The occurrence and geoscience of natural hydrogen: A comprehensive review

Using an interdisciplinary approach, this paper reviews current knowledge in the field of natural hydrogen. For the first time, it combines perspectives on hydrogen from the literature of the former Eastern bloc with that of the West, including rare hardcopies and recent studies. Data are summarized and classified in three main sections: hydrogen as a free gas in different environments, as inclusions in various rock types, and as dissolved gas in ground water. This review conclusively demonstrates that molecular hydrogen is much more widespread in nature than was previously thought. Hydrogen has been detected at high concentrations, often as the major gas, in all types of geologic environment. A critical evaluation of all the proposed mechanisms regarding the origin of natural hydrogen shows that a deep-seated origin is potentially the most likely explanation for its abundance in nature. By combining available data, an estimate of 23 Tg/year for the total annual flow of hydrogen from geologic sources is proposed. This value is an order of magnitude greater than previous estimate but most likely still not large enough to account for recently discovered worldwide diffusive seepages. Hydrogen could play a critical role in mechanisms taking place in both the shallow and deep geospheres and it can influence a very wide range of natural phenomena. Hydrogen is an essential energy source for many microorganisms. Sampling for hydrogen can be a useful tool in studying natural environments, geologic mapping, monitoring of earthquakes, plotting fault traces and resource exploration. Hydrogen of geologic origin has the potential to become the renewable energy source of the future, with exploratory projects ongoing at the present time. The topic of natural hydrogen is therefore relevant from many different perspectives.