ABSTRACT

Coalbed gases in the Velenje basin are highly variable in both their concentrations and stable isotope composition. Major gas components are CO₂ and methane. The CDMI carbon dioxide-methane index [CDMI = (CO₂/(CO₂ + CH₄))·100(%)] varies from 0 to 98.8%, δ¹³C_CO2 from -34.1 to 2.9‰ and δ¹³C_CH4 from -74.0 to -34.7‰. According to the geochemical results (recalculated on an air-free basis) for coalbed gases, several types of origin of CO₂ and methane could be recognized: thermogenic methane, endogenic and thermogenic CO₂, microbial methane and CO₂. The phenomenon of thermogenic methane in the lignite seam could be explained by microbial CO₂ reduction and/or methane oxidation processes, causing enrichment in the heavy ¹³C isotope of methane. Considering geological events at the time of formation of the lignite seam in the Velenje basin, it was found that most of the gas in the lignite seam is of bacterial origin, indicating CO₂ reduction processes mixed with external CO₂. A considerable fraction of the CO₂ in the lignite seam is of external origin. A crucial factor determining the formation of coalbed gases in the Velenje basin was bacterial activity. The distribution of coalbed gases (CO₂ and methane) is a result of the different physicochemical properties of CO₂ and methane. Methane accumulates at the subsurface of the Velenje basin, while carbon dioxide remains adsorbed on the coalbed surface and in micropores of the coal structure. CO₂ is the major gas component in the lignite seam and represents a permanent danger for gas outbursts. Isotopic fractionation due to migration of methane from the lignite seam to the surface is reflected in isotopically light methane in the water saturated layers. CO₂ accumulating at the subsurface of the Velenje basin is mainly derived from hinterland water and oxidation of methane.