Controlling factors and development patterns of fractures in deep tight sandstone in Xihu Sag, East China Sea Shelf Basin: a case study from W Structure in south central region of Central Depression Belt

Most of the oil and gas resources in the Xihu Sag are concentrated in deep low-porosity and permeability reservoirs below 3500 m. Previous studies on deep fractured "sweet spot" reservoirs in this area are relatively rare. Taking the W structure in the central depression belt as the research target area, the fracture prediction method is summarized, and the controlling factors and development patterns of deep fracture formation are studied. Results show that the fractures in the deep tight sandstone reservoir of the W structure could be classified into diagenetic fractures, shear fractures, and tension fractures in fracture genesis type. Among them, the diagenetic fractures is controlled by sedimentary bedding and pressure release. The shear fractures are controlled by the intensity of stratum deformation and are mainly developed in the parts with strong deformation of the Huagang Formation. The tension fractures are related to the horizontal extension stress generated after the stratum uplift. After analyzing the reservoir physical properties, we believe that the development of deep fractures is one of the important factors controlling the "sweet spot" reservoirs of tight sandstone gas reservoirs. Through tectonic stress field simulation combined with single well core and thin section analysis, the concentrated development area of fractures can be effectively predicted, providing a basis for the prediction of deep fractured "sweet spot" reservoirs.