Unravelling abiogenic and biogenic sources of methane in the Earth's deep subsurface

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, H-2 and higher hydrocarbons (ethane, propane, butane), the most C-13-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 geologic settings, including the longstanding controversy concerning the possible contribution of abiogenic gases to economic petroleum hydrocarbon reservoirs. The association of high concentrations of H-2 with C-13-enriched CH4 end-members, and H-2 depletion in the C-13-depleted methanogenic end-members further suggests the possibility that abiogenic gases may support H-2 autotrophy linked to methanogenesis in the deep subsurface. (c) 2005 Elsevier B.V. All rights reserved.