Distribution, composition and origin of coalbed gases in excavation fields from the Preloge and Pesje mining areas, Velenje Basin, Slovenia

• Coalbed gas distribution and origin were investigated in Velenje Basin.
• CO2 is a major gas component at investigated excavation fields.
• Rate of advancement of the working face influences coalbed gas distribution.
• Methane was formed through microbial processes, while CO2 is endogenic in origin.
• CH4 is accumulated at no pre-mined areas, while CO2 accumulates in pre-mined areas.

Coal gas outbursts (especially CO2) present a high risk in mining of lignite in the Velenje Coal Mine, located in the Velenje Basin in northern Slovenia. A programme of monitoring geochemical parameters was set up to help understand the behaviour of the coalbed gas distribution in advance of the working face using mass spectrometric methods to study its molecular and isotopic compositions and origin. Coalbed gas samples from four different excavation fields (G2/C and K.-130/A from the north and south Preloge mining area and K.-5/A and K.-50/C from the Pesje mining area), which were operational between the years 2010 and 2011 were investigated. The major gas components are CO2 and methane. Temporal changes in the chemical and isotopic composition of free seam gases were observed within boreholes as a function of the advancement of the working face. The study also revealed that at a distance of around 120 m from the working face, the influence of coal exploitation by the Velenje Longwall Mining Method causes coalbed gas to migrate. At a distance of 70 m the lignite structure is crushed causing desorption of fixed CO2 from the coal. Differences in coalbed gas composition at the longwall panels which underlie the unmined area or under previously mined areas were found. A high CDMI {=[CO2/(CO2 + CH4)]100 (%)} index with values up to 95.6% was typical for areas of pre-mined excavation fields (South Preloge K.-130/A and Pesje area K.-5/A), while in excavation fields with no previous mining activity (North Preloge G2/C and Pesje area K.-50/C) up to 61.9 vol % of CH4 was detected. The concentration measurements and isotopic studies revealed endogenic CO2 (including CO2 originating from dissolution of carbonates) with δ13CCO2 values ranging from − 7.0‰ to 5.5‰, microbial methane and CO2 with values ranging from − 70.4 to − 50‰ and from − 11.0 to − 7.0‰, respectively. Higher δ13CCH4 values ranging from − 50 to − 19.8‰ could be attributed to so-called secondary processes influencing the δ13CCH4 value, such as migration due to lignite excavation (escape of isotopically lighter methane). In excavation fields (G2/C and K.-50/C) with no-premining activity higher δ13CCH4 values could also be explained by migration of methane from deeper strata. The δ13CCH4 value also depended on the depth of the excavation field; at shallower levels of the excavation field (K.-5/A) a lower δ13CCH4 value was traced indicating microbial gas, while at deeper levels higher δ13CCH4 values were found.