Assessment of natural hydrogen system in the São Francisco Basin (Brazil) and the Northern Apennines (Italy)

Studies on natural hydrogen have increased significantly in recent years, driven by its potential as a low-carbon energy source and by the diverse geological contexts in which it can occur. However, the topic remains poorly explored, requiring progress in several disciplines to enable a broader evaluation of its potential use. This study aimed to evaluate the natural hydrogen system, focusing on its generation mechanisms, as well as on migration pathways, reservoirs, traps, and seals in two distinct geological contexts: the intracratonic São Francisco Basin (Brazil) and the Northern Apennines fold-and-thrust belt (Italy). The São Francisco Basin has high potential for hydrogen exploration and has been the subject of previous studies, particularly those focused on surface data. In contrast, the present study integrates surface and subsurface data, including seismic, magnetic, and well data, bringing new elements to the discussion. The western sector of the Northern Apennines represents a new exploratory frontier, without previous studies focused on natural hydrogen. Following an initial investigation at several sites within the area, dissolved hydrogen concentrations were detected in association with the Monte Prinzera ultramafic body and the Bobbio Tectonic Window. Oxidation processes involving iron-bearing phases are like to play an important role in both studied areas. As most of the potentially iron-rich rocks are not exposed, geophysical surveys, including reflection seismic, gravimetric, and magnetic surveys, are essential to predict the location of iron-rich bodies at depth and to investigate the fault network and flow pathways capable of transporting H₂ toward the surface. Gravimetric and magnetic anomalies, such as those in the São Francisco Basin and within the Bobbio Tectonic Window, are probably associated with subsurface iron-bearing bodies, which represent a potential hydrogen-generating rocks. At Monte Prinzera, rock characterization, focusing on mineralogical, whole-rock compositional, and redox-state analyses, together with thermodynamic modeling, provides evidence of both past and ongoing hydrogen generation. In addition, subsurface hydrogen accumulations were observed in low-permeability reservoirs within structural traps in the São Francisco Basin. In the Northern Apennines, hydrogen has been detected only in dissolved form within water, implying a relatively low potential for hydrogen exploration. The results obtained in this research provided material for four manuscripts addressing different aspects of natural hydrogen exploration. These contributions not only investigate the potential of the studied areas for natural hydrogen exploration but also advance knowledge of this emerging energy resource and support the identification of the most favorable geological frameworks for its occurrence.