TSR origin of sulfur in Permian and Triassic reservoir bitumen, East Sichuan Basin, China

Chunfang Cai*, Kaikai Li, Yangming Zhu, Lei Xiang, Lei Jiang, Tenger, Xunyu Cai, Liu Lu Cai

*Corresponding author for this work

    Research output: Contribution to journalArticle

    51 Citations (Scopus)

    Abstract

    Solid bitumen, H2S, pyrite and anhydrite samples from the Lower Triassic Feixianguan Formation and Upper Permian Changxing Formation, and source rock kerogen samples from the Cambrian, Ordovician, Silurian and Permian in the East Sichuan Basin were analyzed for their sulfur content and δ34S values, to determine if the measurements were consistent with a thermochemical sulfate reduction (TSR) origin of the produced H2S and solid bitumen. The results show that the solid bitumen samples have sulfur contents from 2.6 to 12.5wt.% and δ34S values from+13.4‰ to+22.1‰. The δ34S values are significantly heavier than those of potential Upper Permian source rock kerogens that average+1.3‰ (n=3), suggesting that most of the sulfur in the solid bitumen is derived from isotopically heavy H2S of TSR origin (from+12.0‰ to+13.7‰). Interestingly, two gases produced from gas to water transition zones during drill stem testing (DST) show significantly lighter δ34S values (+3.3‰ and+4.1‰) than the H2S from gas-producing intervals. The isotopically light H2S may be genetically linked to fracture-filling isotopically heavy anhydrite with δ34S values from+38.5‰ to+39.6‰, which are significantly heavier than those of coeval seawater sulfate (+11.8‰ to+21.7‰) as measured for bedded anhydrite. The link may have resulted from reduction of part of the dissolved sulfate by hydrocarbons under sulfate-limited conditions. Source rock kerogens show a secular change in δ34S values relative to source rock age. A dramatic decrease and then increase in δ34Skerogen was found toward the Permian/Triassic boundary for the first time in the East Sichuan Basin.

    Original languageEnglish
    Pages (from-to)871-878
    Number of pages8
    JournalOrganic Geochemistry
    Volume41
    Issue number9
    DOIs
    Publication statusPublished - Sep 2010

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