Study on Fracture Characteristics and Controlling Factors of Tight Sandstone Reservoir: A Case Study on the Huagang Formation in the Xihu Depression, East China Sea Shelf Basin, China

Natural fractures are not only the key to affect the natural productivity of tight sandstone reservoirs but also one of the important factors to control the fracturing effect. If the local strata are affected by multistage tectonic movement and diagenesis, it will lead to the complexity of fracture types and characteristics, which makes it difficult to accurately identify natural fractures, thus, seriously affect the development of fractured reservoirs and the fracturing effect. In this paper, the macroscopic fractures were studied by combining core, conventional logging, imaging logging, petrophysical logging, logging, drilling, and gas test data. The microscopic fractures were analyzed by casting thin sections and scanning electron microscopy and cathodoluminescence. The effect of fractures on hydrocarbon migration was analyzed by fluid inclusions in fractures. The core can be used to visually study the opening fracture, but when the color of the filling inside the fracture is similar to the surrounding, the filling fracture is not easy to be found. Imaging logging has high identification accuracy for filling fractures, but it can hardly be used as a means of identifying high-angle opening fractures. The opening fracture zone in the noncoring section can be studied by conventional logging, mud logging, drilling, and rock mechanical parameter. The macroscopic structural fractures are obviously directional, which is controlled by the tectonic stress field. The occurrence and density of macroscopic structural fractures are controlled by buried depth, lithology, rock thickness, sedimentary facies, distance from the fault, and structural location. The opening fractures are all of high angle, indicating that the inclination angle is an important factor affecting the filling degree of fractures. The direction of intragranular fractures is chaotic, which is affected by the mechanical weak surface of the mineral particles, particle morphology, and particle contact relationship and stress. Effective fractures provide channels for the migration of oil and gas and acidic fluids in the historical period, which promote the occurrence of hydrocarbon accumulation and dissolution.