DETERMINING THE HELIUM FLUX FROM THE DEEP CRYSTALLINE BASEMENT AND THE PROCESSES CONTROLLING ITS TRANSPORT IN OVERLYING PALEOZOIC SEDIMENTARY BASINS (Invited Presentation)

Understanding subsurface fluid migration is crucial for the exploration of natural resources as well as environmental projects such as carbon sequestration and nuclear waste repositories. Noble gases, due to their chemical inertness, are useful tools to identify and quantify subsurface physical processes and can provide a predictive understanding of the fluid origin, residence times and transport.

The Williston Basin is an intracratonic basin with multi-layered aquifers and aquitards aged from Cambrian to Pleistocene. Here we present noble gas isotopic and concentration data from twenty-six natural gas samples collected from different locations and geological formations. Except for Bakken Weyburn samples, which are crustal dominated, air-normalised ³He/⁴He varies between 0.08 Ra to 0.36 Ra, higher than the crustal endmember of ~0.02 Ra and consistent with a resolvable mantle ³He component. By plotting the ²⁰Ne/²²Ne and ²¹Ne/²²Ne ratios against each other, the neon ratios of Deadwood samples (contact unit with the basement) fall on a mixing line between air and a radiogenic production endmember which is distinct from the modern crust and has been ascribed to an Archean source [1, 2].

The initial radiogenic ⁴He concentration in the groundwater can be estimated from ⁴He/²⁰Ne ratios [3]. Compared to calculated in-situ radiogenic production, ⁴He is in excess in shallower units and depleted in deeper units, evidence of cross-formational transport. Cross-formational transport of noble gases and crystalline basement flux is further supported by the mantle-derived ³He observed in various units and the Archean neon signature in the Deadwood Formation.

A vertical scale diffusional model has been constructed to estimate ⁴He flux into the basin. The reference models assume static groundwater and a dominant diffusion transport. Diversion from diffusion profiles is observed for the Viking, Mannville and Mississippian formations, indicating ⁴He loss due to water disturbance, likely related to anthropogenic water flooding in the oil industry common in these formations of the Williston.

1. Lippmann-Pipke, J., et al., Chemical Geology, 2011. 283(3): p. 287-296.
2. Holland, G., et al., Nature, 2013. 497(7449): p. 357-360.
3. Ballentine, C.J., et al., Reviews in mineralogy and geochemistry, 2002. 47(1): p. 539-614.