Rapid consumption of H2 in the deep subsurface.

Subsurface natural hydrogen is an emerging clean energy resource, but its origins, loss mechanisms and cycling timescales, particularly at greater depths, remain poorly constrained. Here, we report large disequilibrium in the clumped isotopes of H2 (2H2H) and hydrogen isotope fractionation between water and H2, measured in gas samples (containing 0.10%-1.59% H2) collected from deep (3000-7000 m) wells in the central Sichuan Basin. Because molecular hydrogen can equilibrate isotopes rapidly in the presence of water, such large disequilibrium indicates active kinetically controlled production and/or consumption. Analyses of geological and chemical evidence demonstrate that isotopic disequilibrium is driven by consumption by chemical reactions in host rocks coupled with partial hydrogen exchange with co-existing water. The results imply that measured compositions do not preserve the information on original sources and processes. More broadly, the dynamic cycling of hydrogen renders sediment-hosted deep gas reservoirs unfavorable for long-term or large-scale accumulation of natural hydrogen.