川西南部须二段低渗透砂岩储层裂缝类型及其形成序列

裂缝是川西坳陷须二段低渗透砂岩天然气藏分布的重要控制因素.利用地表露头、岩心、薄片和成像测井等资料, 对该区裂缝的成因类型、分布特征及其形成序列进行了分析.须二段低渗透砂岩储层主要发育有构造裂缝、成岩裂缝以及与异常高压有关的裂缝等3种裂缝类型, 其中以构造剪切裂缝为主, 主要有近南北向、北东向、近东西向和北西向4组, 它们的发育程度依次由强变弱.但在现今应力场作用下, 北西向裂缝表现为主渗流裂缝.此外, 该区还发育有大量被沥青或碳质充填的裂缝扩张脉群, 结合岩石的声发射等实验结果, 表明该区在白垩纪中晚期曾经经历过异常高压作用, 其最大压力系数可达1.6~2.1, 在新近纪中晚期释放以后的压力系数为1.2~1.3.在中粗粒砂岩中, 还发育大量在构造挤压作用下由于断层的逆冲作用或层间滑动造成的剪切作用有关的近水平剪切裂缝.构造裂缝主要在三叠纪末、白垩纪末和新近纪末—早更新世末形成. 

Type and Sequence of Fractures in the Second Member of Xujiahe Formation at the South of Western Sichuan Depression

Fracture is an important controlling factor for the distribution of natural gas reservoirs in the second member of Xujiahe Formation at the south of western Sichuan depression. Using the data of outcrops, cores, thin sections and image logs, we analyzed the fracture origin types, distribution characteristics and formation sequences. There are three types of fractures, i.e. tectonic fractures, diagenetic fractures and fractures related to overpressure in the low-permeability sandstone reservoirs. Among them, the tectonic fractures are the main. They are mostly shears with four sets of S-N, NE-SW, E-W and NW-SE orientations. Their development degree dies down in turn. But influenced by modern stress field, the fracture of NW-SE orientation is the primary seepage one. Moreover, there are many tension veins filled with asphaltum in the study area. Combined with the analysis of rocky memorial information measured by acoustic emission, it shows there were overpressures in the reservoir rocks in the Middle and Late Cretaceous where the maximum coefficient of formation pressure could reach 1.6-2.1. After the pressure was released in the Middle and Late Neogene, the pressure coefficient decreased to 1.2-1.3. In the basal part of medium-granular sandstone and grit, there developed many approximately horizontal shear fractures related to the thrusting of faults and interlayer sliding under tectonic compression. The tectonic fractures mainly formed in the end of Triassic, Cretaceous and Neogene-Early Pleistocene.