Fracture is an important controlling factor for the distribution of tight sandstone gas reservoirs in the second member of Xujiahe Formation in Jiulongshan structure at the north of western Sichuan Basin. Using the data of outcrops, cores, thin sections, image logs and experimental analysis, we analyzed the fracture origin types, development characteristics and formation mechanism. There are two types of fractures in the study area,i, e. tectonic fractures and diagenetic fractures. Among them, tectonic fractures are the dominant. They are mostly shear fractures with three sets of NEE-SWW, NNE-SSW and NW-SE orientations. According to the dip angle,tectonic fractures can be subdivided into shear fractures with high dip angle and horizontal shear fractures. The shear fractures with high dip angle formed under the tectonism of the end of Indosinian,Yanshan and Himalaya stage. The forces that formed tectonic fractures came from horizontal tectonic compression of Longmen Mountain and Micang-Daba Mountain, overpressure caused by deep bury and stress from the uplift. The horizontal shear fractures were related to the thrusting of faults and interlayer sliding under tectonic compression.
