Geochemical evidence for an Early Cambrian origin of the 'Q' oils and some condensates from North Oman

Emmanuelle Grosjean, Gordon D. Love, Amy E. Kelly, Paul N. Taylor, Roger E. Summons*

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    20 Citations (Scopus)

    Abstract

    The Q family of oils from Oman was previously recognized as highly unusual and distinct from the more prevalent Huqf oil family by having an extraordinarily high C 27/C 29 sterane ratio and δ 13C values near -30‰. A re-evaluation of the hydrocarbon constituents of the Q oils using GC-MS-MS analyses resulted in identification of other useful geochemical discriminators that include gammacerane/hopane ratios between 0.5 and 0.9, C 24T/C 23T≥0.7 combined with C 22T/C 21T cheilanthane ratios <0.4. A relatively simple branched alkane, 17-methylpentatriacontane, identified on the basis of its mass spectrum is especially characteristic. A suite of C 19, C 20 and C 26 norsteranes, characterized by a dominant 203Da fragment in their mass spectra, were also present with an abundance pattern that distinguished the Q oils from other south Oman oil families and their source rocks. These features enabled us to establish a Q-source rock as the origin of a suite of highly mature condensates from north Oman despite them having exceptionally low concentrations of conventional biomarkers.Although we were unable to identify a source facies within the sediments of the South Oman Salt Basin that convincingly correlated with the hydrocarbons of the Q oils, the geochemistry of Q oils provided valuable clues about their origin. Sterane and terpane patterns indicated that the Q source rock is most likely a marine shale deposited under intermittently anoxic and hypersaline conditions. The absence of a carbon isotopic ordering anomaly for normal alkanes and isoprenoids points to a Cambrian age for the Q oils. Moreover, trends in the patterns of norsteranes in south Oman carbonate stringer oils suggest the Q oil source originates from one of the upper units of the Ara carbonate and evaporite sequence. More specifically, our results point to a siliciclastic source rock lying stratigraphically above the A6 carbonates from the Dhahaban Formation. The high relative abundances of 24-isopropylcholestanes in Q oils likely originate from demosponges and we hypothesize that the 17-methylpentatriacontane, which is so abundant in Q oils, is a biomarker for early arthropods that inhabited the salt basins of Oman at the very beginning of the Cambrian Period.

    Original languageEnglish
    Pages (from-to)77-90
    Number of pages14
    JournalOrganic Geochemistry
    Volume45
    DOIs
    Publication statusPublished - Apr 2012

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