Fracture Development, Mechanisms, and Effectiveness in Tight Sandstone Reservoirs: Case Study of the Xujiahe Formation, Northeastern Sichuan Basin

Tight sandstone gas reservoirs are an important type of gas reservoir in the Sichuan Basin. Multi-stage tectonic movements have generated abundant natural fractures that control the migration and accumulation of natural gas. Due to the complexity of natural fracture types, research on the development regularity and effectiveness evaluation of natural fractures is of great significance. Taking the Xujiahe Formation in the Tongnanba Gas Field as an example, this study clarifies the parametric characteristics of different types of natural fractures by using data from field outcrops, cores and image logging. Filled fractures in the tight sandstone of the Xujiahe Formation in the Tongnanba area were formed under NW-SE compression during the Late Yanshanian movement. Effective fractures were developed in two stages: NW-SE compression during the Early Himalayan movement and NE-SW compression during the Late Himalayan movement. The development of effective fractures is controlled by tectonics, lithology, sedimentary microfacies and mechanical layer thickness. Hinge zones of folded anticlines, high-grade fault zones, beach bars and underwater distributary channels are favorable for the development of effective fractures. The effectiveness of natural fractures in the Malubei Anticline is higher than that in the slope zone and the Tongjiang Syncline. Specifically, EW-striking natural fractures in the Malubei Anticline show the best effectiveness; in the slope zone, EW-striking natural fractures are more effective than NW-striking ones, and NS-striking fractures are the poorest; in the Tongjiang Syncline, NW-striking natural fractures are more effective than NS-striking ones.