The identification of crocetane in Australian crude oils

The C20 irregular isoprenoid crocetane (2,6,11,15-tetramethylhexadecane) has been unambiguously identified in a series of Western Australian crude oils from the Canning (Ordovician/Devonian) and Perth (Lower Triassic) Basins. PMI (2,6,10,15,19-pentamethylicosane) was found to co-occur with crocetane in the same samples. In addition, a newly observed series of C22–C24 irregular isoprenoids possessing the same carbon skeleton have also been tentatively identified in each of these crude oils on the basis of GC retention times and mass spectral characteristics. This may indicate a source for crocetane, in the Canning and Perth Basin crude oils, from the diagenesis of PMI. The stable carbon isotopic composition for combined crocetane/phytane in these samples suggests crocetane is not unusually depleted in 13C compared with all previous reports. This is the first reported occurrence of biomarkers attributable to methanogenic archaea in crude oils.