Isotope Geochemistry of Natural Gas from the Horn River Basin: Understanding Gas Origin, Storage and Transport in an Unconventional Shale Play

  • Author / Creator
    Norville, Giselle A
  • Devonian shales of the Horn River Basin, northeast British Columbia are recognized key targets for unconventional shale gas production in Canada. Recent studies show gas isotope geochemistry in several unconventional shale plays worldwide differ from conventional hydrocarbon reservoirs and carbon isotope reversal between hydrocarbon gas components is a common phenomenon. Geochemical properties of natural gases hold clues to origin, storage and transport within the shale and may assist in elucidation of the shale gas system; however limited published work exists on stable isotope geochemistry of Horn River Basin shale gases. Over four hundred mud gas and production gas samples were obtained from Phanerozoic formations in the Dilly Creek area of the basin and chemical and stable isotope compositions were measured. Carbon isotope depth profiles of Horn River Basin gases (methane, ethane and propane) from surface depth to the target formation provide a geochemical template applicable for use in environmental remediation. Isotope signatures (δ¹³C methane, δ¹³C ethane and δ¹³C propane) of surface casing vent gas and soil gas at six leaking well sites in the basin were matched to isotope depth profiles which allow identification of formations from which gas leaks occurred. Carbon isotope lateral profiles map the subsurface variation within shale gas target formations and serve as a proxy for flow connectivity within the shale. Inter- and intra-formational variations in δ¹³C methane values of Horn River Group gases are observed and carbon isotope data purports greater connectivity in the Muskwa Formation than in the Otter Park Formation. Time series data show stable isotope compositional changes during shale gas production and fluctuations in δ¹³C values of produced Muskwa, Otter Park and Evie gases occur with unique trends for each well. In the early stage of production Horn River Group gases generally show more negative δ¹³C methane values, while increases in δ¹³C methane values are observed in after periods of well ‘shut in’ and overall δ¹³C methane values ranged from approximately -38‰ to -28‰. Horn River Group gas isotope signatures indicate gases are dry, mature and thermogenic in origin and several shale gas samples exhibit isotopically reversed gas signatures. Few cases of carbon isotope reversal were observed in mud gases while most produced gases showed partial isotope reversal among gas components or full isotope reversals where δ¹³C methane > δ¹³C ethane > δ¹³C propane. It is plausible that carbon isotope fractionation occurs during shale gas production as hydrocarbon molecules traverse tortuous pathways within the complex shale fabric which alters the carbon isotope signature of produced gases. Spatial and temporal stable isotope variations occur in Devonian shale gases in the Horn River Basin and this should be taken into consideration during interpretation of isotope data obtained from unconventional shale gas reservoirs.

  • Subjects / Keywords
  • Graduation date
    Fall 2014
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.