Modelling Radiolytic Natural Hydrogen From a Fractured Basement: Generation, Migration, and Sequestration Potential (Taranaki Basin–New Zealand)

Abstract

The search for natural hydrogen (H2) in sedimentary basins is gaining increasing recognition due to its environmental friendliness. Therefore, as an alternative energy source, natural hydrogen could address the issue of environmental challenges and participate in the energy mix necessary for the energy transition. Despite ongoing research, many uncertainties remain in H2 exploration, which is still at an early stage. In this work, we provide, for the first time, a numerical model of radiolytic natural H2 generation from the fractured basement based on the Taranaki Basin example (New Zealand). This approach uses conventional hydrocarbon exploration techniques, with some adjustments to properly reproduce the subsurface natural H2 behaviour. We calculated the potential radiolytic H2 generation rate to be approximately 10.3 mg/g/Ma. This value was used as a constant rate input in the model. Potential reservoirs within the possible optimal H2 preservation window (80°C–200°C) include the Tane formation sandstone, as well as the Taimana and Tikorangi carbonate formations. The model highlights that H2 mass concentration in water is higher along the faults and in interbedded sand facies of the Rakopi and Wainui formations, implying that hydrogen in solution could migrate both by diffusion and advection along these paths. The density inversion of the seal and the underlying reservoirs began at 9.4 Ma and 6.8 Ma in the Witiora and Taranga boreholes respectively, due to the northwestward progradation of the Mohakatino Formation. This inversion indicates a period during which hydrogen-saturated water could be trapped or experience delayed flow, potentially leading to the exsolution of supersaturated hydrogen into a gaseous phase. The Tane formation gas anomaly reported during the drilling could be due to the conversion of CO2 into abiotic CH4 via the Sabatier reaction at higher temperatures (> 200°C). Consequently, abiotic CH4 could be an accurate proxy for depicting natural H2 generation.