CH4-He-H2-Rich Fluids Associated With the Intrusive Rocks of the Mid-Continent Rift System in Minnesota (USA)

Abstract

Several studies have shown that H₂ and CH₄ emissions are recorded within Precambrian areas, where the origin of gases has yet to be determined. The Mid-Continent Rift (MCR) is an aborted rift aged from 1.1 Ga and composed of volcanic and (ultra)-mafic rocks. Wells presenting free or dissolved gas were sampled in NE-Minnesota along the MCR. High pH water (up to 11.4) and low Eh (down to −300 mV) with elevated concentrations (up to 756 µmolar) of formate and acetate are associated with gases consisting of He, CH₄, CO_2, heavier hydrocarbons, and up to 500 ppmv of H₂. The CH₄ presents different ¹³C and D isotopic values, suggesting that several abiotic and biotic pathways might be active in these shallow systems. The alkaline and reducing waters associated with the igneous rocks of the MCR suggest that H₂ could have been produced through water-rock interactions at deeper levels not reached by the shallow sampled wells. The associated high concentrations of gaseous and dissolved carbon compounds (VOA and methane) suggest that subsequent redox reactions have occurred in most of the rocks crossed by the wells, consuming a part of the H₂ as it was migrating toward the surface. Those results highlight potentially active H₂ production and consumption processes, providing keys for targeting source rocks in Precambrian environments. Those results suggest that direct H₂ detection in soil gas may not be the most effective exploration strategy. Searching for biogenic methane associated with deep He and N₂ may prove to be more effective.