ORIGIN AND USE OF HELIUM IN AUSTRALIAN NATURAL GASES
C. Boreham1, D. Edwards1. R. Poreda2, P. Henson1.
1 Geoscience Australia, Canberra, Australia; 2 University of Rochester, NY, USA
Over 800 natural gases representative of Australia's hydrocarbon-producing sedimentary basins have been analyzed for their helium (He) content and around 150 gases for their helium isotopic composition, supplemented by isotopic compositions of the higher noble gases. Australian natural gases have helium abundances to over 10%, with the highest values in the Amadeus Basin, in central Australia, while 3He/4He ratios range from around 0.01 to 4.2 Ra (Figure 1). The onshore Gunnedah Basin of southeastern Australia and the offshore Bass and onshore/offshore Otway basins in southern Australia show the highest 3He/4He ratios, indicating a significant mantle contribution. Interestingly, the offshore Gippsland Basin, adjacent to the Bass Basin, has slightly lower 3He/4He ratios. In the Gunnedah Basin, the associated CO2 has a relatively low abundance compared to extreme concentrations of CO2 in some Otway Basin wells, which are associated with recent volcanism. The onshore Bowen and Cooper basins of eastern Australia, where natural gases are predominately sourced from Permian coals, show intermediate 3He/4He ratios with the former having a higher mantle contribution. At the other end of the spectrum, low 3He/4He ratios characterize natural gases of the offshore North West Shelf (Bonaparte, Browse, Carnarvon) and onshore/offshore Perth basins in northwestern and southwestern Australia, respectively, and radiogenic helium predominates. Hence the sometimes extensive volcanic activity and igneous intrusions in these western basins is not expressed in the helium isotopes. The accompanying high CO2 contents (up to 44%) of some of these North West Shelf gases, together with the carbon isotopic composition of CO2, infer an inorganic source most likely from the thermal decomposition of carbonates.
The geochemical data suggest that the origin of helium in Australian natural gas accumulations is region specific and complex with the component gases originating from multiple sources. The relative low CO2/3He ratio for many natural gases indicates a systematic loss of CO2 from most basins. The process by which CO2 has been lost from the system is most likely associated with precipitation of carbonates (Prinzhofer, 2013). The age of the source (and/or reservoir) rock has a primary control on the helium content with radiogenic 4He input increasing with residence time (Figure 1).
References:
Prinzhofer, A., 2013. Noble gases in oil and gas accumulations. The Noble Gases as Geochemical Tracers. Springer. 225-245.
