Reis, H.LS.1 (email@example.com)
1HR Consulting Energy and Geosciences Ltda.
Belo Horizonte, MG, Brazil, 30330-230
Cratons comprise the oldest nuclei of continents that made up large portions of shields and platforms worldwide. Corresponding to thick pieces of continental lithosphere, they survived to multiple crustal reworking events through time due to their strength, typical differentiated composition and thermal structure. These characteristics allowed cratons to preserve a complex tectonic architecture and variable rock assemblages, within three major tectonic components: i) an up to c. 4.0 billion-years old basement composed of metamorphic and igneous rocks, greestone belt successions and (iron-rich) metasedimentary strata; ii) intracratonic basins, which comprise poli-historic depocenters recording superimposed basin-cycle episodes that are typically younger than 1,8 Ga; and iii) Phanerozoic fossil rifts. Natural hydrogen seeps and shows have been extensively reported in both basement and sedimentary strata of cratons, including the West Africa, Kalahari, São Francisco, North America, East Europe and Australian cratons, among others. Regardless many still open questions, the gas generation has been attributed mostly to hydration reactions affecting ultramafic to mafic units of greenstone belt sequences of the basement, as well as iron-rich units and basic igneous intrusions of the basement and sedimentary covers. As cratons host large-scale hydrographic basins and hydrogeologic systems, they show good conditions to induce the continuous interaction between shallower waters and deeper Fe(II)-rich rocks. The age of craton interior rocks, the thermal conditions and crustal helium gas concentrations reported in the literature, also favor other hydrogen generation mechanisms such as radiolysis and overmaturation of organic-rich sedimentary rocks. In some cases, migration pathways connecting sources to shallower reservoirs are apparently associated with deep-seated faults cutting the basement and thick intracratonic strata. These faults formed and often reactivated during major Proterozoic and Phanerozoic plate reorganization episodes. In the Taodeni and São Francisco basins of the West Africa (Mali) and São Francisco (Brazil) cratons, reservoirs comprise relatively thick, tight and locally fractured carbonate to siliciclastic sedimentary rocks ranging from Mesoproterozoic to early Paleozoic in age. In the Taodeni basin, Mesozoic igneous rocks that emplaced during the Mesozoic Atlantic Ocean opening and the Pangea breakup seem to play a role as both sources and permeability barriers. Their correlatives are also preserved in cratons of South America, where they have been locally recognized as important elements in gas-prone petroleum systems. The available information suggest that cratons encompass different sets of sources, migration pathways, reservoirs and permeability barriers required to naturally generate and accumulate hydrogen in the crust. These components are intrinsically associated with their long and complex evolution through the geological time, making them major targets for ongoing hydrogen exploration campaigns.
This work was developed with the support from Storengy (Engie). Olivier Lhote, Tiphaine Fargetton, Chaterine Formento and Tiphaine Tual are thanked for the helpful discussions that enriched the work.