Abstract
Offshore the emissions of dihydrogen are highlighted by the smokers along the oceanic ridges. Onshore in situ measurements in ophiolitic contexts and in old cratons have also proven the existence of numerous H2 emissive areas. When H2 emanations affect the soils, small depressions and vegetation gaps are observed. These depressions, called fairy circles, have similarities with the pockmark and vent structures recognized for long time in the sea floor when natural gas escapes but also differences. In this paper we present a statistic approach of the density, size, and shape of the fairy circles in various basins. New data from Brazil and Australia are compared to the existing database already gathered in Russia, USA, and again Brazil. The comparison suggests that Australia could be one of the most promising areas for H2 exploration, de facto a couple of wells already found H2, whereas they were drilled to look for hydrocarbons. The sum of areas from where H2 is seeping overpasses 45 km2 in Kangaroo Island as in the Yorke Peninsula. The size of the emitting structures, expressed in average diameter, varies from few meters to kilometers and the footprint expressed in % of the ground within the structures varies from 1 to 17%. However, globally the sets of fairy circles in the various basins are rather similar and one may consider that their characteristics are homogeneous and may help to characterize these H2 emitting zones. Two kinds of size repartitions are observed, one with two maxima (25 m and between 220 m ± 25%) one with a simple Gaussian shape with a single maximum around 175 m ± 20%. Various geomorphological characteristics allow us to differentiate depressions of the ground due to gas emissions from karstic dolines. The more relevant ones are their slope and the ratio diameter vs. depth. At the opposite of the pockmark structures observed on the seafloor for which exclusion zones have been described, the H2 emitting structures may intersect and they often growth by coalescence. These H2 emitting structures are always observed, up to now, above Archean or Neoproterozoic cratons; it suggests that anoxia at the time the sedimentation and iron content play a key role in the H2 sourcing.
Highlights
Gas in the soil have been measured in Brazil and Russia and H2 has been found in drilled prospects by the Oil and Gas (O&G) industry in two basins in Australia
The depth and the slope have been computed, the accuracy of these values is depending on the available Digital Elevation Model (DEM), in the places where we had the opportunity to do direct measurements the error was less than 10%
We presented the data from three areas in Brazil and one in Russia where the structures observed are known to be the place of H2 leakage
Summary
The landscape is changing since green fuels are looked for to decrease the CO2 impact of the mobility and to improve the air quality of the cities. The use of H2 as fuel is growing and the demand of green hydrogen, not manufactured from hydrocarbons, is following the same trend. The production of native H2 appears as a promising alternative [1,2,3] and exploration is starting in various countries after the fortuitous discoveries of accumulations in different places and the first years of production in Mali [4]. It should be noted that some authors already alerted the scientific community some time ago on that resource [5] without receiving any attention from the extractive industry but times are changing
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