Effective fractures linked with tectonic reactivation and multiple genetic fluids in the ultradeep Paleozoic carbonate buried hills of the Bozhong sag, North China

Paleozoic carbonate buried hills with burial depths of more than 4500 m (14,764 ft) in the Bozhong sag of the Bohai Bay Basin are rich in natural gas resources, categorized as typical ultradeep fracture-related reservoirs. Based on the data of (sidewall) cores, thin sections, electrical image logs, three-dimensional seismic observations, and physical properties, we evaluated the fracture characteristics and elucidated the dominant controls on effective (unfilled) fractures. Tectonic fractures are the main components in these reservoirs. The intersecting relationships, widths, and strikes of the fractures as well as the carbon and oxygen isotope ratios of the fill materials indicate that NWW- and NE-trending tectonic fractures developed. The NWW-trending fractures are related to the compressional tectonic stress field of the Indosinian orogeny and account for nearly 80% of the fractures, with widths ranging from 40 to 160 μm; 43% of them are filled with carbonate and quartz. The dissolution of meteoric water mainly occurred with this set of tectonic fractures and formed an upper dissolved zone within 200 m beneath the weathering crust. Related fill materials have −2‰ to 2‰ δ13C and −8‰ to −13‰ δ18O. The group of NE-trending fractures accounted for approximately 20% of all fractures, with widths ranging from 200 to 280 μm, and are related to the faulting during the Himalayan orogeny, which contributed to the formation of subsequent NE-trending fractures and reopened prior NWW-trending fractures. Some fill materials present negative δ13C and δ18O values, distributed from −2‰ to −6‰ and −13‰ to −20‰, respectively. These negative values revealed the influence of Cenozoic hydrothermal fluids and organic acids, which contributed to the development of an internal fracture-related dissolved zone that developed beneath 400–600 m from the top of the basement. The analysis demonstrates that the direction of in situ maximum principal stress is subparallel to the fracture trends, which is the key factor for maintaining the effectiveness of the fractures. The coupling of Cenozoic tectonic reactivation and multiple genetic fluids controls the effectiveness of ultradeep carbonate reservoirs.