The Overpressure Genesis,Evolution Characteristics,and Preservation Control Of The X Area In Western Sichuan,Xujiahe Formation

In order to clarify the causes and evolution characteristics of overpressure in X area of western Sichuan and its influence on oil and gas preservation,the causes of overpressure in the target layer of the study area were analyzed by using logging data,drilling data,oil and gas test data,logging data,measured pressure and other analysis data,and various overpressure identification charts were combined with the actual geological conditions to clarify the causes of overpressure.Based on the recovery of the formation pressure evolution process and the tectonic stress distribution in the key period of the study area through numerical simulation,the hydraulic fracture of the caprock,the fracture opening pressure of the caprock,and the overpressure sealing ability of the caprock are studied,and the influence of overpressure on the preservation ability of the caprock in the study area is comprehensively analyzed.The results show that hydrocarbon generation pressurization,undercompaction,tectonic compression and pressure transfer are the main causes of overpressure in the study area.The source rocks of the first,third and fifth members of Xu1、Xu3、Xu5 Formation are mainly undercompaction,hydrocarbon generation pressurization and tectonic compression,while the reservoirs of the fourth and fifth members of Xu2 、 Xu4 Formation are mainly tectonic compression and overpressure transfer.In the middle and late Yanshan movement from the late Jurassic to the Cretaceous,the strata in the study area subsided rapidly and the temperature rose rapidly.At the same time,it was subjected to relatively strong tectonic compression.The source rocks of the first and third members of the Xu1、Xu3 Formation in the study area were formed under the combined action of hydrocarbon generation,undercompaction and tectonic compression.The formation overpressure has increased significantly and reached its peak in the Middle Jurassic.The overpressure of the second and fourth members of the Xu2、Xu4 Formation has also increased under the action of tectonic compression and pressure transfer.From the end of the Early Cretaceous to the present,the strata have been uplifted and denuded under tectonic action.The decrease of vertical load and the decrease of formation temperature cause the rebound of formation pores and the contraction of fluids.It causes obvious pressure reduction,and the formation and opening of faults in the process of formation uplift will also reduce the formation pressure.The overpressure of Xujiahe Formation in the study area has an important influence on the sealing ability of the caprock,and shows obvious differences in the northern and southern parts of the study area,The specific performance is;in the northern part of the study area,the caprock was affected by overpressure and physical sealing during the middle and late Yanshan movement and Himalayan movement.The overpressure of the reservoir did not cause the caprock to break hydraulically,and its sealing ability was good.The caprock preservation ability was always affected by the caprock overpressure sealing ability;in the southern part of the study area,the caprock was controlled by overpressure and physical sealing ability in the middle and late Yanshan movement,and the caprock preservation ability was affected by the overpressure sealing ability of the caprock.In the early Himalayan movement,the overpressure of the reservoir caused the caprock to hydraulic fracture,and the caprock preservation ability was affected by hydraulic fracture.In the late Himalayan movement,the overpressure in the caprock caused the opening of the internal pre-existing fractures,which led to the deterioration of the caprock sealing ability in this period,and the caprock preservation ability was affected by the fracture opening pressure.