Comparison of isotopic compositions of hydrocarbon gas in shallow groundwater and a deep oil and natural gas reservoir in southeastern New Brunswick, Canada

Abstract

Isotopic analyses of natural gas from the Stoney Creek oil field in New Brunswick indicate carbon (δ¹³C) and hydrogen (δ²H) values in methane (C₁) of -42.4 ± 0.7‰ VPDB and -220.9 ± 3.2‰ VSMOW, respectively. Isotopic data and a gas molecular ratio of 12 ± 1 indicate a wet thermogenic gas formed with oil near the onset of the oil-gas transition zone. The isotopic profiles of the C₁–C₅ hydrocarbon gases are consistent with kinetic isotope effect models. The Albert Formation of the Horton Group hosts the Stoney Creek oil field (SCOF) and the McCully gas field (MCGF) the only other gas-producing field in the province. Both are thermogenic in origin; however, the SCOF gas has a lower thermal maturity than the MCGS. Hydrocarbon gas composition in shallow aquifers across southeastern New Brunswick was also evaluated. Gas source interpretations based on δ¹³C and δ²H values are uncertain; oxidation and biogenic overprinting are common and complicate interpretation. The effect of oxidation on δ¹³C and δ²H values was apparent when C₁ concentrations were ≤1 mg/L. In some samples with C₁ concentrations >5 mg/L, isotopic discrimination methods point to a biogenic origin. However, the molecular ratios <75 and the presence of >C₃ fractions, indicate a thermogenic origin. This suggests a thermogenic isotopic signature has been overprinted by biological activity.