Origin of sulfides in the Middle and Lower Ordovician carbonates in Tahe oilfield, Tarim Basin

Middle and Lower Ordovician carbonate reservoirs in Tahe area, northern Tarim Basin, NW China, are shown to have been diagenetieally altered by meteoric water and hot brines from the deeper underlying strata. Some of the reservoirs contain up to 8.3% H₂S in natural gas composition and up to 11% H₂S in gas-phase fluid inclusions in fracture- and vug-filling calcites, respectively. The host calcites have homogenization temperatures of fluid inclusions mainly from f 10. 2 to 198. 9°C, and the H₂S and pyrite aggregate have δ³⁴ S values mainly from 18‰ to 22‰. These features suggest that the sulfides may have generated by thermochemical sulfate reduction by organic matter ( TSR). Oxidization of organic matter is evidenced by light δ¹³ C values from -4. 3‰ to - 8. 3‰ for the calcite and from - 6.0‰ to - 13.8‰ for present formation water HCO₃ ^-. The fact that H₂S concentrations in the associated gas are significantly lower than those of fluid inclusions may indicate that the H₂S may have precipitated as pyrite, and/or incorporated into crude oils to generate sulfur enriched oils. Some pyrite shows light δ³⁴S values as low as - 26‰, indicating a result from bacterial sulphate reduction. However, distribution of pyrite of BSR origin is relatively limited.