Fracture distribution characteristics of 4th member of Xujiahe Formation in Xiaoquan-Xinchang area, western Sichuan

Electro-imaging logging was carried out in 19 key wells to study the natural fractures in the 4th member of the Xujiahe Formation in the western Sichuan Basin. There are 4 sets of fractures as followed: the NE-SW trended fractures from the late Indosinian to the early Yanshanian, the SN trended fractures during the middle and late Yanshanian, the NW-SE trended fractures during the late Himalayan, and the ESE-WNW to ENE-WSW trended fractures during the 4th phase of Himalayan. Low-angle and high-angle diagonal fractures account for a large proportion, while horizontal fractures less and vertical fractures seldom develop. The fractures in the southern and northern flanks have bigger linear densities than those in the hinge, where the linear densities of fractures increase as closing to fault system, showing obvious heterogeneity. Medium sandstones, fine sandstones, conglomerates and coarse sandstones are favorable carriers for fractures. In contrast, fractures are underdeveloped in shales. In shales, pelitic siltstones and siltstones, the linear density of fractures decreases obviously as formation thickness increases. As to silty shales and coarse sandstones, there is no apparent congruent relationship.