Thermochemical and bacterial sulfate reduction in the C ambrian and L ower O rdovician carbonates in the T azhong A rea, T arim B asin, NW C hina: evidence from fluid inclusions, C , S , and S r isotopic data

Petrographic features, C , O , S , and S r isotopes were determined, and fluid inclusions ( FI ) were analyzed on various stages of vug‐ and fracture‐fillings from the C ambrian and L ower O rdovician reservoirs in the T azhong area, T arim basin, NW C hina. 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 δ 34 S values from −22‰ to +31‰. The pyrites with low δ 34 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 ‐ T h) from 55.7 to 73.2°C, salinities from 1.4wt% to 6.59wt% N a C l equiv and δ 13 C values from −2.3‰ to −14.2‰, indicating an origin from bacterial sulfate reduction by organic matter. Other sulfides with heavier δ 34 S values may have formed by thermochemical sulfate reduction ( TSR ) during two episodes. The earlier TSR in the M iddle and L ower C ambrian resulted in pyrites and H 2 S having δ 34 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 δ 13 C values as light as −17.7‰ and FI ‐ T h of about 120–145°C, and pyrite and H 2 S with δ 34 S values close to those of the U pper C ambrian 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 S r/ 86 S r values of anhydrite and TSR calcites from 0.7091 to 0.7125 suggests a source from the underlying E diacaran seawater sulfate and detrital S r contribution.