Geochemical evidence for an Early Cambrian origin of the ‘Q’ oils and some condensates from north Oman

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 C27/C29 sterane ratio and δ13C 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, C24T/C23T⩾0.7 combined with C22T/C21T 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 C19, C20 and C26 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.