Fluid evolution in strike-slip fault zones of lower Paleozoic tight carbonate reservoir in NW Tarim Basin, China

Ultra-deep carbonate reservoirs buried more than 7000 m, and the matrix has no effective reservoir space in the northwestern margin of Tarim Basin. Strike-slip fault-related fractures and cavities are the primary reservoir space and control fluid migration. In this study, using outcrops, thin section, δ¹³C, δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr isotopes were applied to determine the characteristics of the strike-slip fault and origin of the fluid, combined with the homogenization temperature of fluid inclusions, burial history and thermal history in calcite veins, the formation time and evolution of multi-phase fluid were determined. The strike of the strike-slip fault comprises calcite bands, fault breccias, and fractures filled with minerals such as bitumen, dolomite, and quartz. The study area mainly developed three stages of diagenetic fluids. The first stage was meteoric water in the middle Caledonian, which showed a negative δ¹⁸O value, low homogenization temperature, and salinity. The second stage is the Late Hercynian (Early Permian) brine, characterized by more negative δ¹⁸O value, high ⁸⁷Sr/⁸⁶Sr ratios, and high homogenization temperature and salinity. The third stage is the late Permian meteoric water, characterized by a negative δ¹⁸O value, lower homogenization temperature, and salinity than seawater. This study focuses on the fluid activity in the fault zone and the reservoir transformation, making great significance in revealing the formation and development mechanism of strike-slip fault-related fracture and cavity reservoirs.