Fracture development characteristics and main controlling factors of ultra-tight sandstone reservoirs in third member of Upper Triassic Xujiahe Formation, Dayi structure, Western Sichuan Depression

Influenced by multi-phase tectonic superimposition and complex diagenetic evolution, fractures are widely developed in deep tight sandstone reservoirs within the basin. Taking the third member of the Xujiahe Formation (Xu-3 member) in the Dayi structure of the Western Sichuan Depression, Sichuan Basin, as the research object, and integrating multiple technical methods including outcrop surveys, core observations, cast thin sections and scanning electron microscopy analyses, imaging logging interpretation, homogenization temperature measurement of fluid inclusions, and carbon-oxygen isotope analysis, this study systematically characterized the macro- and micro-scale fracture development features, determined the fracture development stages, and further revealed the main controlling factors and reservoir modification effects. The results indicated that fractures in the Xu-3 member of the Dayi area were diverse in types, including high-angle, oblique, low-angle, and horizontal fractures. Overall, tectonic fractures were dominant, with only a small proportion of sedimentary-diagenetic fractures. The dominant fracture orientations were NNE-SSW, NE-SW, and nearly S-N. The filling was mainly semi-filled to fully filled, and the filling minerals primarily consisted of calcite, quartz, and argillaceous matter. Three phases of fracture tectonic events were recorded in the Xu-3 member of the study area, corresponding to the Indosinian, the Late Cretaceous (Middle to Late Yanshanian), and the Oligocene-Miocene (Himalayan period). Fracture development was jointly controlled by structural setting and lithological assemblage, among which the distance to faults, formation curvature, sand-mud interbedding structure, and brittle mineral content were the key influencing factors. The effectiveness of fractures from different periods varied significantly. Early-stage fractures, affected by intense diagenetic modification, had a high filling degree and their effectiveness was significantly reduced. In contrast, Himalayan-stage fractures formed relatively later, experienced weak subsequent damage, and closely aligned with the current stress field, making them the most effective seepage pathways in the present reservoir and playing a core role in enhancing reservoir physical properties.