25th International Meeting on Organic Geochemistry IMOG 2011

P-266
Geochemical and isotopic characterization of coal-related gas
from the SE Upper Silesian Basin, Czech Republic
Philipp Weniger 1 , Juraj Francu 2 , Frantisek Buzek 3 , Petr Hemza 4 , Bernhard M. Krooss 1
1 Institute of Geology and Geochemistry of Petroleum and Coal, RWTH Aachen University, Aachen,
Germany, 2 Czech Geological Survey, Brno, Czech Republic, 3 Czech Geological Survey, Prague, Czech
Republic, 4 GreenGas, DPB a.s., Paskov, Czech Republic (corresponding author:weniger@lek.rwthaachen.de)
Based on their chemical and isotopic composition,
coal-related gases in the Upper Silesian Coal Basin
(USCB) are commonly classified as either
thermogenic or microbial (Kotarba 2001). In the
Ostrava-Karviná coal district in the NE of the Czech
Republic gas occurs in the coal seams of the Ostrava
formation below the nappes of the Carpathian
overthrust, in the coal seams of the Karviná
Formation and the overlying sediments of the
Miocene Foredeep. The subthrust reservoirs contain
predominantly thermogenic methane, whereas
microbial methane prevails below the Carpathian
Foredeep.
In the present study, coal-related gas from different
mines in the Ostrava-Karviná Coal District was
analyzed to evaluate the influence of sorption and
desorption processes on gas composition. For this
purpose, the chemical and stable carbon isotopic
composition of methane and carbon dioxide,
desorbed from coal samples, as well as from mine
face extraction/degassing was compared.
Additionally, gas contents and methane sorption
capacities of related bituminous coal samples have
been determined. Gas desorbed from the Karviná
coal contained mostly methane with δ 13 C values
between -44‰ and -71‰. Gas desorbed from
Ostrava coal had higher C2+ hydrocarbon
concentrations with δ 13 C of methane between -35‰
and -45‰. The geochemical composition of the mine
face gas differed only slightly from the corresponding
desorbed gas composition. However, somewhat
larger differences were apparent in their isotopic
compositions.
According to the classification scheme by Whiticar
(1999), both, the mine face gas and the gas desorbed
from the Karviná coal are attributed to methyl
fermentation, while δ 13 C values of CO2 in the
desorbed gas indicate partial microbial methane
oxidation. The chemical and isotopic composition of
gases from the Ostrava formation indicates a
thermogenic generation process.
During up to 40 days canister desorption, the isotopic
composition of the desorbed CH4 did not show any
consistent trend and fluctuated in a range of +/- 5‰. A
substantial enrichment in 13 C methane by up to 15‰
was observed in a few instances. These could be due
to contamination and need to be verified. Even such a
degree of fractionation, however, would not be
sufficient to significantly alter the original isotopic
signature. It is concluded that the observed variation
in chemical and isotopic composition during
desorption alone cannot explain the differences
between coal-related gases of the Karviná and
Ostrava formations.
C1/(C2+C3)
100000
10000
1000
100
Staric mine face
CSM mine face
CSM desorbed gas
10
1
Lazy desorbed gas
Staric desorbed gas
Polish USCB (Kotarba 2001)
-100 -90 -80 -70 -60 -50 -40 -30 -20
δ13C-CH4 (‰)
Fig.1. Genetic characterization of coal-related gas
from the Upper Silesian Coal Basin using the scheme
from Whiticar (1999)
References
Kotarba, M.J., (2001) Organic Geochemistry 32. 163-
180
Whiticar, M.J., (1999) Chemical Geology 191. 291-
314
400