A QUEST FOR A NEW PARAMETER IN PETROLEUM EXPLORATION GEOCHEMISTRY

Primary Australian terreestrially-derived crudes are characterised by high wax and n-alkane contents.These characteristics, as determined by hydrogenation and hydrous pyrolysis, appear to be unrelated to either the chemical or petrographic compositions of Victorian brown coal lithotypes. Furthermore, since relationships between chemical and petrographic composition are obscure, a re-examination of current concepts which relate these established source rock parameters to liquid hydrocarbon generating potentials is warranted. The content of thermally stable, longer chain, n-alkyl components in source rocks is introduced as the critical factor in determining whether these rocks have the potential to generate typical Australian waxy crudes or hydrocarbon gases. Modifications to this general concept are required by the thermal stability of directly substituted longer chain n-alkyl aromatics and hydroaromatics. These appear to be sources of light hydrocarbons and gases, rather than oils. Inherent weaknesses in the experimental techniques of hydrogenation and hydrous pyrolysis have hindered the collection of data, but the concept that n-alkane potential is a critical factor in determining the petroleum-generating potential of immature source rocks is being pursued using techniques modified for the determination of their total heteroatom-bonded n-alkyl contents.