九龙山构造须二段致密砂岩储层裂缝特征及成因

Characteristics and origin of fractures in tight sandstone reservoirs of the second member of Xujiahe Formation in Jiulongshan structure

裂缝是川西北地区九龙山构造上三叠统须家河组二段致密砂岩天然气藏分布的重要控制因素。利用野外露头、岩心、成像测井和薄片等资料,并结合实验分析,对该区裂缝的成因类型、发育特征、形成期次及其形成机理进行了分析。研究区须二段致密砂岩储层发育有构造裂缝和成岩裂缝两种类型,其中以构造裂缝为主,主要有北东东—南西西向、北北东—南南西向和北西—南东向3组。按照裂缝的倾角,构造裂缝又可分为高角度构造裂缝和低角度构造裂缝。其中,高角度构造剪切裂缝主要是在印支晚期、燕山晚期和喜马拉雅期3期构造作用下形成,形成构造裂缝的力源来自于龙门山以及米仓山—大巴山的水平构造挤压作用、深埋藏造成的异常高压流体压力以及抬升剥蚀造成的应力作用。低角度构造裂缝主要与构造挤压作用下断层的逆冲作用或层间滑动造成的近水平剪切作用有关。

Fracture is an important controlling factor for the distribution of tight sandstone gas reservoirs in the second member of Xujiahe Formation in Jiulongshan structure at the north of western Sichuan Basin. Using the data of outcrops, cores, thin sections, image logs and experimental analysis, we analyzed the fracture origin types, development characteristics and formation mechanism. There are two types of fractures in the study area,i, e. tectonic fractures and diagenetic fractures. Among them, tectonic fractures are the dominant. They are mostly shear fractures with three sets of NEE-SWW, NNE-SSW and NW-SE orientations. According to the dip angle,tectonic fractures can be subdivided into shear fractures with high dip angle and horizontal shear fractures. The shear fractures with high dip angle formed under the tectonism of the end of Indosinian,Yanshan and Himalaya stage. The forces that formed tectonic fractures came from horizontal tectonic compression of Longmen Mountain and Micang-Daba Mountain, overpressure caused by deep bury and stress from the uplift. The horizontal shear fractures were related to the thrusting of faults and interlayer sliding under tectonic compression.