Origin of free and bound mid-chain methyl alkanes in oils, bitumens and kerogens of the marine, Infracambrian Huqf Formation (Oman)

The Huqf oil from Oman and other time-equivalent oils are characterized by relatively large contents of mid-chain branched monomethyl alkanes. The Huq f oil, the saturated hydrocarbon fractions, the desulfurized polar fraction s of the bitumens and the kerogen pyrolysates of three potential source roc ks from the Huqf Formation (Oman) were analyzed in order to resolve the ori gin of these mid-chain branched monomethyl alkanes. Using off-line pyrolysi s and chemical degradation in combination with compound specific carbon iso tope analysis, it was assessed that the midchain branched monomethyl alkane s are not rearrangement products but are derived from a biological source, probably from lipids with C28+ carbon skeletons and methyl branching at the 12- or 13-position. These compounds were incorporated in the kerogen throu gh reactions of sulfur with functional groups at different positions in the precursor lipid(s). (C) 1999 Elsevier Science Ltd. All rights reserved.