δ13C signatures of a natural gas zone with high H2S content in a major unconventional petroleum accumulation, Montney Formation, western Canada

Abstract

We report geochemical data from mud gas, produced gas and produced hydrocarbon liquid samples for three horizontal wells that produced hydrocarbons from the Montney unconventional petroleum accumulation in west-central Alberta. Mud gas data from the deepest well provide a rare opportunity to characterize the trends of C₁-C₃ δ¹³C values in a high-H₂S (∼13 %) zone at high-resolution increments (50–100 m) along ∼1,500 m of a horizontal well. The results suggest that H₂S in this zone was generated by thermochemical sulfate reduction (TSR) close to the time of maximum burial (∼60–80 Ma) and ceased during subsequent uplift. Produced gas samples from the horizontal well in the high-H₂S zone have abnormally enriched ethane and propane δ¹³C values compared to those from other wells in the Montney unconventional play. This strong isotopic enrichment reflects oxidation of ethane and propane during TSR reactions and produces an anomalous, strongly convex-upward profile on a Chung plot. Mud gas samples from the horizontal well in the high-H₂S zone have δ¹³C values that increase laterally from south to north by 7.5 ‰ for ethane and 5.6 ‰ for propane, while the corresponding methane δ¹³C values are essentially invariant. This suggests that ethane and propane were the main hydrocarbon reactants oxidized during TSR reactions, and that methane was not significantly involved. Our findings are consistent with those of previous studies on TSR-influenced gas systems in western Canada and other basins showing there is limited variation in methane δ¹³C values with progressive TSR until ethane and propane are wholly consumed.