Crude oil, H2S and pyrite samples from Palaeozoic formations in the Tarim Basin have been analyzed for δ34S values together with oil samples for biomarkers, and fluid inclusions for chemical composition, to elucidate the origin of sulfur in the oil and to address potential applications in oil-source rock correlation. The results show that crude oil samples have sulfur contents from 0.02 to 2.47% and δ34S values from + 11.9 to + 28.2‰. Non-degraded oils without associated H2S gas have δ34S values from + 11.9 to + 20.5‰, and are relatively rich in C28 ααα 20R sterane, aryl isoprenoids and/or gammacerane. The features are well-correlated with the Cambrian and Lower Ordovician source rocks with δ34S values from + 10.4 to + 19.4‰. Non-degraded oils with associated H2S gas have δ34S values from + 15.1 to + 19.1‰, close to the H2S (from + 15.0 to + 18.5‰), suggesting that the H2S was incorporated into the oils, leading to the generation of 2-thiaadamantanes and likely alkyl-thiolanes in the oil with a δ34S value of + 18.5‰. Heavily biodegraded oils have the highest sulfur contents, and show δ34S values from + 20.0 to + 28.2‰, significantly heavier than those of any of the potential source rocks (- 15.3 to + 19.4‰). Thus, it is likely that isotopically heavy sulfides have been incorporated into the biodegraded oils, resulting in δ34S values of the oils becoming closer to Cambrian and Ordovician age seawater sulfates. The sulfides may have originated from thermochemical sulfate reduction (TSR), as evidenced by the occurrence of H2S-rich fluid inclusions and late-stage and fracture-filling pyrite with δ34S values mainly from + 25 to + 34‰ in the Palaeozoic reservoirs.
- Oil-source rock correlation
- Organic sulfur
- Tarim Basin
- Thermochemical sulfate reduction (TSR)