Stable isotope ratios in methane containing gases in the United Kingdom

The gas explosions at Loscoe and Abbeystead have highlighted the dangers associated with the migration of methane in the subsurface, and the need to quickly and accurately establish the source of the methane in order to instigate appropriate measures for control and remediation. Sources of methane include leaking gas mains, sewers, natural gas reservoirs, gas associated with oil and coal, landfills and the products of anaerobic degradation of organically contaminated groundwater. Although public opinion may demand rapid action following the discovery of methane hasty remedial action which does not consider the source of the gas or its mode of transmission, may at best be a waste of resources, or, at worst, may exacerbate the spread of the gas. To confirm a methane source three approaches may be used. The source can be identified by comparing the composition of the gas with known source compositions: identifying a migration pathway between the putative source and the point at which methane is observed and confirming the existence of a mechanism (such as diffusion or convection) capable of moving the gas along the pathway (Williams and Hitchman, 1989). In most cases only the first approach is taken: usually by comparing major and trace components with those of known gases, or by determining the ^^C content to distinguish 'geological' from recent biogenic methane. Identifying the origin may be very much more complicated if more than one source of methane exists, and, where compositional evidence alone is used, it is important to have a well defined database for the likely composition of possible source gases and to evaluate whether changes in composition may have occurred during gas migration. Stable isotope ratios of carbon and hydrogen (ie ^^C/^'^C, ^H/ ^H) potentially provide an additional means by which methane can be characterised but such data are scant for sources in the United Kingdom. In order to assess whether stable isotope ratios could be diagnostic of particular sources, methane containing gases from a variety of origins were collected and analysed. These included coal fields. North Sea gas fields, deep biogenic reservoirs, landfills and anaerobic digesters. Results show that there is some overlap on the B'^HQH^ versus 3^^CCH4 graph, notably between coal and landfill gas, but this was resolved with 8^^Cco2 plotted against 3^^CCH4- These results indicate that the determination of stable isotope ratios of carbon and hydrogen in methane and carbon dioxide is a useful additional technique for source characterisation but that isotopic firactionation may occur if the methane is oxidised by microbial processes.