Exploring the 2013-2018 degassing mechanism from the Pesje and Preloge excavation fields in the Velenje Coal basin, Slovenia: insights from molecular composition and stable isotopes.

Gas samples were collected from 25 m long horizontal boreholes drilled into the excavation field at 10° inclination to the longwall face in two mining areas, Pesje and Preloge, in the Velenje Coal Mine, Slovenia, from 2013 to 2018. The degassing mechanism of coalbed gas and its stable isotopic composition (C, C, and H) were investigated in boreholes in advance of eight working faces. The major coalbed gas constituents were CO and methane. Gas concentrations and isotope values revealed that the methane is biogenic in origin with C values of -69.4 to -29.5 ‰, H values of -301 to -222 ‰, and a fractionation factor (α-) of 0.998-1.073, suggesting that methane derives from microbial acetate fermentation and CO reduction. The carbon dioxide methane index values ranged from 50.0-98.3 vol.% and C values from -11.8 to -0.5 ‰, indicating that CO is biogenic and endogenic in origin. The degassing mechanism results in isotope fractionation of methane and CO for carbon isotopes up to 39.9 ‰ and up to 8.5 ‰, respectively, depending on the position of the excavation fields in space, e.g. under pre-mined coal area, fresh overburden.