CHEMICAL AND ISOTOPIC COMPOSITION OF GASES FROM CRETACEOUS COALBEDS AND SHALLOW AQUIFERS IN ALBERTA, CANADA

Conventional gas resources in Canada are expected to decline within the next decade, which has led to the exploitation of other sources of energy, such as natural gas from coal (NGC) or coalbed methane (CBM). There may be up to 500 TCF of NGC in Alberta (Canada), but it is still uncertain as to how much of this gas can be extracted. To better understand gas formation processes and to monitor for potential impacts on shallow groundwater, we have determined the chemical and isotopic compositions of produced fluids and dissolved gases from 73 NGC wells completed in the Lower Cretaceous Mannville Formation and Upper Cretaceous Horseshoe Canyon Formation of Central Alberta, Canada. Shallow groundwater and its free gases from over 90 wells in the vicinity of NGC operations in Alberta have also been analyzed chemically and isotopically. The Mannville Formation contains subbituminous coal deposited in a freshwater to brackish water environment based on its boron content (256-1144 mg/kg). Coal from this formation is rich in vitrinite that formed under anaerobic conditions. Dissolved hydrocarbon gases in produced fluids from this formation had average carbon isotope values of -49.4±3.6‰ for methane (n=24), -28.8±2.1‰ for ethane (n=24) and -26.9±1.1‰ for propane (n=23) suggesting a significant thermogenic gas component. The Horseshoe Canyon Formation contains subbituminous coal deposited in a mildly brackish environment (B content 217 to 266 mg/kg). Coal from this formation is rich in vitrinite that did not form under anaerobic conditions. Dissolved gases from the Horseshoe Canyon Formation fluids had average δ¹³C values of -54.0±4.1‰ (n=45), -36.5±2.4‰ (n=42) and -29.4±1.0‰ (n=38) for methane, ethane and propane respectively, indicating a higher proportion of biogenic gas. Free gas contained in shallow groundwater had a mean δ¹³C value of methane of -67.5±9.4‰ (n=85) and a mean δ¹³C value of ethane of -44.4±13.5‰ (n=41), indicating that the gases are predominantly of biogenic origin. These data suggest that coalbed-derived methane and ethane are often isotopically distinct from methane and ethane in shallow groundwater. Therefore, carbon isotope measurements on dissolved or free methane and ethane in shallow aquifers may serve as a suitable tool for monitoring potential contamination of shallow groundwater resources with produced gases in Alberta, Canada.