HTHP packet floating in the basin. It can be seen from the isopach maps of the tops of abnormal overpressure that it is shallower and thicker in the central part of the basin, and deeper and thinner towards the margin of the basin before eventually disappearing. Additionally, a paleo-overpressure interval has been discovered which is inconsistentwith the present day one. We believe that the interval between the paleo and present overpressure intervals was an energy release zone of paleogeotherm and paleopressure. All of the gas fields recently discovered in the basin occur within this zone, belonging to the petroleum system above the overpressure system. (The petroleum system above the HTHP packet in Yinggehai basin is called the upper petroleum system, below it called the lower petroleum system). The HTHP packet with its associated mud diapirs contain the source rocks for the upper petroleum system. The pressure of the HTHP packet changed continuously with the hydrothermal and young fluid supercharges. When the pressure increased and broke the overlying formations, hydrocarbons were released intermittently from the HTHP packet and migrated mainly in water phase from a high potential field towards the surrounding low potential field. The energy release zone allowed hydrocarbons to migrate vertically along mud diapirs and gas chimneys fromothe HTHP packet. Horizontally, hydrocarbons migrated from the HTHP packet into the energy release zone through large faults and unconformities , i. e. migrated into and accumulated in fault-structural zones along the margin of the basin and into large deltaic sand bodies. Therefore, it is important to accurately identify traps within the energy release zone which is would accumulate hydrocarbon.
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