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 ult rasonically with boiling toluene, from coals from the Tara-1 well in the Gr eat South Basin, New Zealand. The coals represent an evolutionary trend fro m immature to post oil expulsion. In all samples the C40+ alkanes were char acterised by dominant straight-chain members up to about C-65, which decrea sed significantly in abundance in bitumen with increasing maturity. They ar e present at an early stage of maturity, prior to the main phase of n-alkan e generation, and may be liberated from cutan/cutin sources within kerogen by thermal decarboxylation of esters or have an earlier origin in the bitum en inherited from diagenesis. An odd-over-even predominance in the C40+ n-a lkanes was most marked in the C51-C57 range and decreased with increasing m aturity, suggesting that minor amounts of C40+ n-alkanes with a CPI approac hing 1 are also generated during catagenesis. However, the observed C40+ n- alkane distributions may be affected to a degree by decreasing efficiency o f solvent extraction with increasing n-alkane molecular weight, and by furt her long-chain n-alkanes generated during catagenesis being partially inacc essible to solvent extraction. (C) 2000 Elsevier Science Ltd. All rights re served.