Relationship between natural gas accumulation and overpressure in Xujiahe Formation, Xiaoquan—Fenggu structural belt, Western Sichuan Depression

The Western Sichuan Depression is a typical overpressure foreland basin.The forming mechanism of overpressure and its relationship with natural gas accumulation still remain discussing in researches.Interval transit time has been applied to calculate formation pressures of 29 wells in the Xiaoquan—Fenggu structural belt.Combined with the measured formation pressures of 37 wells,it has been concluded in this paper the forming mechanism and classification of overpressure as well as the relationship between overpressure and natural gas accumulation and preservation.1) Obvious overpressure has been found in Xujiahe Formation of the Xiaoquan—Fenggu structural belt.The 2nd and the 4th members of Xujiahe Formation belong to different pressure system.The neighbor-source overpressure mechanism is obvious.2) Undercompaction and hydrocarbon generation effects are the main causes for overpressure in the study area.3) 3 types of overpressure distribution have been found,including the Xinchang,the Hexingchang and the Fenggu ones.4) Natural gas accumulation is closely related to overpressure in Xujiahe Formation of the Xiaoquan—Fenggu structural belt.The PLS (pressure leakage strata) close to the top of OPC (overpressured compartment) is rich in natural gas.5) Studies of residual pressure for caprock breakthrough have indicated that the 2nd member of Xujiahe Formation is favorable for natural gas pre-servation all around the study area while the 4th member is unfavorable for preservation in the northeast of Xinchang and the southeast of Fenggu.