PRIMARY HELIUM GAS FIELD FORMATION (Invited Presentation)

Developing exploration strategies to discover new helium reserves (a resource that has been discovered, of known size, and extracted at a profit) is critical in mitigating a global hiatus in helium supply security caused by both a limited number of global sources and present day geopolitics.

At concentrations greater than ~0.3%, in North America, helium becomes the primary value gas and the co-value of high-carbon-footprint CH₄ and CO₂ is no longer needed for a viable helium reserve. Nevertheless, helium-rich gas field occurrence is itself reliant on co-located gases to exceed the local groundwater bubble point and generate the gas phase necessary for buoyant migration and geological accumulation. Co-genetic N₂, generated in the same source rocks as the helium, is another gas that can do this, and has no carbon footprint.

In assessing a sedimentary basin or sub-basin for its commercial helium potential, the approach is similar to that for hydrocarbons. This requires a source rock, a primary expulsion mechanism, secondary migration pathway and a geological trapping structure (e.g. Danabalan et al., 2022). Unlike hydrocarbons, helium is both highly diffusive and requires long geological timescales to accumulate from U+Th radioactive decay in source rocks. Primary helium gas fields may be formed by rapid release of helium from source rocks in a few to tens of millions of years, seen in the East African Rift, to forming and charging over hundreds of millions of years in stable intracontinental areas, such as the Williston Basin N America.

Each helium system is a dynamic process operating over multiple time and length scales. This includes: the capacity of the source rock/s to retain helium (helium generative vs retentive capacity); the mechanism and rate of release of helium from the source rock (steady state vs thermal/tectonic enhanced); the form of secondary migration (diffusive vs advective dissolved/gas phase); the role of the geological architecture in focussing the secondary migration (deep faults, effective seals and lateral migration); the role of co-genetic (N₂) or other gases in generating a gas phase, but for CH₄ and CO₂ also diluting the helium; and the trap seal efficiency relative to helium charge rate.

Danabalan et al., (2022) The Principles of Helium Exploration, Petroleum Geoscience 28 (2), petgeo2021-029