Origins of Palaeozoic oils in the Tarim Basin: Evidence from sulfur isotopes and biomarkers

Abstract Crude oil, H 2 S and pyrite samples from Palaeozoic formations in the Tarim Basin have been analyzed for δ 34 S 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 δ 34 S values from + 11.9 to + 28.2‰. Non-degraded oils without associated H 2 S gas have δ 34 S values from + 11.9 to + 20.5‰, and are relatively rich in C 28 ααα 20R sterane, aryl isoprenoids and/or gammacerane. The features are well-correlated with the Cambrian and Lower Ordovician source rocks with δ 34 S values from + 10.4 to + 19.4‰. Non-degraded oils with associated H 2 S gas have δ 34 S values from + 15.1 to + 19.1‰, close to the H 2 S (from + 15.0 to + 18.5‰), suggesting that the H 2 S was incorporated into the oils, leading to the generation of 2-thiaadamantanes and likely alkyl-thiolanes in the oil with a δ 34 S value of + 18.5‰. Heavily biodegraded oils have the highest sulfur contents, and show δ 34 S 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 δ 34 S 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 H 2 S-rich fluid inclusions and late-stage and fracture-filling pyrite with δ 34 S values mainly from + 25 to + 34‰ in the Palaeozoic reservoirs.