HIGH HELIUM CONCENTRATIONS IN WELL GASES FROM THE DULUTH COMPLEX, MINNESOTA, USA

Helium is an important industrial gas with a wide array of applications. It is used extensively in medical magnetic resonance imaging (MRI) scanners, for scientific and cryogenic research, welding, refrigeration, gas for aircraft and as a coolant for nuclear reactors. Over the past decade, the global supply chain has struggled to meet increasing demand. This has led to significant disruptions to the global helium market, resulting in volatile helium prices.

Radiogenically produced ⁴He accumulates in ancient crust during quiescent periods and is subsequently released during periods of active tectonism or volcanism, or gradually via diffusion¹. Gas reservoirs can trap and store ⁴He that is produced by radiogenic decay of U and Th in the crust. This occurs as another gas (CH₄, CO₂ or N₂) or fluid (i.e., groundwater) interacts with a ⁴He rich source rock, which acts to concentrate the helium in subsurface reservoirs^2,3. Helium is most commonly extracted as a byproduct of natural gas production, when CH₄ is the dominant gas in the crust, but can also be concentrated by CO₂ and N₂ when stable continental cratonic regions interact with volcanic systems or become tectonically active.

We report gas chemistry data from an exploration borehole recently drilled in the Duluth Complex (DC), Minnesota, USA . We find that well gases contained 10.5% He, 13.2% N₂, 73.8% CO₂ and 2.4% CH₄. The DC is the major intrusive component of the 1.1 Ga Midcontinent Rift System, composed predominantly of 1096 Ma anorthositic to troctolitic mafic intrusions emplaced into the 1108-1083 Ma North Shore Volcanic Group (NSVG) and above 1.85 Ga Animikie Basin sediments⁴. The DC has been tectonically quiescent since emplacement of the youngest intrusion, but was proximal to the longer duration NSVG thermal anomaly⁵.

These findings are an example of how helium can be focused in continental settings. The continental crust provides the source of ⁴He and intrusive volcanics provide the source of CO₂ and heat to release and concentrate the helium⁶. The major element chemistry of the gases from the Duluth well is consistent with their release from the crystalline basement following interaction with a CO₂ rich magmatic body.

¹Warr et al., 2022

²Ballentine and Burnard 2002

³Ballentine and Sherwood Lollar, 2002

⁴Miller et al., 2002

⁵Schmidt et al., 2021

⁶Danabalan et al., 2022