Development characteristics of structural fractures in tight sandstone reservoirs under multi-level configuration interfaces: a case study of second member of Xujiahe Formation in Western Sichuan Depression

Fractures are crucial for the high and stable production of natural gas from the tight sandstone reservoirs in the second member of the Xujiahe Formation in the Western Sichuan Depression, Sichuan Basin. The development of these fractures is controlled by the configuration interfaces of sand bodies formed under different sedimentary environments. Through field outcrop surveys, core observations, logging identification, and detailed characterization of lithofacies, a classification scheme for sand body configuration interfaces was proposed for the second member of the Xujiahe Formation in the Western Sichuan Depression. These interfaces were identified and classified into second to fourth levels. The study clarified the characteristics and geological significance of fractures under different levels of configuration interfaces. These interfaces controlled the occurrence, opening degree, development degree, and storage-permeability capacities of reservoir fractures. The effective control ranges of the second and third level interfaces were 0-0.35 m and 0-3 m, respectively, while the fourth level interface had a larger control range (0-11 m). The development characteristics of fractures differed under different configuration interfaces. Specifically, fractures were underdeveloped at the second level interfaces; fairly developed at the fourth level; and most developed at the third level, primarily as low-angle dipping fractures. Fracture orientations were predominantly NNE-SSW, SEE-NWW, and SSE-NNW, with more fractures having larger openings (>0.04 mm) and larger dip angle. There was a clear positive correlation between core porosity and permeability in the fracture development under the control of the multi-level interfaces. Fractures at the third level interface had the strongest improvement effect on reservoir physical properties and made the highest contribution to oil and gas production, followed by the fourth level interface, with the second level interface being the least effective. Clarifying the control of different levels of configuration interfaces on fracture development is conducive to the effective development of tight sandstone gas reservoirs in the study area.