Fluid mixing induced by hydrothermal activity in the ordovician carbonates in Tarim Basin, China

Permian hydrothermal activity in the Tarim Basin may have been responsible for the invasion of hot brines into Ordovician carbonate reservoirs. Studies have been undertaken to explain the origin and geochemical characteristics of the diagenetic fluid present during this hydrothermal event although there is no consensus on it. We present a genetic model resulting from the study of C-13, O-18, S-34, and Sr-87/Sr-86 isotope values and fluid inclusions (FIs) from fracture- and vug-filling calcite, saddle dolomite, fluorite, barite, quartz, and anhydrite from Ordovician outcrops in northwest (NW) Tarim Basin and subsurface cores in Central Tarim Basin. The presence of hydrothermal fluid was confirmed by minerals with fluid inclusion homogenization temperatures being >10 degrees C higher than the paleo-formation burial temperatures both in the NW Tarim and in the Central Tarim areas. The mixing of hot (>200 degrees C), high-salinity (>24 wt% NaCl), Sr-87-rich (up to 0.7104) hydrothermal fluid with cool (60-100 degrees C), low-salinity (0 to 3.5 wt% NaCl), also Sr-87-rich (up to 0.7010) meteoric water in the Ordovician unit was supported by the salinity of fluid inclusions, and C-13, O-18, and Sr-87/Sr-86 isotopic values of the diagenetic minerals. Up-migrated hydrothermal fluids from the deeper Cambrian strata may have contributed to the hot brine with high sulfate concentrations which promoted thermochemical sulfate reduction (TSR) in the Ordovician, resulting in the formation of C-12-rich (C-13 as low as -13.8 parts per thousand) calcite and S-34-rich (S-34 values from 21.4 parts per thousand to 29.7 parts per thousand) H2S, pyrite, and elemental sulfur. Hydrothermal fluid mixing with fresh water in Ordovician strata in Tarim Basin was facilitated by deep-seated faults and up-reaching faults due to the pervasive Permian magmatic activity. Collectively, fluid mixing, hydrothermal dolomitization, TSR, and faulting may have locally dissolved the host carbonates and increased the reservoir porosity and permeability, which has significant implications for hydrocarbon exploration.