H213-10
The vertical transport of helium, hydrogen and methane within the Witwatersrand Basin, South Africa

Wednesday, 16 December 2020: 17:57
Virtual
Rūta Karolytė1, Oliver Warr2, Esta van Heerden3, Johanna Lippmann-Pipke4, Dirk Opperman5, Phillip Armand Bester6, Fanie de Lange7, Susan Jane Webb8, Tullis C Onstott9, Chris J Ballentine10 and Barbara Sherwood Lollar2, (1)University of Oxford, Department of Earth Sciences, Oxford, United Kingdom, (2)University of Toronto, Department of Earth Sciences, Toronto, ON, Canada, (3)North-West University, Mahikeng, South Africa, (4)HZDR, Research Seite Leipzig, Leipzig, Germany, (5)University of the Free State, Department of Microbial, Biochemical, and Food Biotechnology, Bloemfontein, South Africa, (6)University of the Free State, Department of Medical Microbiology and Virology, Bloemfontein, South Africa, (7)University of the Free State, Bloemfontein, South Africa, (8)Univ Witwatersrand, Johannesburg, South Africa, (9)Princeton University, Department of Geosciences, Princeton, NJ, United States, (10)University of Oxford, Earth Sciences, Oxford, United Kingdom
Abstract:
The production of helium and hydrogen over millions to billions of years within stable cratonic regions can lead to high concentrations in fluid-bearing fractures. These fluids have been discovered within the deep mines in South Africa, Canada and Fennoscandia, however, potential mechanisms of vertical fluid transfer towards the surface remain poorly understood.

Here we present major gas concentration, stable isotope, and gas flow rate data from 19 mine exploration boreholes (514 – 1750 m depth) within the Witwatersrand and Ventersdorp Supergroups in the Free State, South Africa. Gas flow rates were monitored for 14 consecutive months in 12 gas-emitting boreholes. The average observed gas flow from all boreholes was 2.6 x 106 L/day and remained stable throughout the observation time. The gases were predominantly methane in composition (62 – 99%), with high helium (0.1 – 15.6%) and nitrogen (3 – 27%) component, and trace amounts of C2-C4 hydrocarbons. The minimum estimate of pore and fracture fluid degassed per borehole is 0.3 – 14 x 106 L/day. The calculated helium residence times are 0.2 – 16 Ma, significantly younger than the host formations (2.7 – 2.9 Ga), indicating active vertical volatile transport within the Archean basement and to the overlying Karoo sediments. The total helium balance is a function on in-situ production, flux from the deeper basement, where fluids with >2 Ga component have been previously discovered1, and diffusive or advective loss to the overlying formations. C1 isotopic signatures and C1/C2+ ratios suggest microbial methanogenesis, likely by carbon dioxide reduction to methane with hydrogen gas as an electron-donating reductant, in line with previous observations in shallower mines2. Likewise, the hydrogen concentrations were below the detection limit in the gas phase and up to 351 ppm in the water phase, suggesting efficient hydrogen autotrophy within the sampled formations2. This is in contrast to the hydrogen-rich (up to 54% in the gas phase) deep mine fluids2.

  1. Lippmann-Pipke, J. et al. Neon identifies two billion year old fluid component in Kaapvaal Craton. Chem. Geol. 283, 287–296 (2011).
  2. Sherwood Lollar, B. et al. Unravelling abiogenic and biogenic sources of methane in the Earth’s deep subsurface. 226, 328–339 (2006).
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