Overpressure Features and Its Influences on Hydrocarbon Accumulation in the W Structure of West Slope of Xihu Sag

Overpressure feature was analyzed based on drilling data, logging data and analysis assay data in the W structure of west slope of Xihu Sag, distribution of overpressure compartment and origin of overpressure were clarified, the influences on hydrocarbon accumulation were summarized by studying the thermal evolution of hydrocarbon source rock in overpressure compartment, characteristics of reservoir diagenesis, and features of hydrocarbon migration and accumulation. The results show that the overpressure compartment was mainly distributed in lower member of Pinghu Formation with the thickness of 200～550 m. The mudstone overpressure system was widely distributed in W structure while the reservoir overpressure system was only distributed in the central and south parts of study area with a maximum pressure coefficient of 1.5. Origin of mudstone overpressure was mainly determined as disequilibrium compaction and hydrocarbon-generating pressurization by log data analyses, acoustic velocity-density plots, and geological background analysis. Origin of reservoir overpressure was mainly determined as pressure transmission, which could be from Santan Sub-sag. The influences of overpressure on hydrocarbon accumulation are mainly manifested as: overpressure inhibits kerogen thermal degradation in source rocks and widens oil generation window; overpressure inhibits reservoir compaction, promoted reservoir cementation and reservoir dissolution; overpressure provides migration forces of oil and gas and large lithologic reservoirs can be formed in overpressure compartment.