Physical processes occurring in tight gas reservoirs from Western Canadian Sedimentary Basin: Noble gas signature

Noble gases have become a powerful tool to constrain the origin of fluids as well as the rates of fluid migration in sedimentary basins. The aim of this study was to apply these tracers to understand the genesis and the evolution of unconventional gas reservoirs, basin-centered gas reservoirs (also called tight-gas reservoirs). A natural laboratory for this study is the methane-rich gas field in Cretaceous tight sands from the Western Canadian Sedimentary Basin (WCSB). The selection of the WCSB was motivated by an easy access to wells that cover an extensive area from deep to shallow parts of the basin, as well as the relatively well documented geology and hydrology of the area. The elemental noble gas signature (He, Ar, Kr and Xe) of natural gas from 18 wells of the basin shows a mixture between the original low content noble gas signature of the hydrocarbon gas (after its charge into the reservoir) and water (trapped in intergranular volumes (IGV)), which is comparatively rich in noble gases. This difference in relative contents can be used to estimate the geographical position of the contact between the gas reservoir and the shallower IGV water. Furthermore, the 'original' noble gas signature, which is defined as the noble gases generated in the source, later transferred, and mixed with in situ produced noble gases in the reservoir, is mainly composed of radiogenic isotopes (40Ar and 4He). The comparison of 4He/40Ar ratios with an average value for potential source rocks and reservoir rocks production ratio (estimated with U, Th and K concentrations) allows us to understand how the gas was transferred from the source rock to the reservoir rock. These results, combined with the geological and hydrological knowledge of the WCSB, affords a new method to better understand the dynamics of unconventional gas reservoirs.