Comment on “Molecular hydrogen in surface and subsurface natural gases: Abundance, origins and ideas for deliberate exploration” by Alexei V. Milkov [Earth-Science Reviews, 230, 2022: 104063]

In the article entitled “Molecular hydrogen in surface and subsurface natural gases: Abundance, origins and ideas for deliberate exploration”, Alexei V. Milkov leverages a large database to address the occurrence of molecular hydrogen (H2) in natural gases. The stated aims involve getting new insights into the mechanisms that lead to the production of H2 in geological habitats as well as producing tools for its deliberate exploration. This commentary discusses the definition and practical utility of some concepts employed by the author. It also provides observations suggesting that the diagnostic value of the isotopic signatures of H2 and associated gases is more limited than what Milkov (2022) contends. I further believe the examination of the hydrogen isotopic systematics in the CH4–H2–H2O ternary system presented by Milkov (2022) to be incomplete, and outline some potentially complicating factors to the proposed interpretations.