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Abstract

DOI:10.1306/01031110103

Gas isotope reversals in fractured gas reservoirs of the western Canadian Foothills: Mature shale gases in disguise

Barbara Tilley,1 Scott McLellan,2 Stephen Hiebert,3 Bob Quartero,4 Byron Veilleux,5 Karlis Muehlenbachs6

1Department of Earth and Atmospheric Sciences, 1-26 Earth Sciences Building, University of Alberta, Edmonton, Alberta, Canada T6G 2E3; [email protected]
2Talisman Energy Inc., Suite 3400, 888-3rd St. Southwest Calgary, Alberta Canada T2P 5C5; [email protected]
3Talisman Energy Inc., Suite 3400, 888-3rd St. Southwest Calgary, Alberta Canada T2P 5C5; [email protected]
4Talisman Energy Inc., Suite 3400, 888-3rd St. Southwest Calgary, Alberta Canada T2P 5C5; [email protected]
5Talisman Energy Inc., Suite 3400, 888-3rd St. Southwest Calgary, Alberta Canada T2P 5C5; [email protected]
6Department of Earth and Atmospheric Sciences, 1-26 Earth Sciences Building, University of Alberta, Edmonton, Alberta, Canada T6G 2E3; [email protected]

ABSTRACT

Isotopically reversed gases (delta13C methane gtdelta13C ethane gtdelta13C propane) occur in fractured mixed clastic-carbonate reservoirs of the Permian and the Triassic in the foothills at the western edge of the Western Canada sedimentary basin (WCSB). The delta13C methane values (–42 to –24permil), gas dryness, and organic maturity (Rogt 2.2) are indicative of mature gases, and gas maturity generally increases with reservoir age and from the southeast to the northwest. The delta13C ethane values range from minus44 to minus25, with the less negative values in isotopically normal gases to the northeast of the gas fields we studied. To explain the gas isotope reversals observed in the WCSB foothills, we adopt the concept of a closed-system shale, in which simultaneous cooking of kerogen, oil, and gas yields gas with light delta13C ethane and heavy delta13C methane. This gas was released from shales and trapped in fractured folds of brittle clastic-carbonate rocks during deformation and thrust faulting of the Laramide orogeny, creating some of the most prolific gas pools. These gases are actually mature shale gases. Local high abundances of H2S and CO2 are most likely the products of thermochemical sulfate reduction (TSR) reactions in anhydrite-rich interbeds and underbeds that admixed to the released shale gas during the tectonic event. No evidence exists that TSR is responsible for the isotope reversals. Variations in delta13C ethane are likely caused by local differences in thermal history, the timing of gas release from shale, and the timing of the fault and fold development. Less negative delta13C ethane values (resulting in isotopically normal gases) to the northeast of the fields and in the underlying Devonian carbonates likely reflect a more open shale system where the earliest generated gas was lost. We suggest that isotopic reversals are restricted to closed-system maturation, and that their magnitude may be related to the relative volume of gas retained in shales.

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