The role of porosity in H2/He production ratios in fracture fluids from the Witwatersrand Basin, South Africa

Abiotic H2 produced in the Precambrian lithospheric crust is a key substrate at base of metabolic chain chemosynthetic and photosynthesis-independent microbial communities, significant to our understanding life on early Earth other planets. cycling processes are also relevant recent hydrogen exploration efforts engineered subsurface environments such as radioactive waste disposal sites. In crust, through water-rock reactions (serpentinisation) radiolysis; latter directly linked He radioelement decay (U, Th). The Witwatersrand Basin South Africa an ideal place study radiolytic production pathway particular, because low abundance ultramafic mafic minerals therefore potential for serpentinisation reactions. Gas samples gas flow rate data (n = 12) were collected from surface boreholes tapping Ventersdorp Supergroups. predominantly composed CH4 (65–99%), N2 (3–27%), (0.1–15%), trace amounts C2+ hydrocarbons. Notably, these was below detection limit, despite presence - providing critical indicator removing system. Using Bayesian modelling approach, we test hypothesis that observed fluids generated in-situ, driven by subsequent methanogenesis, controlled porosity host rock. consistent with this hypothesis, can be accounted variation between 0.3 2.2% (typical values basement) across different sampling These He-rich hydrocarbon gases originate hydrogeological system porosity-constrained isolated externally-sourced fluids. Radioelement primary process driving generation energy system, utilised hydrogenotrophic methanogens deep carbon cycle. Microbial utilisation mechanism consumptions and, conversely, preservation, suggesting conditions favourable commercial discoveries likely constrained hypersaline where activity inhibited. model results under proposed (consistent N2/H2 ratio boreholes) raises possibility N2, which often co-occurs fluids, radiolysis, future work needed fully evaluate hypothesis.