**1. Introduction**

The natural production of molecular hydrogen (hereafter hydrogen or H2) has drawn increasing scientific attention due to the central role this molecule plays in fueling the deep subsurface biosphere or promoting the abiotic synthesis of organic molecules (Truche et al., 2020 [1]). Natural H2 sources may also represent a new attractive primary carbon free energy resource (Smith, 2002 [2]; Smith et al., 2005 [3]; Truche and Bazarkina, 2019 [4]; Gaucher, 2020 [5]). This latter industrial perspective has motivated recent H2 exploration studies in ophiolite, peralkaline, Precambrian shields and intra-cratonic geological settings (see review by Zgonnik, 2020 [6]).

Natural hydrogen (also known as native hydrogen) sources have been identified for several decades in seafloor hydrothermal vents, and hyperalkaline springs in ophiolite massifs. Serpentinization of ultramafic rocks is the water–rock interaction process responsible for H2 generation in these contexts (Neal and Stranger, 1983 [7]; Coveney et al., 1987 [8]; Abrajano et al., 1990 [9]; Charlou et al., 1996 [10]; Seewald et al., 2003 [11]). However, the recent discoveries of intra-cratonic H2 seepages and accumulations with no obvious link to an ultramafic formation challenge our current understanding of H2 production and fate in the crust (Larin et al., 2015 [12]; Zgonnik et al., 2015 [13], Prinzhofer et al., 2018 [14]). To date, there is no in-depth understanding of the hydrogen system from source to seep in these latter geological settings. When not fortuitous, as in the Taoudeni Basin in Mali (Prinzhofer et al., 2018 [14]), the discoveries of new H2 seepages were made thanks to the satellite detection of sub-circular soil depressions displaying vegetation anomalies, e.g., Borisoglebsk in Russia (Larin et al., 2015 [12]) and Carolina Bay in the US (Zgonnik et al., 2015 [13]). These surface features are the only evidence used to detect these H2 seepages. This limited understanding of the H2 systems, and this lack of robust pathfinders prevents the development of a methodic exploration strategy or resource assessment in these environments.

The São Francisco Basin belongs to this short list of intra-cratonic basins where H2 seepages have been discovered. There, hydrogen gas vents from slight topographic depressions that are circular and barren of vegetation and, in one of them, the recorded H2 concentrations range from 50% to 80% (Prinzhofer et al., 2019 [15]; Cathles and Prinzhofer, 2020 [16]). In the area of the São Francisco Basin, Flude et al., (2019 [17]) also, recorded up to 20% H2, mostly accompanied with N2 and several percent of CH4, in the gas mixture from the head of exploration wells and natural gas seeps.

The São Francisco basin provides one of the first H2 case studies where geological information can be collected with a sufficient level of detail to provide the primary elemental bricks that may compose the H2 system in intra-cratonic basins. Here, we review the different layers of information that compose a supposed H2 system in this basin and lay the foundation of a H2 exploration guide.
