Natural fractures in ultra-deep reservoirs of China: A review

In recent years, China has made significant progress in exploring oil and gas in sandstone and carbonate rocks with burial depths of more than 6000 m in the Tarim, Sichuan, and Junggar Basins. These ultra-deep reservoirs have widely developed natural fractures that are important storage spaces and primary flow pathways influencing hydrocarbon enrichment and production. For ultra-deep tight sandstone, fractures caused by tectonic, diagenetic, tectonic-diagenetic, and overpressure processes are widespread. In the foreland thrust belt, which is divided into five fracture domains, fault-related folds affect fracture distribution in ultra-deep sandstones. For ultra-deep carbonate rocks, strike-slip faults control scale, structural style, and linkage evolution of fractures. Fractures formed over multiple periods. Many fractures in Cretaceous sandstones formed recently whereas some of those in carbonate rocks formed in the Paleozoic when strata were buried less than 5000 m. With the increase in burial depth, fracture porosity (and, probably) connectivity is reduced by mineral deposits. Fracture-forming processes include overpressure, present in situ burial and tectonic loading. Other variables modifying fractures include when fractures formed (timing), regional and local structural position, and scale and fluid flow leading to heterogeneity in whether fractures are mineral filled, open, or enhanced by dissolution. Our review provides insights into the origins, characteristics, evolution, and effectiveness of fractures in ultra-deep sandstone and carbonate rocks.