Natural hydrogen in low temperature geofluids in a Precambrian granite, South Australia. Implications for hydrogen generation and movement in the upper crust

Abstract

Natural hydrogen (H2) has the potential to be a low-carbon fuel and energy source, playing a crucial role in achieving a clean, secure, and affordable energy future. However, our understanding of the generation, migration, and accumulation of H2 in the subsurface remains poorly constrained. Increasing evidence suggests that H2 is abundantly and widely present in various geological settings, making them viable targets for exploration of this emerging resource. Reports have indicated the presence of hydrogen-dominated natural gas in vintage boreholes located in the Yorke Peninsula and Kangaroo Island, South Australia. These boreholes were drilled in Cambrian sedimentary cover over Precambrian terranes that host iron ore, copper or gold ore deposits. To gain insight into these occurrences, rock samples were examined from the Roxby Downs granite, a Mesoproterozoic pluton associated with the Hiltaba Suite event (1.59 Ga), belonging to the same magmatic origin as the Yorke Peninsula basement.Our investigation revealed metasomatic fluid circulation, evidenced by alteration of feldspar and mafic grains, as well as discrete cemented fractures within the granite. In some of these fracture-fill cements, we discovered water and gas inclusions containing hydrogen. The formation of the hydrogen-bearing cements occurred at paleo-temperatures ranging from 170 °C to the present-day 55 °C. Analysing the oxygen isotope values of the quartz cements and considering their fluid inclusion temperatures, a marine water signature as the source of water in equilibrium with the quartz cements was identified.To constrain the fluid history, neon and argon isotope signatures from the fluid inclusion gases were measured from the granite and separated quartz grains. Nucleogenic neon isotopes increased with depth along specific production lines. While this is consistent with long-lived fluid within the granite and increasing fluid residence with depth, separated quartz grains show less variation with depth and suggest that younger fluids were introduced, triggering alteration of less robust minerals, visible in bulk granite. Petrographic analyses identified hydrolytic alterations of mafic minerals into magnetite, and subsequently magnetite to hematite, as sources of hydrogen and contributors to the observed increase in pore water salinity. Our findings highlight that natural hydrogen can be produced and retained in granites at low temperatures, expanding beyond the commonly reported alteration processes observed in mafic and ultramafic rocks.