Geological factors and fracture distribution in deep and ultra-deep sandstones in Kuqa Depression, Tarim Basin, China

Deep (> 4500 m) and ultra-deep (> 6000 m) sandstone reservoirs hold great potential for hydrocarbon resources, yet complex geological challenges hinder the successful exploitation of oil and gas. Fractures in deep and ultra-deep sandstones are prevalent and significantly enhance rock permeability, and critically impact fluid flow and hydrocarbon productivity. Relationships between geological factors and fracture distribution in deep sandstone reservoirs, despite its significance, have remained poorly understood. This study utilizes core, thin section, acoustic emission tests and geophysical well logs to elucidate the interplay between geological elements and fracture occurrences in tight sandstones of the Kuqa Depression, which is a tectonically active foreland basin. The controls of sedimentation, sandbody distribution and earth stress on fracture distribution are analyzed. The research then unravels the effects of lithology units, earth stress fields, and broader tectonic context on fracture distribution patterns. Geological factors, including sedimentary factors (lithology, sandbody thickness and sandbody distribution), earth stress, and tectonic structure are integrated to comprehensively evaluate the fracture distributions in Kuqa Depression. The different lithologies are identified, and fractures in different lithologies are characterized. High-angle fractures and vertical fractures are the main fracture types in Bozi-Dabei area. Fracture density exhibits an inverse relationship with sandbody thickness. The presence of thinner sandstones in conjunction with thin mud layers facilitates the formation of fractures. Paleostress affects the generation of natural fractures, and high fracture density is associated with high paleostress magnitudes. In situ stress affects the subsequent modification of natural fractures. However, some filled fractures with wider aperture oriented at high angles to SHmax reveal cement-stress interactions jointly controlling apertures, not stress dominance. Structure factors including the position at folds and the proximity to faults are crucial for the fracture distribution. Fractures are more abundant in the hinge areas of anticlines compared to the limb areas, and fracture density above the neutral planes is notably higher. In addition, fracture density is higher in the formation adjacent to the fault due to the effect of the regional stress field. This study helps unravel the geological controlling factors and distribution of fractures by integrating geological and geophysical data, and has implications for hydrocarbon resource exploration in deep and ultra-deep sandstones.