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Petrographic features, C, O, S, and Sr isotopes were determined, and fluid inclusions ( FI) were analyzed on various stages of vug- and fracture-fillings from the Cambrian and Lower Ordovician reservoirs in the Tazhong area, Tarim basin, NW China. The aim was to assess the origin of pyrite and anhydrite and the processes affecting sulfur during diagenesis of the carbonates. Pyrite from seven wells has δ34 S values from −22‰ to +31‰. The pyrites with low δ34 S values from −21.8‰ to −12.3‰ were found close to fracture-filling calcites with vapor-liquid double-phase aqueous fluid inclusions homogenization temperatures ( FI- Th) from 55.7 to 73.2°C, salinities from 1.4wt% to 6.59wt% Na Cl equiv and δ13 C values from −2.3‰ to −14.2‰, indicating an origin from bacterial sulfate reduction by organic matter. Other sulfides with heavier δ34 S values may have formed by thermochemical sulfate reduction ( TSR) during two episodes. The earlier TSR in the Middle and Lower Cambrian resulted in pyrites and H2 S having δ34 S values from 30 to 33‰, close to those of bedded anhydrite and oilfield water (approximately 34‰). The later TSR is represented by calcites with δ13 C values as light as −17.7‰ and FI- Th of about 120-145°C, and pyrite and H2 S with δ34 S values close to those of the Upper Cambrian burial-diagenetic anhydrite (between +14.8‰ and +22.6‰). The values of the anhydrite are significantly lighter than contemporary seawater sulfates. This together with 87 Sr/86 Sr values of anhydrite and TSR calcites from 0.7091 to 0.7125 suggests a source from the underlying Ediacaran seawater sulfate and detrital Sr contribution. [ABSTRACT FROM AUTHOR]

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 δ13C, δ18O, δ34S, and 87Sr/86Sr 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°C higher than the paleo-formation burial temperatures both in the NW Tarim and in the Central Tarim areas. The mixing of hot (>200°C), high-salinity (>24 wt% NaCl), 87Sr-rich (up to 0.7104) hydrothermal fluid with cool (60-100°C), low-salinity (0 to 3.5 wt% NaCl), also 87Sr-rich (up to 0.7010) meteoric water in the Ordovician unit was supported by the salinity of fluid inclusions, and δ13C, δ18O, and 87Sr/86Sr 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 12C-rich (δ13C as low as −13.8‰) calcite and 34S-rich (δ34S values from 21.4‰ to 29.7‰) 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. [ABSTRACT FROM AUTHOR]