Hydrocarbon Gases in Hydrogeologically Isolated Fractures in Au Mines of the Witwatersrand Basin, South Africa: Potential Substrates for Deep Subsurface Microorganisms
Abstract
Deep subsurface microbial communities are the subject of a multi-disciplinary study in the Witwatersrand Basin, of South Africa. Hydrocarbon and H2 gases found in the mines were investigated to determine their origin and role as potential substrates for long-term survival of microorganisms. Large quantities of gas (up to 30L/min/borehole) are released when sealed fracture systems are opened by exploration drilling. Two compositionally and isotopically distinct types of gases have been identified in the Witwatersrand and Ventersdorp sequences beneath the Transvaal Sequence. Group 1 gases correspond to major fault intersections within the Witwatersrand Supergroup and contain methane that is isotopically depleted in 13C (-43.5 to -59.4 permil), and enriched in 2H (-162 to -214 permil) consistent with either bacteriogenic and/or thermogenic orgins. Group 2 gases discharges in association with saline fissure waters from fractures within the andesitic lava of the Ventersdorp Sequence. They are isotopically depleted in 2H (to -327 permil), and enriched in 13C (to -34.4 permil) falling outside the conventional bacteriogenic or thermogenic isotopic fields. Group 2 gases do however resemble those reported from Precambrian Shield mines in Canada for which an abiogenic origin has been suggested. Differences in the microbial communities associated with these different gas types and geologic strata are under investigation.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFM.B22B0149W
- Keywords:
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- 0400 BIOGEOSCIENCES;
- 1040 Isotopic composition/chemistry;
- 1055 Organic geochemistry