Molecular hydrogen in surface and subsurface natural gases: Abundance, origins and ideas for deliberate exploration

Abstract

Geologic molecular hydrogen (H2) occurs in the subsurface and vents and seeps at the surface. However, this valuable natural resource is under-utilized in the economy because the distribution, abundance and origins of H2 are poorly understood. I studied a global dataset of 6246 natural gases with reported H2 concentrations from 16 different geological habitats. The average H2 concentration in all gas samples is 3.5%, but the median concentration is only 0.01%. Gases sampled in Mid-Ocean Ridges and in serpentinites have the highest average concentrations of H2 (~24% and ~21%, respectively). More than 30 different processes may produce H2 observed in natural gases. Hydrogen isotopic composition (expressed as δ2H-H2 values) may indicate crustal (<-650‰) or mantle and primordial (from -650‰ to -100‰) sources of H2, or may result from temperature-dependent equilibration of H2 with water. Much of crustal H2 may be sourced by the reactions of serpentinization, while the quantitative significance of other H2-generating processes such as radiolytic decomposition of water and hydrocarbons, fracture-induced reduction of water, petroleum cracking and coal metamorphism remain speculative. Primordial H2 perhaps vents in some volcanic settings. Provided better understanding of H2 abundance and origins in different geological settings should enable the purposeful exploration for geologic H2 and the assessment of its economic resources.