Origin of Nama Basin bitumen seeps: petroleum derived from a Permian lacustrine source rock traversing southwestern Gondwana

Roger E. Summons*, Janet M. Hope, Roger Swart, Malcolm R. Walter

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    77 Citations (Scopus)

    Abstract

    Biodegraded bitumens associated with quartz and calcite veins in the Cambrian Fish River Subgroup sediments of the Nama Group of southern Namibia have a geochemical signature diagnostic for organic matter that was deposited in a saline lacustrine palaeoenvironment. In particular, they contain abundant gammacerane, β-carotane and 3β-methylhopanes while 24-isopropyl cholestanes and dinosteroids are not detectable. Sealed tube hydrous pyrolysis of asphaltene and polar fractions yielded saturated hydrocarbons amenable to C isotopic analysis, and these analyses show unusually low δ 13C values. These combined characteristics are also present in immature bitumens from the Permian Irati Formation of Brazil and a saline lacustrine facies of the Whitehill Formation in the Karoo Basin, South Africa. We conclude that the bitumens originated from Whitehill equivalent strata of the Kalahari Basin deposited in what was an extensive saline lacustrine basin in southwestern Gondwana during the Early Permian. In southern Africa, source rocks of the Whitehill Formation are generally immature for petroleum generation and it is therefore likely that the Nama bitumens were expelled by contact metamorphism during emplacement of Karoo dolerite sills and dykes in the Jurassic.

    Original languageEnglish
    Pages (from-to)589-607
    Number of pages19
    JournalOrganic Geochemistry
    Volume39
    Issue number5
    DOIs
    Publication statusPublished - May 2008

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