Controlling factors for fracture distribution in the low-permeability sandstone reservoir of the Second Member of the Upper Triassic Xujiahe Formation in the south of western Sichuan

There are tectonic fractures,diagenetic fractures and fractures related to overpressures in the low-permeability sandstone reservoir in the Second Member of the Upper Permian Xujiahe Formation in the south of the western Sichuan depression. Among them,tectonic fractures are dominant. Under the stresses resulting from horizontal tectonic compressions in the Indosinian,Yanshanian and Himalayan periods,overpressures formed by deep burial in the Cretaceous and tectonic uplift and erosion since the Neogene,four sets of fractures formed in the study area. The development degree of these fractures was controlled by the lithology,layer thickness,structural locations,stresses and fluid pressures. The zones of fractures are mainly distributed along the N-S and NE-SW-trending structural belts. In the N-S-trending structural belt,nearly N-S and nearly E-W sets of fractures are better developed than the other two sets of fractures; whereas in the NE-SW-trending structural belt,the NE-SW and NW-SE sets of fractures are better developed than the other two sets. The fracture development zones control the concentration of natural gas in the study area.