Developmental characteristics and dominant factors of fractures in marine–continental transitional facies tight sandstone reservoirs in heavily deformed areas: a case study

Fractures enhance secondary porosity and permeability of tight sandstone and in turn promote fluid migration and well recovery. The developmental characteristics and dominant factors of tight sandstone reservoir fractures in the lower Permian Shanxi Formation, southern Qinshui Basin, were systematically studied by combining qualitative observations, quantitative characterizations, logging interpretations, and tectonic analysis. The results show that fractures are extensively developed in the Shanxi Formation tight sandstone. The primary factors controlling these fractures include tectonic position, proximity to faulting, rock brittleness, single sand body thickness, formation anisotropy, and diagenesis. In crest or flank portions of the anticline and the bottom or low regions or well-developed faults, the fracture density is generally greater than 2 per meter. The scale and intensity of faulting both have a significant impact on the fracture development. Near some faults, “crushed zones” or “weak zones” were observed in the cores. These areas have a moderate- to low-angle or near-horizontal dips of less than 15° and widths of less than 50 cm. The rock rupture of the crushed zone typically occurred in a certain direction. It was found that the first fractures to form in a tight sandstone reservoir are related to tensile failure or shear-tensile failure. A negative exponential correlation exists between the linear fracture density and the single sand body thickness. When the single sand body thickness is less than 3 m, the linear fracture density is generally higher than 4 per meter; when the single sand body thickness is greater than 6 m, the fracture density is generally lower than 2 m −1 . The influence of fracture density on rock anisotropy is stronger than that of the geostress. Minerals with unstable chemical properties, such as carbonate cements and feldspar, provide favorable conditions for the migration of acidic fluids and the formation of dissolution fractures.