Natural hydrogen production from serpentinized mantle rocks

Mantle rocks undergoing serpentinization at upper-crustal levels can generate significant amounts of natural hydrogen. Although the potential for substantial hydrogen generation during fluid-rock reactions of these rocks is documented by lab experiments and occurrences of hydrogen-rich natural gas seeps, the exact production rates and the factors limiting production remain poorly understood. Here, we demonstrate that in two mantle rock systems with distinct compositions, the fertile lherzolites of the Western Pyrenees and the depleted harzburgites of Northern California, continuous fluid infiltration may sustain hydrogen generation, but only at rates of approximately 500 tonnes per year. Furthermore, by integrating three-dimensional geophysical inversion with a semi-coupled thermo-hydro-chemical model, we show that the main limiting factors controlling hydrogen yield are H₂ saturation in the fluid, porosity evolution at the reaction front, and reaction kinetics. The generation rates determined within the physico-chemical limits suggest that, although natural hydrogen from mantle rock serpentinization is a clean energy source, it may not be renewable on a large scale. Instead, its generation times for substantial accumulations more closely resemble those of fossil-type resources.