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

Chun Fang Cai*, Guang Hui Wu, Kai Kai Li, Li Xin Chen, Mei Li, Hong Tao Li

*Corresponding author for this work

    Research output: Contribution to journalArticle

    11 Citations (Scopus)

    Abstract

    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.

    Original languageEnglish
    Pages (from-to)44-48
    Number of pages5
    JournalBulletin of Mineralogy Petrology and Geochemistry
    Volume26
    Issue number1
    Publication statusPublished - Jan 2007

    Keywords

    • δS
    • Organic sulfur
    • Tarim basin
    • Thermochemical sulfate reduction

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