Chemical Geology, 2009, 268: 197-210

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

Chunfang Cai, Chunming Zhang, Liulu Cai, Guanhui Wu, Lei Jiang, Zhiming Xu, Kaikai Li, Anlai Ma and Lixin Chen

Abstract

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.