Tracing the sources of noble gases for helium prospectivity in the Western Canada Sedimentary Basin

Constraining the origin and understanding how fluids move through the Earth's crust is critical in assessing the accumulation of globally important gas reserves such as helium and hydrogen. Noble gases serve as useful tracers in natural gases due to their inertness and are routinely used to identify the origin and migration of subsurface fluids in order to resolve the physical migratory processes that occur in the subsurface. Here we report a new compilation of 53 noble gas isotope and concentration measurements from the Western Canada Sedimentary Basin (WCSB) obtained from 46 natural gas wells that sample the regional stratigraphy from the Upper Cretaceous to the Cambrian. Previous results show evidence of radiogenic noble gases present in the Alberta basin formations. Helium isotope ratios indicate resolvable mantle derived helium ( 3 He), likely from Eocene age intrusions within the basin. Increases in the predominantly radiogenic derived 4 He, 21 Ne* and 40 Ar* are identified in the deepest samples, while ratios of 21 Ne/ 22 Ne, 40 Ar*/ 36 Ar and 21 Ne*/ 36 Ar also increase with proximity to the Precambrian basement. We infer this indicates a radiogenic flux is present, which most probably originates from the Precambrian basement. Variation exists throughout the basin, specifically in the youngest formations which exhibit enrichments in the lightest noble gases ( 4 He and 21 Ne) when compared with the heaviest ( 40 Ar). We propose that this is due to a combination of solubility-controlled fractionation and the preferential release of lighter noble gases from minerals in a lower temperature regime. The calculation of initial 4 He in groundwaters in a closed system model demonstrates that the deepest WCSB formations (Cambrian to Upper Devonian) have some of the oldest 4 He ages compared to their formation ages, suggesting an external supply of radiogenic He is required, likely from a basement source. Higher in the stratigraphy, 4 He ages both older and younger than the respective lithologies are observed, providing evidence of frequent groundwater recharge or glacial flushing events. This also suggests that some of the shallowest formations require an external source of He, implying an open-system cross-formational flow of radiogenic noble gas within WCSB. We identify the most prospective formation for possible He accumulation in WCSB to be the porous Devonian or deeper Cambrian formations, due to their proximity to basement lithologies.