Vivian Azor de Freitas

Located in a Brazilian craton, the São Francisco Basin (SFB) presents a proven natural hydrogen (H2)system, evidenced by high H2 concentrations in boreholes and numerous surface H2 emanations, somealready long-term monitored. Despite over six years of study, no detailed research has been carriedout to correlate these superficial observations with subsurface geological features. Therefore, thisstudy aims to evaluate the elements of the H2 system, including generating rocks, reservoirs, traps,and seals, focus on the central and southern parts of the SFB, using public geophysical and well datapreviously acquired by the oil and gas industry.Wells drilled in structural traps contain natural gas and H2 at various depths, from 300 m to almost 4km, particularly in Neoproterozoic units. The H2 rich layers exhibit very low permeability, classifyingthem as tight reservoirs. The H2 amounts vary from a few percent to 41%. These intervals can beidentified using neutron log, especially where the H2 concentration exceeds that of CH4. Overlappingimpermeable zones within the same lithostratigraphic units act as a caprock. These seals seem to beefficient to stopping or delaying the H2 flux, since the H2-hosting wells do not coincide with areas ofhigh density of the sub-circular depressions (SCD), which are typically associated with surface H2emanation. Alternatively, the absence of SCD may be attributed to soil characteristics that are notfavorable for their formation, since we noticed the SCD are predominantly found in soft sediments ofthe Cenozoic cover of the SFB.The Archean and Paleoproterozoic basement containing ferrous and radioactive rocks, as well as theCretaceous ultramafic intrusions, are the main candidates for the H2 generation rocks. These unitsmostly outcrop around the basin, such as the Banded Iron Formation from the Quadrilátero Ferrífero(QF) in the southeast, and the ultramafic Alto do Paranaíba Igneous Province (APIP), in the southwest.Significant magnetic anomalies observed in and around the basin may indicate potential H2 kitchens,as they can be considered indirect indicators of ferrous bodies. Among them, it must be highlightedthe Pirapora Anomaly, in the central part of the basin, and anomalies beneath the APIP and QF,extending to the internal part of the basin. These areas may correspond to the main generating rocksof the basin, even if a large distance from the H2 evidences is required. Furthermore, the contributionof radiolysis cannot be excluded.The multiphase structural history of the SFB allows gas migration over short and long distances, whichmay have enabled the widespread H2 occurrences in the basin, originated by multiple generating rocksboth inside and outside he basin.In summary, the presence of all the elements of the H2 system in the central and southern SFB makesit a proxy area for H2 attractive discoveries. Additionally, the existing gas pipeline network to majorpopulation centers can promote a future H2 production and distribution.​Co-authors: ​Vivian Azor de Freitas, Alain Prinzhofer, João Batista Françolin, Francisco José Fonseca Ferreira andIsabelle Moretti