The character and habitat of natural hydrogen resource systems

Hydrogen use today is mostly as a chemical feedstock, producing ammonia used in fertiliser production amongst other hard to abate uses. Today’s hydrogen is produced directly from hydrocarbons with the resulting CO₂ contribution ca 2.4% of global emissions. Hydrogen as a future clean energy vector could see hydrogen demand increase from ca 95 Mt H₂ today, to 540 Mt H₂ by 2050.

The mass of hydrogen generated within the continental crust is only recently being appreciated as a potential societal resource. Accumulation and preservation of a small portion of the natural hydrogen, in accessible parts of the continental crust, is required. The dominant sources of natural hydrogen are through water-rock reactions with mafic or ultramafic rocks and the radiolysis of water from the radioactive decay of U and Th in rocks.  The timescales and environments that enable significant hydrogen generation occur in geological different terrane. These vary from dominantly Phanerozoic ophiolite complexes; Proterozoic-Phanerozoic alkaline granite complexes; Mesoproterozoic-Phanerozoic large igneous provinces (LIP) to dominantly Archean TTG and greenstone belts. The tectonic evolution in each setting, and capacity to form traps, is required alongside the porosity and permeability history that exposes the rock to water. To form a commercial reserve, an environment that produces and preserves a free gas phase from the ubiquitous water over the timescale of the system is required. Helium (⁴He) provides an analogue for natural hydrogen behaviour and the processes that control both deep-seated flux to the near surface and gas phase formation. Loss due to microbial utilisation remains a high preservation risk.

C Ballentine, R Karolytė, A Cheng, B Sherwood Lollar, J Gluyas, M Daly. Natural hydrogen resource accumulation in the continental crust, In review