Gas maturity and alteration systematics across the Western Canada Sedimentary Basin from four mud gas isotope depth profiles

Carbon isotope and molecular compositions of Mississippian to Upper Cretaceous mud gases have been examined from four depth profiles across the Western Canada Sedimentary Basin (WCSB). The profiles range from the shallow oil sands in the east (R₀ = 0.25) to the very mature sediments in the overthrust zone to the west (R₀ = 2.5). In the undisturbed WCSB, δ¹³C₁–δ¹³C₂ and δ¹³C₂–δ¹³C₃ cross-plots show three maturity and alteration trends: (1) pre-Cretaceous gas sourced from type II kerogen; (2) Cretaceous Colorado Group gas; and (3) Lower Cretaceous Mannville Group biodegraded gas. A fourth set of distinctly different maturity trends is recognized for Lower Cretaceous gas sourced from type III kerogen in the disturbed belt of the WCSB. Displacement of these latter maturity trends to high δ¹³C₂ values suggests that the sampled gas was trapped after earlier formed gas escaped, probably as a result of overthrusting. Unusually ¹³C-enriched gas (δ¹³C₁ = −34‰, δ¹³C₂ = −13‰, and δ¹³C₃ = 0‰), from the Gething Formation in the disturbed belt, is the result of late stage gas cracking in a closed system. In general, gas maturity is consistent with the maturity of the host sediments in the WCSB, suggesting that migration and mixing of gases was not pervasive on a broad regional and stratigraphic scale. The ‘Deep Basin’ portion of the WCSB is an exception. Here extensive cross-formational homogenization of gases has occurred, in addition to updip migration along the most permeable stratigraphic units.