Tracing the migration of mantle CO2 in gas fields and mineral water springs in south-east Australia using noble gas and stable isotopes

Abstract Geochemical monitoring of CO 2 storage requires understanding of both innate and introduced fluids in the crust as well as the subsurface processes that can change the geochemical fingerprint of CO 2 during injection, storage and any subsequent migration. Here, we analyse a natural analogue of CO 2 storage, migration and leakage to the atmosphere, using noble gas and stable isotopes to constrain the effect of these processes on the geochemical fingerprint of the CO 2 . We present the most comprehensive evidence to date for mantle-sourced CO 2 in south-east Australia, including well gas and CO 2 -rich mineral spring samples from the Otway Basin and Central Victorian Highlands (CVH). 3 He/ 4 He ratios in well gases and CO 2 springs range from 1.21 to 3.07 R A and 1.23 – 3.65 R C /R A , respectively. We present chemical fractionation models to explain the observed range of 3 He/ 4 He ratios, He, Ne, Ar, Kr, Xe concentrations and δ 13 C(CO 2 ) values in the springs and the well gases. The variability of 3 He/ 4 He in the well gases is controlled by the gas residence time in the reservoir and associated radiogenic 4 He accumulation. 3 He/ 4 He in CO 2 springs decrease away from the main mantle fluid supply conduit. We identify one main pathway for CO 2 supply to the surface in the CVH, located near a major fault zone. Solubility fractionation during phase separation is proposed to explain the range in noble gas concentrations and δ 13 C(CO 2 ) values measured in the mineral spring samples. This process is also responsible for low 3 He concentrations and associated high CO 2 / 3 He, which are commonly interpreted as evidence for mixing with crustal CO 2 . The elevated CO 2 / 3 He can be explained solely by solubility fractionation without the need to invoke other CO 2 sources. The noble gases in the springs and well gases can be traced back to a single end-member which has suffered varying degrees of radiogenic helium accumulation and late stage degassing. This work shows that combined stable and noble gas isotopes in natural gases provide a robust tool for identifying the migration of injected CO 2 to the shallow subsurface.