Study on Reservoir Characteristics, the Tightening Process and Reservoir Quality in Source-to-Sink Systems in the Xu-2 Member of the Xujiahe Formation in the Western Sichuan Basin, Western China

The Upper Triassic Xujiahe Formation in the western Sichuan Basin is rich in natural gas resources and is one of the main tight sandstone gas-producing layers in the Sichuan Basin. Taking the tight sandstone of the second member of the Xujiahe Formation (Xu-2 Member) in the western Sichuan Basin as the study target, based on the analysis of the rock sample, a thin section, scanning electron microscopy, inclusion, the carbon and oxygen isotope, the petrological characteristics, the reservoir properties, the diagenetic sequences, and the pore evolution processes were revealed. The tight sandstones are composed of litharenite, sublitharenite, and feldspathic litharenite with an average porosity of 3.81% and a permeability mainly ranging from 0.01 to 0.5 mD. The early to late diagenetic stages were revealed, and the diagenetic evolution sequence with five stages was constructed. The Xu-2 sandstones were subdivided into three different types, and each type has its own tightening factors and processes. In the quartz-rich sandstone, the compaction and pressure solution were the primary causes of reservoir tightening, while late fracturing and dissolution along fractures were the main factors improving reservoir properties. In the feldspar-rich sandstone, early dissolution was a primary factor in improving porosity, while carbonate and quartz cements generated by dissolution contributed to a decrease in porosity. In the rock-fragment-rich sandstone, chlorites formed in the early stage and dissolution were the main factors of reservoir quality improvement, while the authigenic quartz formed in the middle diagenetic stage was the primary cause of reservoir tightening. Four major source-to-sink systems were identified in the western Sichuan Basin and they have different reservoir characteristics and reservoir quality controlling factors. This study will contribute to a deeper understanding of the characteristics, diagenetic evolution, and tightening process of tight sandstone reservoirs, effectively promoting scientific research and the industrial development of tight sandstone gas in the Xu-2 Member of the Sichuan Basin.