Thermochemical sulfate reduction and origin of sulfur in crude oils in Palaeozoic carbonates

Different sulfur-bearing species from Palaeozoic formation have been analyzed for δ³⁴S values aiming to addressing their origin and potential application in oil-source rock correlation. The results show that oils produced from the Ordovician are sulfur-poor and have δ³⁴ S values from 13.6 to 19.9‰. The values are close to those of associated H₂S gas and authigenic pyrite in the range from 15 to 18.5‰. Oils produced from the Carboniferous and Silurian are sulfur-rich and have δ ³⁴S values from 20‰ to 25.8‰ whilst associated pyrite has much wider values from 9.5‰ to 34‰. Interestingly, anhydrite and barite veins in enlarged fractures show much higher δ³⁴S values (from 44.2‰ to 46.62%) than those of Phanerozoic seawater, suggesting that the sulfates may have been the residue of thermochemical sulfate reduction (TSR) due to kinetic sulfur isotope fractionation. Small amount of pyrite was found to precipitate near TSR site in the Ordovician but did not inhibit TSR proceeding, whilst much higher pyrite occurs in the overlying strata. Relationship between oil sulphur content and δ³⁴S value shows that TSR-derived inorganic sulfur may have been incorporated into petroleum. However, sulfur-poor oils have low δ³⁴S values distinct from inorganic sulfur. The oils show 2.5‰ to 7.5‰ lower than the Cambrian potential source rock, but much higher than the Upper Ordovican potential source rock, indicating that they may have derived from the Cambrian. Thus, it is reasonable to apply sulfur-poor oil δ³⁴S value to petroleum-source rock correlation.