Tight gas accumulation caused by overpressure: Insights from three-dimensional seismic data in the western Sichuan Basin, southwest China

Abstract

Tight gas is an important unconventional gas resource, but numerous aspects of its accumulation and the distribution of high-quality reservoirs remain unclear. The Mesozoic–Cainozoic strata in the Sichuan Basin in southwest China are a focus for tight gas exploration, but have a complex pore pressure distribution that makes exploration challenging. In this study, the pore pressure in Upper Triassic and Jurassic strata in the West Sichuan Depression was predicted from three-dimensional seismic data using the Fillippone method. The results indicate that overpressure has developed in the Upper Triassic Xujiahe Formation in the Xinchang Structural Belt, with pressure coefficients of 1.2–2.4, which are highest in the third member, moderate in the fourth and fifth members, and lowest in the second member. Faults have a significant effect on the pore pressure. Overpressure along regional primary faults is typically weak, but it increases with distance from the faults. The overpressure resulted from disequilibrium compaction and hydrocarbon generation during the Mesozoic, and was intensified and modified by the Himalayan Orogeny. We infer that differential overpressure in tight sandstones is a key factor in gas migration and accumulation. Weakly overpressured zones along primary faults are dominant gas accumulation zones. Secondary faults and related fractures have a limited effect on overpressure, but improve the physical properties of reservoirs by increasing the permeability and promoting water–rock interactions. We suggest that the areas with relatively low pressure around faults and fractures are the most favourable targets for hydrocarbon exploration in tight sandstones.