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 Huqf oil, the saturated hydrocarbon fractions, the desulfurized polar fractions of the bitumens and the kerogen pyrolysates of three potential source rocks from the Huqf Formation (Oman) were analyzed in order to resolve the origin of these mid-chain branched monomethyl alkanes. Using off-line pyrolysis and chemical degradation in combination with compound specific carbon isotope analysis, it was assessed that the mid-chain branched monomethyl alkanes 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 through reactions of sulfur with functional groups at different positions in the precursor lipid(s).