Migration of Natural Hydrogen from Deep-seated Sources in the São Francisco Basin, Brazil

Abstract

Hydrogen gas is seeping from the sedimentary basin of São Franciso, Brazil. The seepages of H2 are accompanied by helium whose isotopes reveal a strong crustal signature. Geophysical data indicates that this intra-cratonic basin is characterized by i) a relatively high geothermal gradient, ii) deep faults delineating a horst and graben structure and affecting the entire sedimentary sequence, iii) an archean to paleoproterozoïc basements enriched in radiogenic elements and displaying mafic and ultramafic units, and iv) a possible karstic reservoir located 400 m below the surface. The high geothermal gradient could be due to a thin lithosphere enriched in radiogenic elements, which can also contribute to a massive radiolysis process of water at depth, releasing an important amount of H2. Alternatively, ultramafic rocks that may have generated H2 during their serpentinization are also documented in the basement. The seismic profiles show that the faults seen at the surface are deeply rooted in the basement, and can drain deep fluids to shallow depths in a short time scale. The carbonate reservoirs within the Bambuí group which forms the main part of the sedimentary layers are crossed by the fault system and represent good candidates for temporary H2 accumulation zones. The formation by chemical dissolution of sinkholes located at 400 m depth might explain the presence of sub-circular depressions seen at the surface. These sinkholes might control the migration of gas from temporary storage reservoirs in the upper layer of the Bambuí formation to the surface. The very high fluxes of H2 escaping out of these structures which have been recently documented are, however, in disagreement with the newly developed H2 production model in the Precambrian continental crust. They either question the validity of these models or the measurement methodology.

Geochemical characteristics of geothermal and hot spring gases in Beijing and Zhangjiakou Bohai fault zone

“…H 2 would escape into the atmosphere along the weak areas of the Earth (Neal and Stanger, 1983); (2) When the fracture broke, waterrock reaction occurred on the fresh silicate rock surface to produce hydrogen (Kameda et al, 2003). The more active the active fault zone was, the more developed the fresh silicate rock fracture surface in the fault zone was, and the more H 2 was produced (Kita et al, 1982); (3) U and Th elements in rocks produced high concentrations of hydrogen with water during radioactive decay (Lin et al, 2005;Donze et al, 2020); (4) A large amount of hydrogen was produced in the serpentinization process of olivine (Katayama et al, 2010;Donze et al, 2020); (5) Soil organic matters produced hydrogen during anaerobic bacterial fermentation (Libert et al, 2011). H 2 is the lowest density gas known in the world, featuring strong diffusivity and penetrability.…”