Distribution Characteristics and Genesis of Overpressure in K Structure of Baochu Slope in Xihu Sag

More than half of the oil and gas discoveries in the Middle-Upper Eocene Pinghu Formation of K structure in Baochu slope zone of Xihu Sag have the characteristics of overpressure, which is closely related to oil and gas accumulation in this area. Based on the analysis of cast thin section, physical properties, vitrinite reflectance, drilling and logging data, the distribution characteristics of overpressure in K structure are analyzed, and the causes of overpressure in middle-high zone, middle-low zone and low zone are clarified. The overpressure of the K structure is mainly developed in the middle-lower part of Pinghu Formation. While the burial depth of the top interface of overpressure increases gradually from the middle-high zone to the low zone. The predicted pressure coefficient of mudstone is slightly greater than the measured pressure coefficient of sandstone in the middle-high zone. The two pressure coefficients are equivalent in the middle-low zone, and opposite in the low zone. The main genesis of overpressure in the middle-high zone of the study area is disequilibrium compaction, the middle-low zone is the composite cause of disequilibrium compaction and fluid expansion, and the main cause of abnormal high pressure in the low zone is fluid expansion. Both hydrocarbon generation pressurization and clay mineral dehydration pressurization contribute to the overpressure in the middle-low and low zone. The overpressure zone is favorable for oil and gas enrichment.