Petroleum migration in the Georgina Basin

evidence from the geochemistry of oil inclusions and bitumens

H. Volk, S. C. George, R. H. Kempton, Keyu Liu, M. Ahmed, G. J. Ambrose

    Research output: Chapter in Book/Report/Conference proceedingConference proceeding contribution

    Abstract

    Although commercial petroleum occurrences are yet to be discovered in the largely unexplored Georgina Basin, petroleum generation and migration are indicated by the occurrence of conventional oil and gas shows, as well as solid bitumen in exploration wells. The Quantitative Grain Fluorescence on Extract (QGF-E) technique was used to detect residual oil zones in the wells Elkedra-2, -6, -7A and Ross-1. Fluorescence microscopy confirmed the presence of oil-bearing fluid inclusions hosted in replacement dolomites in samples between 960.7 m to 970.1 m (Middle Cambrian Thorntonia Limestone) in Ross-1, and vug-filling dolomite crystals between 188 m and 195 m (Late Cambrian Hagen Member of the Chabalowe Formation) in Elkedra-7A. The sections with inferred residual oil from QGF-E intensities above baseline (10 photometer counts) are broadly coincident with the occurrence of oil shows and solid bitumens in core and the presence of oil inclusions in dolomite. The molecular compositions of oil inclusions in Elkedra-7A (188.3–188.8 m and 190.6–191.1 m) and Ross-1 (965.5 m) were analysed off-line by breaking up cores into small rock chips (1 to 5 mm) and solventrinsing, prior to crushing into a fine powder and Soxhlet solvent extraction. Complementary on-line thermal extraction of extracted rock chips provided information on low-molecular-weight hydrocarbons preserved in oil inclusions. The carbonate lithology of the host rocks inhibited the use of oxidising acids that are used for sample clean-up of siliciclastic grains containing oil inclusions. Furthermore, individual dolomite crystals containing oil inclusions could not be separated due to the tightly cemented lithology of the host rocks. Consequently, the thermal and solvent extracts are interpreted to be a mixture of biodegraded residue or bitumen extracted from the surface of the dolomite crystals, and a non-biodegraded oil charge containing abundant n-alkanes believed to be mainly from the inclusion oils. The thermal maturities of the Elkedra-7A and Ross-1 hydrocarbon extracts are very similar and correspond to a mid-oil window maturity level, equivalent to a vitrinite reflectance of about 0.7% to 0.8%. The Elkedra-7A extracts are interpreted to have been derived from a carbonate source rock, dominated by algal and/or bacterial organic matter rich in organic sulfur and deposited in an anoxic environment. They contain abundant 2,3,6-aryl isoprenoids, a series of monoaromatic hydrocarbons that may be a degradation product of isorenieratane, a photic zone anoxia indicator of a shallow and very restricted source rock environment. The very pronounced predominance of n-C₁₇ and n-C₁₉ is consistent with source input from Gloeocapsomorpha prisca, an extinct microorganism with enigmatic phylogeny that is common in Ordovician source rocks, but also occurs in rocks and oils of different ages. However, there are also some differences to the classic examples of G. prisca-derived oils in the Amadeus Basin. Regular steranes are dominated by C₂₉ isomers, typical for oils sourced from lower Palaeozoic strata. Although the Ross-1 (965.5 m) hydrocarbon extract shares some similarities, such as the predominance of n-C₁₇ and n-C₁₉ alkanes, with the two Elkedra-7A extracts, many of its molecular characteristics are quite different. Aryl isoprenoids could not be detected in the Ross-1 sample. However, this negative evidence for a specific depositional environment may be influenced by the low yield of aromatic hydrocarbons in this sample. Tricyclic terpanes are abundant in the Ross-1 sample, but to a lesser degree and with a different relative distribution than in the Elkedra-7A samples. C₂₉ steranes are not as dominant as in the Elkedra-7A samples, whereas rearranged diasteranes and diahopanes are more abundant. These data suggest a less restricted and more clayrich depositional environment for the source rock that generated the Ross-1 hydrocarbons, compared to that which generated the Elkedra-7A hydrocarbons. A comparison with published data indicates that the Elkedra-7A fluid inclusion oils are most likely derived from the underlying Arthur Creek Formation, whereas the fluid inclusion oil in Ross-1 is probably derived from organic-rich intercalations within the Thorntonia Limestone.
    Original languageEnglish
    Title of host publicationProceedings
    Subtitle of host publicationCentral Australian basins symposium
    EditorsT.J. Munson, G.J. Ambrose
    Place of PublicationDarwin
    PublisherNorthern Territory Geological Survey
    Pages282-303
    Number of pages22
    ISBN (Print)9780724571123
    Publication statusPublished - 2007
    EventCentral Australian Basins Symposium (CABS) - Alice Springs
    Duration: 16 Aug 200518 Aug 2005

    Conference

    ConferenceCentral Australian Basins Symposium (CABS)
    CityAlice Springs
    Period16/08/0518/08/05

    Keywords

    • Georgina Basin
    • Early Palaeozoic
    • depositional environment
    • Thorntonia Limestone
    • Arthur Creek Formation
    • Hagen Member
    • petroleum geology
    • petroleum exploration
    • oil inclusions
    • Gloeocapsomorpha prisca
    • aryl isoprenoids

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