CHARACTERISTICS AND STAGES OF FRACTURES IN THE SECOND MEMBER OF XUJIAHE FORMATION IN QIONGXI STRUCTURE,WESTERN SICHUAN DEPRESSION

Gas exploration and development began in the second member of Upper Triassic Xujiahe Formation in Qiongxi structure, western Sichuan depression, since the well Qiongxi-3 tested high rate gas flow in 2002. The gas reservoirs in the second member of Xujiahe Formation in the study area are of fractured-porous type and are dominated by tight clastic rocks, which become effective reservoirs due to the development of fractures. The mechanism, controlling factors and stagis of fracture development are discussed through comprehensive quantitative analysis of fracture types, mechanical mechanism, and characteristic parameters. Four sets of fractures, including near NS-trending, NE-trending, near EW-trending and NW-trending fractures, are mainly developed in the second member of Xujiahe Formation in Qiongxi structure and the near NS-trending fractures are better developed than the NE-trending ones. The factures in the study area are dominated by high angle structural shear fractures, followed by near horizontal diagenetic fractures, gliding fractures and anomalous pressure-related fractures. The structural fractures are mainly developed in the Indo-Chinese epoch, late Yanshan epoch and Himalayan epoch. The formation of diagenetic fractures has experienced a very long geologic history. The anomalous pressure-related fractures are mainly formed in the Cretaceous. The formation, development stage and scale of fractures are mainly controlled by regional tectonic events. In addition, lithology, layer thickness and faults also have direct influences on the development of fractures.