Formation of coalbed methane and water-dissolved gas in Kushiro Coal Mine, Japan, based on isotopic compositions of gas, groundwater, and calcite

Abstract Kushiro Coalfield is one of the largest coalfields in Japan, and Kushiro Coal Mine (KCM) is currently the only underground coal mine in the country. At KCM, coalbed methane (CBM) has been extracted from the Eocene Harutori Formation and water-dissolved gas from the Paleocene Shiomi Formation. This study investigates the origin of these gases and their formation environments. A positive correlation between δD(CH4) of the sampled gas and δD(H2O) of the groundwater, following δD(CH4) = δD(H2O) − 180 ± 20, indicates that CBM and water-dissolved gas in KCM are classified as secondary biogenic methane and were formed within in situ groundwater. The lower δ13C(CO2) values (less than −6.9‰) from KCM compared with representative CBM of biogenic origin from other coalfields imply the contribution of oxidants (e.g., SO42−) in the biodegradation of sedimentary organic matter (SOM) to CO2 in KCM. Conversely, high δ13C(CaCO3) values (>12.5‰) of calcite deposited in the coal seams of KCM imply negligible contributions of oxidants for SOM biodegradation when the calcite was formed. The methanogenic conditions at the time of calcite formation may have arisen when the rate of groundwater flow in the coal seams was higher than at present.