An isotopic study of hydrocarbon generation processes

The sources of reservoired liquid hydrocarbons in Australia differ substantially from their Northern Hemisphere counterparts. Not only are Australian source rocks largely of terrigenous origin but the concepts accepted as defining the hydrocarbon potential of source rocks have recently been challenged. Although isotopic measurements on Australian source rocks and liquid hydrocarbon products closely parallel those published from elsewhere, new interpretations of these data have been made, particularly with regard to D/H ratios in coals. As interesting as such results may be, the most important findings relate to the origins and histories of methane, the higher gaseous hydrocarbons and carbon dioxide, and the interpretation of these data in terms of hydrocarbon genesis. Simple Northern Hemisphere concepts which characterize gases as being biogenic, oil-associated, etc., on the bases of their isotopic and chemical compositions are clearly not applicable to the Australian scene. Indeed justification of such precise classifications in the Northern Hemisphere on the available evidence is also questioned.Isotopic and compositional data from gases from the Cooper, Bowen, Sydney and Gippsland Basins and from the North West Shelf (Australia) are reviewed, and assessed in terms of generation pathways.