Spatial distribution and origin of coalbed gases at the working faces of the Velenje Coal Basin, Slovenia, since the year 2000

Geochemical and isotopic monitoring of coalbed gases at the excavation fields of mining areas in Velenje Coal Basin, Slovenia, has been ongoing since the year 2000 with the aim of obtaining better insights into the distribution and origin of coalbed gases. Results from the mining areas Pesje and Preloge (active excavation fields) are presented here from the year 2000 up to the present. Composition and origin of coalbed gases were determined using mass spectrometry at the Jožef Stefan Institute. From a larger database of geochemical samples, 119 samples were used for analysis and spatial presentation in a geographical information system (GIS) environment. We have used geochemical (CH₄, CO₂ and N₂) and isotopic (δ¹³C_CO2 and δ¹³C_CH4) tracers for geochemical and isotopic characterisation of coalbed gases from the active excavation fields. Concentrations of CO₂ and the carbon dioxide–methane indices in the southern part of the basin are higher than in the northern part of the basin due to the vicinity of the active Šoštanj Fault. The value of δ¹³C_CH4 at the active excavation field indicates a bacterial origin, with values greater than –50‰, and only some boreholes show elevated δ¹³C_CH4 quantities as a consequence of the CO₂ reduction process in Velenje Coal Basin. The value of δ¹³C_CO2 indicates the bacterial and endogenic origin of carbon.