Origin and distribution of an under-pressured tight sandstone reservoir: The Shaximiao Formation, Central Sichuan Basin

Abnormal pressure is of great significance for hydrocarbon generation, migration and accumulation. The tight gas reservoirs in the Middle Jurassic Shaximiao Formation of the Sichuan Basin are characterized by considerable underpressure, which origin and evolution are still unclear. In this study, the pressure distribution was characterized by repeat formation test and Bowers' method using sonic and density well log data. The origins of underpressure were analysed by geological analysis and quantitative calculation. Basin modelling method was also applied for pressure evolutionary process reconstruction. The results showed that underpressure developed at depths of 1900-2300 m with pressure value of 9.24-17.19 MPa and the pressure coefficients of 0.45-0.89. Laterally, the pressure coefficient showed a decreasing tendency from the western to the eastern portion of the study area, which was characterized by considerable underpressure. The results of the underpressure origin analysis indicated that the elastic rebound and temperature reduction led to pressure reductions of approximately 12-15 and 3-4 MPa, respectively. However, the source rocks of Lower Jurassic Ziliujing Formation and Upper Triassic Xujiahe Formation provided pressure increasing by hydrocarbon generation as two of the main sources in the Sichuan Basin. The gas generating process in the source rocks of the Xujiahe Formation increased the pressure by more than 20 MPa which compensated for the pressure decrease in the western part of the Central Sichuan Basin. However, the gas generation of the Ziliujing Formation only provided less than 10 MPa of pressure compensation for the eastern part, which was then characterized by widely distributed underpressure. Consequently, the western and eastern portions of the Central Sichuan Basin underwent different pressure evolution processes, which lead to different pressure distributions at present. This study provided a basis for underpressure analysis of gas reservoirs in quality and quantity in the Central Sichuan Basin and may be helpful for similar gas fields worldwide.