Hydrocarbon Gas and Microbial Geochemistry in the Deep Subsurface of the Witwatersrand Basin, South Africa
Abstract
Field investigations in the deep mines of South Africa, targeting gas and fluids under extreme conditions of temperature, pressure, salinity, and locally radioactivity indicate the existence of micro-organisms (in particular methanogens). Gases associated with the saline water pockets (>3km in depth) within the Precambrian Witwatersrand Basin, South Africa are rich in hydrocarbons (methane, ethane, propane and butane). Compositional analyses and stable carbon and hydrogen isotopes of individual hydrocarbon components are used to identify the origin of the gases. Based on the depleted δ13C signatures and high methane/higher hydrocarbon ratios, 38-100% of the gas encountered in the Witwatersrand quartzite and shale sequences is estimated to be microbial in origin. In contrast, an isotopic pattern consistent with abiogenic gas is found in samples from the Ventersdorp lava sequences, although some degree of mixing with microbial gases is also indicated. Mixing between abiogenic and microbially produced hydrocarbons explains the range of isotopic signatures and compositions obtained for the samples. Confirmation of the presence of methanogens based on microbial evidence will be presented.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2002
- Bibcode:
- 2002AGUFM.B11B0743W
- Keywords:
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- 0400 BIOGEOSCIENCES