1. Department of Resource and Environment, Southwest Petroleum University, Chengdu 610500, China; 2. Research Institute of Geology Exploration and Development, Sichuan-Changqing Drilling Engineering Company, Chengdu 610051, China; 3. Central Sichuan Oil and Gas Mines, Southwest Oil and Gas Field Branch, Suining 629000, Sichuan, China

Sandstone reservoir in the 4th Member of Xujiahe Formation in Upper Triassic of Guangpan area in Sichuan is one of the main gas reservoirs in Sichuan Province. Its reservoir rocks are fine to medium-grained lithic sandstone, feldspathic sandstone and quartz sandstone with typical low porosity and permeability. Based on the study of macroscopic depositional characteristics of the gas reservoirs in Upper Triassic Xujiahe Formation in this area, the methods with rock slides, casting thin section, scanning electronmicroscope and X ray diffraction, are used to study the diagenesis, pore evolution of gas reservoirs and the influences on storage capacity, which indicates that the sandstone in the studied area went through the diagnesis stages of compaction and p ressure solution, cementation, metasomatism, dissolution and fracturing, and then reached the B substage of middle diagenetic stage. Diagenesis is a key factor in controlling the physical property of reservoir in this area. During the process of pore evolution, the compaction and pressure solution and cementation played destructive function, while the dissolution andrim chlorite cementation played constructive function.