*2.8. H2 Basin Resources*

The last three papers discuss a completely new and surprising kind of nonhydrocarbon basin energy resource. Hydrogen gas has been observed venting from circular depressions (fairy circles) in many basins. As described by Cathles and Prinzhofer [14], H2 concentrations at 1 m depth in the Sao Francisco Basin in Brazil occur mainly at the margins of a circular depression ~550 m in diameter, and are nonzero for about 6 h a day with peak concentrations occurring at ~1:00 pm. The periodic venting could be caused by atmospheric pressure tides, which have a very regular diurnal cycle at this location. The maximum rate of atmospheric pressure decrease occurs at 1:00 pm. The changing atmospheric pressure pushes air into and pulls it out of the subsurface in an accordion-like fashion. The volume of the gas pulled in and out of the shallow subsurface vents must be 1000 times less than the volume of gas in the subsurface reservoir that is compressed and decompressed. H2 losses to the atmosphere can be supplied by a H2 flux of ~0.1 m<sup>3</sup> m<sup>−</sup><sup>2</sup> d−1.

Donzé et al. [15] address the critical question of whether the H2 venting observed in many basins could constitute a significant energy resource by placing the venting rate analyzed by Cathles and Prinzhofer in contest with the local geology and global venting rate estimates. The H2 generation rate expected from the 10<sup>8</sup> km<sup>2</sup> of Precambrian lithosphere, scaled to the 300,000 km<sup>2</sup> area of the Sao Francisco basin, suggests the entire basin could generate 90 to 266 tons H2/y by radiolysis and/or 113 to 1018 tons H2/y by serpentinization. Both generation mechanisms are possible in and under the Sao Francisco Basin: faults cut deeply into the crust, and storage reservoirs are present. However, the H2 venting rate estimated by Cathles and Prinzhofer of 200 to 5400 tons H2 per year from a single vent is as high as that expected from the entire Sao Francisco Basin, which suggests either the global estimates are too low or the venting at the study site is presently unusually strong.

Finally, Simon et al. [16] discount the possibility that solid Earth tides could be the cause of the variable venting in the Sao Francisco Basin by showing that solid Earth tides at the site have two co-equal peaks per day, but the H2 venting has only one. We are just beginning to understand the H2 system. We are at a stage similar to when we knew of a few hydrocarbon seepages, but had no concept of the magnitude or importance of the petroleum system.
