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Coalbead methane is part of the non-conventional gas reservoirs and makes a significant contribution to gas production in some parts of the world. Initially, it was assumed that coalbed methane was of thermogenic origin, but most recent studies based on isotope and chemical data and taking into account the hydrogeology of the basin have demonstrated that secondary biogenic gases are formed in many coal-bearing basins. This study, using a similar approach, evaluated the origin of the gas in the Elk Valley coalfield located in British Columbia, Canada. Isotope data in methane samples collected from testhole wells and piezometers show a range that varies from −51.8‰ to −65.4‰ and −303‰ to −415‰ for δ13C and δ2H, respectively. The δ2H data, which are among the most depleted data reported for coalbed methane, have to be related to the very depleted δ2H values of the groundwater (−148‰ to −163‰). Isotope and chemical data collected from DIC show a trend of increasing δ13C values (−11.9‰ to +34.9‰) associated with an increase in DIC concentration (216 to 1650 mg/l). The most 13C depleted DIC and low DIC waters are found in the shallow groundwater flow system representing conditions close to recharge areas, while the most 13C enriched DIC and high concentration DIC waters are found in the discharge areas associated with a deep groundwater flow system. The DIC pattern, which is typical for methanogenic aquifers, and the isotope data obtained in methane samples clearly indicate that the gas found in the Elk Valley coalfield is mainly biogenic in origin. This study reaffirms that an approach that combines an evaluation of the groundwater flow system, the isotopic characterization and concentration pattern of the main carbon pools (CH4 and DIC), and the isotopic characterization of the groundwater is needed to fully evaluate the origin of gases in coal basins. |
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