Constraining the Geochemical Fingerprints of Gases from the UK Carboniferous Coal Measures at the Glasgow Geoenergy Observatories Field Site, Scotland

Usage of thermal energy contained in abandoned, flooded, coal mines has the potential to contribute to low carbon heating or cooling supply and assist in meeting net-zero carbon emission targets. However, hazardous ground gases, such as CH 4 and CO 2 , can be found naturally in superficial deposits, coal bearing strata and abandoned mines. Determining the presence, magnitude, and origin of subsurface gases, and how their geochemical fingerprints evolve within the shallow subsurface is vital to developing an understanding of how to manage the risk posed by ground gases in geoenergy technology development. Here, we present the first CH 4 and CO 2 concentration-depth profiles and stable isotope (δ 13 C CH4 , δ 13 C CO2 , and δD CH4 ) profiles obtained from UK mine workings, through analysis of headspace gas samples degassed from cores and chippings collected during construction of the Glasgow Observatory. These are used to investigate the variability of gas fingerprints with depth within unmined Carboniferous coal measures and Glasgow coal mine workings. Stable isotope compositions of CH 4 (δ 13 C CH4 = −73.4‰ to −14.3‰; δ 13 C CO2 = −29‰ to −6.1‰; δD CH4 = −277‰ to −88‰) provide evidence of a biogenic source, with carbonate reduction being the primary pathway of CH 4 production. Gas samples collected at depths of 63–79 m exhibit enrichments in 13C CH4 and 2 H, indicating the oxidative consumption of CH 4 . This correlates with their proximity to the Glasgow Ell mine workings, which will have increased exposure to O 2 from the atmosphere as a result of mining activities. CO 2 gas is more abundant than CH 4 throughout the succession in all three boreholes, exhibiting high δ 13 C CO2 values relative to the CH 4 present. Gases from unmined bedrock exhibit the highest δ 13 C CO2 values, with samples from near-surface superficial deposits having the lowest δ 13 C CO2 values. δ 13 C CO2 values become progressively lower at shallower depths (above 90 m), which can be explained by the increasing influence of shallow groundwaters containing a mixture of dissolved marine carbonate minerals (∼0‰) and soil gas CO 2 (−26‰) as depth decreases. Our findings provide an insight into the variability of mine derived gases within 200 m of the surface, providing an important ‘time-zero’ record of the site, which is required in the design of monitoring approaches.