Timing of natural fractures formed in the gas reservoirs of the 2nd member of Xujiahe Fm in the Xinchang area, western Sichuan Basin

The 2nd member of the Upper Triassic Xujiahe Fm in the Xinchang area, western Sichuan Basin, experienced multi-stage tectonic movements, resulting in strong heterogeneity of natural fractures, which is difficult to map. Based on the observation and description of fractures in cores and the investigation of fractures on the analogous outcrops, this paper analyzes the development characteristics of the natural fractures in the study area in the respects of fracture occurrence, size, fracture surface features and effectiveness. The vertical growth of the natural fractures is controlled by layer thickness and is commonly within the layers, and the lateral extension of the fractures is generally no more than 5 m. The heterogeneity of fractures is strong with a fracture density of 0.8 fractures per meter, and the development levels of fractures are from medium to high. In combination with fracture development features and tectonic evolution background, experimental tests, such as rock acoustic emission experiment, stable carbon-oxygen isotopic analysis of fracture fillings and fluid inclusion test of fracture fillings, reveal that the natural fractures in the 2nd member of Xujiahe Fm in the Xinchang area were formed in four stages, which corresponds to the Indo-Sinian period, the Yanshanian period, and the 3rd and 4th episodes of the Himalayan period respectively. The early fractures are mostly fully-filled or semi-filled, while the late fractures formed in the 3rd and 4th episodes of the Himalayan period are mainly non-filled. The effectiveness of fractures formed in the 4th episode is particularly high.