AN ISOTOPIC STUDY OF GASES AND HYDROCARBONS IN THE COOPER BASIN

Abstract

Gases and/or liquid hydrocarbons from a total of some 70 locations and stratigraphic levels in the Cooper Basin have been isotopically analysed for D and 13C. Measurements on liquid hydrocarbons show significant variation in 13C contents in accord with the view that the Tirrawarra Formation is the most oil-prone sequence. The constancy in 5D values is interpreted as confirming a similar freshwater environment of maturation for all the condensates and oils examined.

Isotopic and compositional differences between natural gases from humic sources and coal seam gases suggest that dispersed organic matter (DOM) rather than seam coal is the main source of gaseous and liquid hydrocarbons in the Copper Basin.

Carbon dioxide, which occurs in gases in concentrations varying from < 1 per cent to > 50 per cent, proves to be of considerable interest. Although current views appear to favour the thermal decomposition of humic (coaly) material as the major source of this gas, the isotopic data suggest such an origin to be unlikely. On the present evidence a source associated with the underlying granite is preferred.