川西孝泉—新场地区须家河组四段裂缝分布特征

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

通过电成像测井的手段,对研究区内19口重点井须家河组四段发育的天然裂缝资料进行了统计、分析,认为该层位裂缝主要有4组:印支晚期—燕山早期的NE-SW向裂缝,燕山中、晚期的近SN向裂缝,喜马拉雅晚期的NW-SE向裂缝和喜马拉雅期四幕的ESE-WNW至ENE-WSW向裂缝;孝泉—新场地区须家河组四段主要发育低角度斜交裂缝以及高角度斜交裂缝,水平缝发育较少,垂直缝基本不发育;背斜南、北两翼的裂缝线密度要大于轴部,背斜轴部总体上表现为距断裂系统越近、裂缝的线密度越大的特点;裂缝线密度的不均质性较强;中砂岩、细砂岩、砾岩、粗砂岩是裂缝的主要载体,裂缝在中砂岩、细砂岩、砾岩、粗砂岩中发育得更好,而页岩中的裂缝最不发育;页岩、泥质粉砂岩、粉砂岩中的裂缝线密度呈较清晰的随层厚增加而减小的趋势;粉砂质页岩、粗砂岩中裂缝线密度与层厚没有表现出明显的对应关系。

Electro-imaging logging was carried out in 19 key wells to study the natural fractures in the 4th mem- ber 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 forma- tion thickness increases. As to silty shales and coarse sandstones, there is no apparent congruent relationship.