Origin and use of helium in Australian natural gases

Over 800 natural gases representative of Australia's hydrocarbon-producing sedimentary basins have been analyzed for their helium abundance and around 150 gases for their helium isotopic composition (supplemented by isotopic compositions of the higher noble gases Ne, Ar, Kr and Xe). Helium shows abundance up to over 10% with the highest values in the Amadeus Basin (central Australia), while 3He/4He ratios range from around 0.01 to 4.2 Ra (Figure 1). The Gunnedah Basin of south-east Australia and the Bass and Otway basins in southern Australia show the highest 3He/4He ratios, indicating a significant mantle contribution. Interestingly the adjacent Gippsland Basin has slightly lower 3He/4He ratios. The associated CO2 has a relatively low abundance in the Gunnedah Basin (highest 3He/4He ratio) compared to some extreme concentrations of CO2 in the Otway Basin, which are associate 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 Bonaparte, Browse, Carnarvon and Perth basins in northern and south-western Australia where radiogenic helium predominates. The minor mantle contribution that is inferred from the He isotopes in these regions has resulted from the limited volcanic activity and igneous intrusions throughout the basins' evolution. The accompanying high CO2 contents of some of these gases, together with their carbon isotopic composition, infer an inorganic source most likely from 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 (or reservoir) rock has a primary control on the helium content with radiogenic 4He input increasing with residence time (Figure 1). With the recent acceleration in the exploitation of Australia's enormous reserves of natural gas, the LNG processing plants in western, northern and eastern Australia offer the opportunity to commercialize helium in gases with as low as 0.1% He. However, a key factor to the gases monetary value is its inherent N2 content. From the 15N/14N ratios, N2 associated with mineral decomposition severely impacts the economics of helium extraction. Presented at the 2017 International Meeting in Organic Geochemistry (IMOG)