CHARACTERISTICS AND CONTROLLING FACTORS OF FRACTURES IN RESERVOIRS OF XUJIAHE FORMATION IN ZITONG AREA, SICHUAN BASIN

Based on the data of outcrops, cores, thin sections and experimental analysis, the development characteristics and controlling factors of fractures in tight sandstone reservoirs of Xujiahe Formation in Zitong Area, Sichuan Basin were analysed and interpreted. There are three types of fractures, i.e. tectonic fractures, diagenetic fractures and fractures related to abnormal high pressure of fluid in the tight sandstone reservoirs. Among them, the tectonic fractures are the main part. There are main three sets fractures of nearly EW, nearly SN and NW-SE orientations. The fractures in study area have an average density of 0.56/m and are mostly confined within the single layer. In the plane, fractures density decreases along the Lao Guanmiao-Wen Xingchang-Zhe Bachang area. While, the fractures of T₃x⁴ are most developed. The proportion of effective fracture is the lowest around the Lao Guanmiao area and increases along the direction of NW to Zhe Bachang area. The distribution of these fractures in study area were controlled by such factors as the lithology, layer thickness, structures and abnormal high pressure of fluid. Sandstone with fine particle and thin layer are favorable for fractures development. Where near the faults and the high part of the structure also are the favorable areas for tectonic fractures development. The control action of faults on the forming and distribution of fractures plays a dominant role. Moreover, the abnormal high pressure of fluid is beneficial to the development of fractures, especially for the extension fractures, the fractures density in abnormal high pressure belts increases significantly. The abnormal high pressure also causes some fractures which were closed in early stage to open and plays important roles in increasing the extent of mineral filling and corrosion increases of fractures.