Hydrogen exploration: a review of global hydrogen accumulations and implications for prospective areas in NW Europe

From a geological perspective, hydrogen has been neglected. It is not as common as biogenic or thermogenic methane, which are ubiquitous in hydrocarbon basins, or carbon dioxide, which is common in geologically active areas of the world. Nevertheless, small flows of hydrogen naturally reach the Earth’s surface, occur in some metal mines and emerge beneath the oceans in a number of places worldwide. These occurrences of hydrogen are associated with abiogenic and biogenic methane, nitrogen and helium. Five geological environments are theoretically promising for exploration based on field, palaeofluid and theoretical evidence: ophiolites (Alpine, Variscan and Caledonian in order of decreasing prospectivity), thinned-crust basins (failed-arm rifts, aulacogens), potash-bearing basins, basement in cratonic areas and the Mid-Atlantic ridge and its fracture zones. The subsurface areas of these environments are relatively poorly known, compared to hydrocarbon basins. Hydrogen shows may indicate larger reserves in the subsurface, in a similar way to the beginnings of hydrocarbon exploration in the 19th century. The main source of hydrogen is ultramafic rocks, which have experienced serpentinization, although other generation processes have been identified, including biogenic production of hydrogen during very early stages of maturation and radiolysis. There are two main tectonic settings where serpentinization has operated. The main accessible onshore areas are where ophiolites are found tectonically emplaced within fold belts. Potentially much larger investigation areas lie in the subsurface of some ophiolites. These areas generally lie outside hydrocarbon provinces. However, where thrusting has emplaced ophiolites over a hydrocarbon-bearing foreland basin, tests involving sub-thrust conventional hydrocarbon exploration plays could also be employed to search for hydrogen. The other main tectonic setting is in highly extended basins, for example failed rifts or aulacogens, where thick sediments overlie thinned or absent crust above probably serpentinized mantle. These structures occur offshore on continental margins and extend onshore into long-lived rifts which have been reactivated and rejuvenated repeatedly. Conventional seismic reflection data are already available in these areas, but deep subsurface resolution is poor where there are extensive volcanic rocks. Analogues of these occurrences are also found in the deep oceans, along the mid-ocean ridges and offsetting transform faults. Here, thin crust and faulting may facilitate serpentinization of the mantle rocks by seawater ingress. Further research should aim to identify the extent of the hydrogen flux and its probable dominant role in the abiogenic production of hydrocarbons in Precambrian times, a natural process now largely replaced by biogenic participation. A similar industrial process replicates serpentinization, producing hydrogen and ultimately liquid hydrocarbons on a commercial scale in some countries. It remains to be proved whether a contribution from exploration can be made to any future hydrogen economy.