Possibly a trillion tons of mineable hydrogen
Last October Geoffrey Ellis, from the USGS, presented his model for global subsurface natural hydrogen resource potential. Based on oil industry experience, this model works under the assumption that only 10% of all natural hydrogen accumulations might ever be tapped economically, which could nonetheless satisfy world demand for a long, long time. This article does a great job in summarizing the impact of this model, while giving a few important facts about generation, loss mechanisms and extraction possibilities.
Source: Next Big Future