N2-HE GAS FIELD FORMATION IN INTRACRATONIC BASINS

The global shortage of helium supply has stimulated interest in exploration for this critical and non-renewable element. Helium gas pool formation requires natural concentration and extraction of groundwater-dissolved helium, often through gas-water equilibration with sedimentary CH₄ or magmatic CO₂ gas phase. These mechanisms lead to gas accumulations which when discovered and produced have a significant carbon footprint and often a very low concentration of helium in the resulting gas phase. Helium-rich gas fields are also found in ancient intracratonic basins dominated by N₂ with a low or zero carbon footprint.

Helium generated from U and Th decay fluxes out of the continental crust at a rate comparable to the steady state whole crustal production. Nitrogen gas formation is less well understood but a constant range of N₂/He is observed. To understand the formation of N₂ dominated He gas field in ancient intracratonic basins, a conceptual model was built. The model assumes that helium and nitrogen originate from the deep continental crust and are transported to the overlying sedimentary column at steady state. Combined with sedimentary deposition, resulting basin architecture and in situ helium production, we model the dissolved gas groundwater concentration evolution in the sedimentary porsepace through diffusion.

The selected exemplar system is the Williston Basin, North America, which is a classic Cambrian intracratonic sedimentary basin. Our model shows that nitrogen can reach concentrations that exceed its solubility at the base of the sedimentary column, and it is calculated to have formed a free gas phase as early as 140Ma. The formation of a nitrogen free-gas phase results in the partitioning and concentration of helium into the same gas phase. In the Williston Basin, the formion of a primary N₂-He gas pool reduces the diffusive loss of ⁴He into overlying strata by ca 30%. Recognition of this gas phase formation mechanism enables a new quantitative insight into how to explore for helium in similar intracontinental sedimentary basins found worldwide.