RESERVOIR COMPLEXITY REVEALED BY STABLE ISOTOPE COMPOSITION OF PRODUCED GASES FROM HORN RIVER BASIN SHALES

Devonian shales in the Muskwa, Otter Park and Evie Fms. of the Horn River Group (HRB), NE British Columbia, Canada are gas rich and prime targets for shale gas production. Mud and produced gases were obtained from 28 wells located in the Dilly Creek region of the HRB and were analysed for chemical and stable isotope compositions. Carbon and hydrogen isotope lateral profiles were created for 2 horizontal wells completed in the Muskwa and Otter Park Fms. Short (25 to 50 days) and long term (up to 1250 day) times series of production samples from the Muskwa, Otter Park and Evie were also measured.

HRB gases are dry, mature and thermogenic in origin and the mole fraction of methane in shale gas samples ranged from ~80 to 90%. Gases from the Muskwa and Otter Park Fms. are indistinguishable based on chemical composition; however Evie gases contained higher fractions of CO₂. δ¹³C_methane values for the Muskwa and Otter Park mud gases averaged -34.9‰ and -35.0‰ respectively but varied along ~2 km of horizontal sections from -35.5‰ to -34.5‰ in the Muskwa compared to -36.9‰ to -31.9‰ in the Otter Park. Lateral δ¹³C_methane profiles in these adjacent formations imply large differences in the scale of heterogeneity of permeability within these two shale units.

δ¹³C time series of methane, ethane and propane show isotopic variations during gas production. Methane produced at early stages was generally enriched in ¹²C compared to gases sampled later in production. δ¹³C_methane values of gas from the Muskwa, Otter Park and Evie ranged between ~ -38‰ and -28‰. δ¹³C of gas did not vary monotonically with production. Examples of non-reversal, partial and full isotope reversal are all observed in HRB gases produced from a given well. In general enrichment in ¹³C of methane was observed in shale gases after periods of well ‘shut-in’. We conclude that shale reservoirs are far more complicated than heretofore realized.