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

Isotopically reversed gases (delta(13)C methane > delta(13)C ethane > delta(13)C 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 delta(13)C methane values (-42 to 24 parts per thousand), gas dryness, and organic maturity (R(0) > 2.2) are indicative of mature gases, and gas maturity generally increases with reservoir age and from the southeast to the northwest. The delta(13)C ethane values range from -44 to -25, 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 delta(13)C ethane and heavy delta(13)C 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 H(2)S and CO(2) 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 delta(13)C 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 delta(13)C ethane values (resulting in isotopically normal gases) to the northeast of the fields and in