Fate and transport of dissolved methane and ethane in cretaceous shales of the Williston Basin, Canada

Baseline characteristics of dissolved methane (CH4) and ethane (C2H6) and their stable isotopes in thick, low hydraulic conductivity, Cretaceous shales were determined using high‐resolution core profiling at four sites in the Williston Basin (WB), Canada. Positive correlations with the conservative natural tracer Cl− reflected a lack of measureable production or consumption of gases in the shale to the depth investigated (150 m below ground, BG) and suggest CH4 and C2H6 concentrations near the interface with overlying Quaternary sediments are controlled by lateral migration and dilution in permeable zones. Curvilinear increasing concentrations with depth in the shale at all sites coupled with 1‐D solute transport modeling suggest long‐term (over millions of years) upward diffusion of CH4 and C2H6 from deeper WB sources, likely the Second White Speckled Shale Formation (SWSS; ∼790 m BG). δ13C‐CH4 profiles in the shale are consistent with upward diffusional fractionation of isotopes from the SWSS. Distinct CH4 and C2H6 isotope values of gases in the shales versus 13C‐enriched thermogenic isotopic signatures of CH4 and C2H6 in deeper oil‐producing WB intervals could be used to identify fugitive gases originating deeper in the Basin. Key Points: Correlations between dissolved Cl and CH4 and C2H6 in Cretaceous shale suggest a lack of production or consumption of CH4 and C2H6 Depth profiles of Cl, CH4, and C2H6 in the shale and 1‐D solute transport modelling suggest long‐term upward diffusion from deeper sources δ13C‐CH4 profiles in the shale are consistent with upward diffusional fractionation of isotopes from greater depths in the shale