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.
|Number of pages||9|
|Journal||Acta Petrologica Sinica|
|Publication status||Published - 2012|
- Crude oil
- Sulfur isotopes
- Tarim Basin