Reis Humberto

In recent years, natural hydrogen has become a major target for exploring new, low-carbon natural resources. Remarkable occurrences have been reported in almost all continents in different tectonic settings, ranging from oceanic ridges, old and modern orogenic systems, cratons and different types of sedimentary basins. Natural hydrogen shows have been documented in Brazilian onshore sedimentary basins as continuous emissions in circular surface depressions (fairy circles) and as gases produced in old wells drilled during hydrocarbon exploration campaigns. Anomalous concentrations were found in the Paleozoic Solimões, Paraná, and Parnaíba basins, in the Mesozoic Tacutu rift and in the Proterozoic to Mesozoic São Francisco basin. The hydrogen concentrations may vary from a few to up to c.1000 ppm at the surface and from 0,45 up to 40% in hydrocarbon wells, where they are usually associated with remarkable amounts of helium, alkanes and nitrogen. Although showing different ages and geological histories, all sedimentary basins share a series of common features. The studied cases were developed in cratonic lithosphere, corresponding to depocenters underlain by ancient or fossil rifts or representing fossil rifts developed in the Phanerozoic. Showing gas accumulations hosted by different types of conventional and unconventional reservoirs, these basins also host remarkable bodies of Mesozoic sills and dykes emplaced either during the Mesozoic Central Atlantic or the South Atlantic events. In the Paleozoic basins, these intrusions offered the heat to generate hydrocarbons from organic-rich rocks also trapping and sealing natural gas accumulations in some cases. Since most of these sedimentary basins are intracratonic in nature, their strata cover and may also contain large volumes of potential natural hydrogen sources such as ultramafic-rich greenstone belt successions, basic to ultrabasic intrusions, overmature organic-rich rocks and other iron-rich assemblages. Helium and noble gases isotopes measured in samples of the São Francisco basin and samples from its Archean basement also indicate a strong crustal component for the helium and, by consequence, important radiolytic components for the hydrogen (Flude et al. 2020, 2021, Magalhães et al. 2021). When compared with West African and Australian evidence, as well as North American occurrences, they show several similarities including i) the tectonic evolution and styles, ii) architecture and formation mechanisms, iii) potential reservoirs and seals, iv) potential sources, among others. The studied Brazilian cases and correlatives seem to reveal prolific fault- and fold-related natural hydrogen plays that might be associated or not with the compressional reactivation of preexisting normal faults, as well as four-way closing magmatic intrusions. In many observed cases, the structural architecture seems to favor the effective connection with shallower surface and underground water. If confirmed by future natural hydrogen exploration and discoveries, all of these characteristics also confirm the similarities between the natural hydrogen system and hydrocarbon and mineral systems typically described in sedimentary basins.Co-authors:Olivier Lhote, Tiphaine Fargetton, Catherine Formento, Stephane Galibert and Tiphayne Tual​Affiliation:  Storengy-ENGIE, Bois-Colombes, Île-de-France, France