Natural hydrogen: What we know about its genesis and its geological occurrences

Recently, interest in hydrogen as an emission free fuel has increased. So far, hydrogen is produced by steam methane reforming (SMR), water electrolysis or methane pyrolysis. These processes are energy consuming and SMR emits carbon dioxide. Naturally occurring hydrogen might represent an alternative to technologically produced hydrogen. It is therefore worthwhile to explore whether economically viable amounts of hydrogen are present in the earth.

Whereas a wealth of hydrogen on earth is considered to be trapped in the earth’s core and lower mantle and mostly inaccessible to humans, natural hydrogen also occurs in the crust and upper mantle where it is formed from biogenic and abiogenic sources. Important processes for the generation of natural hydrogen include: water-rock interactions involving ferrous iron, e.g. serpentinization, equilibrium reactions associated with volcanic activity and hydrothermal vents, water radiolysis, mechanochemical reactions in cataclastic rocks associated with fault zones and thermal decomposition of organic matter to form graphite at high temperatures in deep sedimentary basins or crystalline basement.

The presence of hydrogen-rich gas was documented at Mid-Ocean-Ridges, ophiolites, sedimentary basins, and Precambrian cratons. A multitude of studies on natural hydrogen exists, but the relationship between generation, fluid migration and potential occurrence of economic accumulations of natural hydrogen is still a matter of debate.

We present an overview of the current published knowledge on natural hydrogen showing selected study locations of previous works and the amount of naturally occurring hydrogen inferred at each site. The methods used to estimate the amount of occurring hydrogen are reviewed.