The chemical structure of a pronounced series of iso-alkanes in South Oman crudes

Gas chromatograms of the saturated hydrocarbon fractions of the South Oman crudes are characterised by a striking series of peaks (called X-peaks throughout this paper) eluting between the n-alkanes mainly in the C₂₀–C₂₆ range. A GS-MS study, combined with optimised GC resolution shows that each peak is a superposition of subpeaks which correspond to a complex series of monomethylalkanes. The X₂₀ peak in the chromatogram, for instance, is found to be composed of virtually co-eluting peaks corresponding to 5-, 6-, 7-, 8-, 9- and 10-methylnonadecane. The larger X₂₂, X₂₃ and X₂₄ peaks are superpositions of a similar set of monomethylalkanes and in addition a series of (presumably) dimethylalkanes of which the structures are only partly solved. The large abundance of these iso-alkanes is very specific for South Oman crudes and may be caused by their (pre-) Cambrian origin or a specific source material.