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

Abstract

Crude oil, H 2S and pyrite samples from Palaeozoic formations in the Tarim Basin have been analyzed for δ 34S 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 34S values from + 11.9 to + 28.2‰. Non-degraded oils without associated H 2S gas have δ 34S 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 34S values from + 10.4 to + 19.4‰. Non-degraded oils with associated H 2S gas have δ 34S values from + 15.1 to + 19.1‰, close to the H 2S (from + 15.0 to + 18.5‰), suggesting that the H 2S was incorporated into the oils, leading to the generation of 2-thiaadamantanes and likely alkyl-thiolanes in the oil with a δ 34S value of + 18.5‰. Heavily biodegraded oils have the highest sulfur contents, and show δ 34S 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 34S 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 2S-rich fluid inclusions and late-stage and fracture-filling pyrite with δ 34S values mainly from + 25 to + 34‰ in the Palaeozoic reservoirs.