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

Abstract

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 (δ 13 C CO2 , δ 13 C CH4 , and δ 2 H CH4 ) were investigated in boreholes in advance of eight working faces. The major coalbed gas constituents were CO 2 and methane. Gas concentrations and isotope values revealed that the methane is biogenic in origin with δ 13 C CH4 values of −69.4 to −29.5 ‰, δ 2 H CH4 values of −301 to −222 ‰, and a fractionation factor (α CO2 -CH4 ) of 0.998-1.073, suggesting that methane derives from microbial acetate fermentation and CO 2 reduction. The carbon dioxide methane index values ranged from 50.0-98.3 vol.% and δ 13 C CO2 values from −11.8 to −0.5 ‰, indicating that CO 2 is biogenic and endogenic in origin. The degassing mechanism results in isotope fractionation of methane and CO 2 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.

Carbon isotopic composition of methane and its origin in natural gas from the Petišovci-Dolina oil and gas field (Pannonian Basin System, NE Slovenia) – a preliminary study

“…If supposing in general a unique source rock and gas formation realm, isotope fractionation can be explained in a way that CO 2 is heavier than CH 4 which is more mobile and migrates easier upwards than CO 2 . This effect quite often occurs in e. g. thick coal beds as for example in the Velenje lignite seam (Kanduč & Pezdič, 2005;Kanduč et al, 2021). 1.…”

“…If we compare gas composition and δ 13 C CH4 and CO 2 from the PDOGF with free coalbed gas sampled from excavation fields from the Velenje lignite-bearing basin (Kanduč et al, 2021) we can conclude that they are completely different in origin. We found out in the Velenje basin that the major coalbed gas constituents were CO 2 , methane, and nitrogen, while in the PDOGF natural gas is by far predominantly composed of methane.…”

“…5). High δ 13 C CH4 values (from -40 ‰ to -29.5 ‰) indicate thermogenic methane, which could be originated in shales forming pre-Pliocene basement of the Velenje Basin (Kanduč et al, 2021). The Carbon Dioxide Methane Index (CDMI) values ranged from 50 to 98.3 % and δ 13 C CO2 values from -11.8 to 0.54 ‰, indicating that CO 2 is biogenic and endogenic in origin.…”

“…In the last 20 years, numerous theoretical and applied studies have been conducted and published on gases in the intermontane Pliocene lignite-bearing Velenje basin (Kanduč & Pezdič, 2005;Kanduč et al, 2011;Sedlar et al, 2014;Kanduč et al 2021), about 100 km west of the PDOGF area. There, the situation is much more complicated.…”