Isotopically reversed gases (δ13C methane > δ13C ethane > δ13C 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 δ13C methane values (–42 to –24‰), gas dryness, and organic maturity (Ro > 2.2) are indicative of mature gases, and gas maturity generally increases with reservoir age and from the southeast to the northwest. The δ13C 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 δ13C ethane and heavy δ13C 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 δ13C 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 δ13C 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.