Geochemical Fingerprinting of UK Carboniferous Coal Measures and Coal Mine Workings at the Glasgow UKGEOS site, Scotland

Abstract:

Interest in constraining the origin of methane, and how its geochemical fingerprint changes through the shallow subsurface; has increased as a result of exploration for unconventional gas sources and ongoing development of geoenergy technologies.

Dissolved methane is often encountered within local groundwater in the UK; and can pose a significant environmental risk. The Glasgow Geothermal Energy Research Field Site (GGERFS) in Scotland, has been constructed to investigate the potential of low temperature geothermal energy from disused shallow mine workings in the UK (Monaghan et al., 2019). Rock core and chipping samples were obtained during construction of the site; and provide an opportunity to investigate the variability of the gas fingerprints with depth within the coal mine workings and unmined Carboniferous coal measures.

The rock core and chipping samples were collected during drilling, with samples being obtained every 10m depth from a 200m depth seismic monitoring borehole; and every three metres from two (max 90m depth) water monitoring boreholes. These core and cutting samples were then stored in gas tight isojars for 3 months, to allow for degassing. Geochemical gas analyses consisted of bulk concentration analysis using gas chromatography to determine gas concentrations; followed by δ¹³C, δD, and δ¹⁸O stable isotope analyses, in order to determine potential gas origin.

Our results indicate that elevated concentrations of CH₄ were encountered within unmined coal seams, and elevated levels of CO₂ were observed within mined seams. The methane encountered is of biogenic origin, with δ¹³C and δD fingerprints indicating a mixed methane source; from bacterial methyl type fermentation, and bacterial carbonate reduction mediated pathways. Preliminary δ¹³C_CH4 and δ¹³C_CO2 stable isotope data indicate that methane oxidation appears to be the primary source of the CO₂; and may be enhanced by elevated levels of oxygen present in the mined seams, relative to unmined.

Hence, the GGERFS site in Scotland shows that coal mining operations have altered the ground gas composition of the subsurface; and introduced elevated levels of oxygen into the former workings. Further work will allow us to fully resolve the gas generation pathways; and enable greater understanding of the change in the gas fingerprints with depth.