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 C-65, 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. (C) 2000 Elsevier Science Ltd. All rights reserved.