Elsevier

Chemical Geology

Volume 226, Issues 3–4, 28 February 2006, Pages 328-339
Chemical Geology

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

https://doi.org/10.1016/j.chemgeo.2005.09.027Get rights and content

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 geologic 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.

Introduction

Recent reports of CH4 in the Mars atmosphere focussed scientific and public attention on possible geological and biological sources of these hydrocarbons (Formisano et al., 2004, Kerr, 2004). Resolving the question of the origin of atmospheric CH4 on Mars is even more challenging given that distinguishing abiogenic versus biogenic sources of CH4 in the terrestrial subsurface is still controversial (Gold, 1979, Kenney et al., 2002, Shock, 1995). Deep subsurface fluids in Precambrian Shield rocks have been shown to be dominated by reduced gases such as CH4 and locally, high concentrations of H2 (up to 30% by volume) (Sherwood Lollar et al., 1993a, Sherwood Lollar et al., 1993b). Sherwood Lollar et al. (2002) used stable isotope signatures to suggest that CH4 and higher hydrocarbon gases (ethane, propane and butane) at Kidd Creek mine on the Canadian Shield are produced abiogenically by water–rock interaction such as surface-catalysed polymerization (Anderson, 1984, Foustoukos and Seyfried, 2004); metamorphism of graphite–carbonate bearing rocks (Giardini and Salotti, 1969, Holloway, 1984, Kenney et al., 2002); and other gas–water–rock alteration reactions such as serpentinization (Berndt et al., 1996, Charlou and Donval, 1993, Horita and Berndt, 1999, Kelley et al., 2001, Kelley et al., 2005, McCollom and Seewald, 2001, Vanko and Stakes, 1991). In this paper, data from 4 new sites in Canada and South Africa suggests that abiogenic hydrocarbon gases are more globally pervasive than has been understood previously. This is due to the fact that at many sites, the distinct abiogenic signature of such hydrocarbons is obscured by mixing with microbial CH4. Based on this data, we present a model for identification of abiogenic hydrocarbons through the resolution of microbial and abiogenic mixing — an approach that is applicable to a wide variety of crustal settings where potential abiogenic hydrocarbon reserves have been suggested (Gold, 1979, Kenney et al., 2002, Shock, 1995).

Section snippets

Geological setting and samples

In gold and base-metal mines throughout the Precambrian Shield rocks of Canada, Finland and South Africa, flammable gases discharge from fractures and exploration boreholes (Nurmi and Kukkonen, 1986, Lahermo and Lampen, 1987, Cook, 1998, Sherwood Lollar et al., 1993a, Sherwood Lollar et al., 1993b). Originally dissolved in saline groundwater (TDS levels range from several 1000 to tens of 1000s of ppm) in sealed fracture systems in the rocks, gases are released via depressurization into mine

Sampling methods

All gas and fracture water samples were collected at the borehole collar after the method of Sherwood Lollar et al. (2002) and Ward et al. (2004). A packer was placed into the opening of the borehole and sealed to the inner rock walls below water level to seal the borehole from the mine air and minimize air contamination. Gas and water were allowed to flow through the apparatus long enough to displace any air remaining in the borehole or the apparatus before sampling. Plastic tubing was

Isotopic patterns suggest an abiogenic origin

Mantle-derived abiogenic hydrocarbons are typically identified based on three criteria: a δ13C value for CH4 more enriched than − 25‰; a “carbon isotopic reversal” trend of increasing isotopic depletion in 13C with increasing molecular weight for CH4–ethane–propane–butane; and a 3He / 4He ratio indicative of mantle-derived helium (R / Ra > 0.1) (Jenden et al., 1993). Based on these criteria, coal bed or thermogenic hydrocarbon gases with anomalously enriched δ13C values and/or anomalously depleted δ2H

Conclusions

This study suggests that the abiogenic hydrocarbon gases first identified at Kidd Creek in fact exist at a variety of Precambrian Shield sites worldwide. Using the mixing models outlined here, the relative contribution of abiogenic versus microbial end-members can be estimated. Verification of an abiogenic component first requires identification of the end-member based on the δ13C and δ2H model illustrated in Fig. 1, followed by evaluation of the extent of mixing with other more conventional

Acknowledgements

This study was supported in part by grants from the Natural Sciences and Engineering Research Council of Canada and Discovery Grants Program, and Canadian Space Agency to BSL and by the National Science Foundation Life in Extreme Environments Program (EAR-9714214) grant to TCO and NASA Astrobiology Institute grant to the IPTAI team. We thank H. Li for compositional and isotopic analyses. Special thanks are due to the geologists and staff of the following mines for providing geological

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