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

Crude oil, H2S and pyrite samples from Palaeozoic formations in the Tarim Basin have been analyzed for delta S-34 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 delta S-34 values from + 11.9 to + 28.2 parts per thousand. Non-degraded oils without associated H2S gas have delta S-34 values from + 11.9 to + 20.5 parts per thousand, and are relatively rich in C-28 alpha alpha alpha 20R sterane, aryl isoprenoids and/or gammacerane. The features are well-correlated with the Cambrian and Lower Ordovician source rocks with delta S-34 values from + 10.4 to + 19.4 parts per thousand. Non-degraded oils with associated H2S gas have delta S-34 values from + 15.1 to + 19.1 parts per thousand, close to the H2S (from + 15.0 to + 18.5 parts per thousand), 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 delta S-34 value of + 18.5 parts per thousand. Heavily biodegraded oils have the highest sulfur contents, and show delta S-34 values from + 20.0 to + 28.2 parts per thousand, significantly heavier than those of any of the potential source rocks (- 15.3 to + 19.4 parts per thousand). Thus, it is likely that isotopically heavy sulfides have been incorporated into the biodegraded oils, resulting in delta S-34 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 delta S-34 values mainly from + 25 to + 34 parts per thousand in the Palaeozoic reservoirs. (C)2009 Elsevier B.V. All rights reserved.