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

Karolytė, R. and Warr, O. and van Heerden, E. and Flude, S. and de Lange, F. and Webb, S. and Ballentine, C. J. and Sherwood Lollar, B. (2022) The role of porosity in H2/He production ratios in fracture fluids from the Witwatersrand Basin, South Africa. Chemical Geology, 595. 120788. ISSN 0009-2541 (https://doi.org/10.1016/j.chemgeo.2022.120788)

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Abstract

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

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