Determining the role of diffusion and basement flux in controlling 4He distribution in sedimentary basin fluids

The transport of helium from the crystalline continental basement and overlying Phanerozoic sedimentary formations to the near surface can be controlled by both diffusive and advective processes. The relative role of each is vital to helium resource prediction, and important in quantifying the residence times of fluids relevant to groundwater resources, hydrocarbon systems, geologic repositories for nuclear waste and carbon sequestration. The Williston Basin, North America, is a prominent sedimentary basin, providing an excellent natural laboratory to assess these processes. Here, we report noble gas isotopic and composition data for 28 gas samples from natural gas wells that sample different stratigraphic horizons down to the basement (Cretaceous to the Cambrian). Helium isotope ratios show a resolvable mantle 3He component (up to 4.7%) in most samples. Neon isotopic compositions of the Cambrian samples are consistent with a crystalline basement gas contribution. Both helium and neon isotopic observations provide evidence for the contribution of conservative noble gases from the crystalline basement or deeper into the overlying sedimentary basin. 4He groundwater concentrations in the sedimentary formations, calculated from 4He/20Ne values in gas samples, are in excess of in situ U+Th 4He production in some shallow units and depleted in others, providing further evidence of cross formation gas contributions. The highest 4He groundwater concentrations can be compared with the results obtained from a fully-coupled vertical scale transport model characterising diffusive-dominated transport through a static groundwater column. The model includes the 4He flux into the basin from the Precambrian basement and quantifies the apparent basement 4He flux to be between 0.8 - 1.6 × 10−6 mol 4He/m2 yr, comparable to the steady-state flux estimated for the average continental crust (1.47 × 10−6 mol 4He/m2 yr) (Torgersen, 2010). The lithologies in which 4He concentrations are significantly lower than the reference model predictions are consistent with a history of water flooding and produced water disposal in those formations over decades of hydrocarbon production. While an advective component cannot be ruled out, this work demonstrates the importance of both diffusion and the basin architecture development in controlling 4He flux into and out of different lithologies. The assumption of negligible 4He loss from the top surface of a lithology is often made when determining the 4He age of its groundwater. In the Williston Basin, this study shows that deeper lithologies may reach steady state at different stages of basin development, with shallower lithologies sometimes also showing significant 4He loss from their top surface. In the Williston Basin, 4He diffusive loss from the target lithology must be considered to accurately interpret 4He groundwater residence times and accumulation potential.

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