『Abstract
At four underground sites in Precambrian Shield rocks in Canada
and South Africa, hydrocarbon and hydrogen gases exsolving from
saline fracture waters are analyzed for compositional and isotopic
signatures. Dominated by reduced gases such as CH4,
H2 and higher hydrocarbons (ethane, propane,
butane), the most 13C-enriched methane end-members
at all four sites show a pattern of carbon and hydrogen isotopic
values similar to abiogenic gases produced by water-rock interaction
that have been identified previously at one site on the Precambrian
Shield in Canada. The abiogenic nature of these gases was not
previously recognized due to mixing with a second methane component
produced by microbial processes. The microbial methane end-member
is identified based on carbon and hydrogen isotopic signatures,
and DNA gene amplification (PCR) data that indicate the presence
of methanogens. A framework is presented to estimate the relative
contribution of abiogenic versus microbial hydrocarbon gases at
these sites. This approach has important implications for evaluation
of potential abiogenic hydrocarbon reservoirs in a wide range
of geological settings, including the longstanding controversy
concerning the possible contribution of abiogenic gases to economic
petroleum hydrocarbon reservoirs. The association of high concentrations
of H2 with 13C-enriched CH4 end-members, and H2 depletion
in the 13C-depleted methanogenic end-members further
suggests the possibility that abiogenic gases may support H2 autotrophy linked to methanogenesis in the deep
subsurface.
Keywords: Abiogenic; Methane; Hydrogen; Autotrophy; Deep biosphere;
Mars』
1. Introduction
2. Geological setting and samples
3. Methodology
3.1. Sampling methods
3.2. Compositional gas analysis
3.3. Isotopic analysis
3.4. DNA gene amplification (PCR)
4. Results and discussion
4.1. Isotopic patterns suggest an abiogenic origin
4.2. Mixing trends
4.3. H2-based autotrophy linked to methanogenesis
5. Conclusions
Acknowledgements
References