High-temperature GC evidence for the early formation of C40+ n-alkanes in coals

High-temperature GC analyses were performed on a coal-sourced waxy oil from the Toka-1 well in the Taranaki Basin and on three bitumens, extracted ultrasonically with boiling toluene, from coals from the Tara-1 well in the Great South Basin, New Zealand. The coals represent an evolutionary trend from immature to post oil expulsion. In all samples the C40+ alkanes were characterised by dominant straight-chain members up to about C65, which decreased significantly in abundance in bitumen with increasing maturity. They are present at an early stage of maturity, prior to the main phase of n-alkane generation, and may be liberated from cutan/cutin sources within kerogen by thermal decarboxylation of esters or have an earlier origin in the bitumen inherited from diagenesis. An odd-over-even predominance in the C40+ n-alkanes was most marked in the C51–C57 range and decreased with increasing maturity, suggesting that minor amounts of C40+ n-alkanes with a CPI approaching 1 are also generated during catagenesis. However, the observed C40+ n-alkane distributions may be affected to a degree by decreasing efficiency of solvent extraction with increasing n-alkane molecular weight, and by further long-chain n-alkanes generated during catagenesis being partially inaccessible to solvent extraction.