Origin and distribution of coalbed gases from the Velenje basin, Slovenia

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))center dot 100(%)] varies from 0 to 98.8%, delta C-13(CO2) from -34.1 to 2.9 parts per thousand and delta(13) C-CH4 from -74.0 to -34.7 parts per thousand. 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 C-13 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.