**2. The São Francisco Basin**

Located in the Brazilian states of Minas Gerais and Bahia, the São Francisco Craton presents rocks dating back to the Paleoarchean, to the Cenozoic, and several Precambrian sedimentary successions (Heilbron, 2017 [18]) (Figure 1). The basement is mostly composed of Archean TTG (Tonalite–Trondhjemite–Granodiorite) rocks, granitoids and greenstones belts (Anhaeusser, 2014 [19]) together with Paleoproterozoic plutons and supra-crustal successions. This polycyclic substratum, assembled during late Neoarchean times under high-grade metamorphic conditions, is intruded by late tectonic K-rich granites, mafic-ultramafic units, and mafic dikes (Teixeira et al., 2017 [20]). The Southern part of the São Francisco Craton consists of several gneiss complexes and greenstone belts from the Mineiro orogeny.

The sedimentary cover is made up of units younger than 1.8 Ga: the São Francisco basin (Southern part), the Paramirin Aulacogen (Northern part) and the Recôncavo–Tucano–Jatoba rift (Northeastern part) (Heilbron et al., 2017 [18]). Besides these Proterozoic sedimentary successions, the São Francisco basin also contains Phanerozoic units (Permo-Carboniferous and Cretaceous rocks).

Along the southern edge of the São Franciso basin, the Bambuí Group fills a series of buried grabens. The Bambuí strata exposed along the area of interest in this study are generally flat lying and cover more than 300,000 km2. The entire basin is covered by 450 to 1800 m-thick Neoproterozoic to Cambrian sedimentary successions, which are unconformably overlying the Archean-Paleoproterozoic basement (Delpomdor et al., 2020 [21]) (Figure 1b).

**Figure 1.** (**a**) Digital elevation model of southeastern Brazil, showing the topography associated with the São Francisco Craton outlined by the pink line (From Heilbron et al., 2017 [18]). (**b**) Simplified geological map (Serviço Geológico do Brasil) of the Southern part of the São Francisco Basin, outlined by the black line, with the Bambuí group in Yellow, the Phanerozoic cover in blue. Gas seepages (H2, N2, He, Hydrocarbons) have been observed during field investigations inside the green ellipse (Curto et al., 2012 [22]) and the triangle H2G corresponds to the gas seepage presented in Figure 2 (Prinzhofer et al.; 2019 [15]). The red markers, including the 1-RD-001-MG well, indicate locations of the exploration wells. The E–W (East–West) black dash line corresponds to the seismic sections presented in Figure 3. The solid black line linking the P1 and P39 white markers corresponds to the Magnetotelluric stations for the MT1 line set up in São Francisco Basin by Solon et al. (2015 [23]).

### **3. H2 Seepages in the Bambuí Group in the Southern Part of the São Francisco Basin**

To constrain the magnitude of the H2 emission, a permanent monitoring station has been installed in a depression located 16 km North-North East of Santa Fé de Minas in the State of Mina Gerais (Prinzhofer et al., 2019 [15]) (Figure 2). The recorded emission rates range from 7000 m<sup>3</sup> to 178,000 m<sup>3</sup> of H2 per day with H2 concentrations in the venting gas in the order of 1000 ppm (Cathles and Prinzhofer, 2020 [16]).

**Figure 2.** (**a**) Magnetotelluric stations for the MT1 line set up in São Francisco Basin by Solon et al. (2015 [23]) with the location of well 1-RF-001-MG, from Figure 1b. (**b**) H2 seepages (H2G) observed in circular depression zones (16◦33.605 S; 45◦20.620 W) (Prinzhofer et al., 2019 [15]).

In the same area, various geophysical data acquisitions have been previously obtained from surface monitoring or exploration wells. Seismic and magnetotelluric sections show the distribution of the main stratigraphic units across the São Francisco basin (e.g., Romeiro-Silva and Zalán, 2005 [24]; Reis and Alkmim, 2015 [25]; Solon et al., 2015 [23]) (Figure 1b).

The Bambuí group includes seven stratigraphic units. Logs from the 1-RF-001-MG well, located near station P1 on line MT1 (Figure 2a), provide quantitative information regarding the thickness of each geological layer to a depth of 1848 m with the sequences listed in Table 1. The depositional age of the Bambuí Group, especially its lower part, remains controversial, e.g., the estimated date varies from 560 Ma to 762 Ma for the lowermost part of the Sete Lagoas (Delpomdor et al., 2020 [21]).

**Table 1.** Simplifiedinterpretation of the well log data from the Petrobras' well 1-RF-1-MG (From Solon et al., 2015 [23]).


### **4. A Possible Deep Origin for H2**

In addition to H2 venting at location H2G (Figure 2b), He concentrations (5 ppm above atmospheric reference value) measured by Prinzhofer et al. (2019 [15]) at a depth of 1 m, sugges<sup>t</sup> a possible gas migration from deep horizons, where He is generated. Other analyses of gas sampled at the surface, from the head of the exploration wells drilled in the São Francisco basin confirmed that, besides high concentrations of H2 (up to ~20%), He (>1%) is also present, in association with methane-dominated hydrocarbons and N2 (Flude et al., 2019 [17]). Stable isotope data also sugges<sup>t</sup> an abiotic origin for the methane, while He isotopes reveal a strong crustal signature (3He/ 4He < 0.02 R/Ra) (Flude et al., 2019 [17]). The nucleogenic 3He from the decay of 6Li could account for the 3He/ 4He ratios found in the head of the exploration wells drilled in the São Francisco basin, i.e., close to R/Ra = 0.01 for an average granitic crust. Moreover, Neon isotope data also sugges<sup>t</sup> the presence of an Archaean crustal component in the gases, indicating that a component of the gas has likely originated from the underlying crystalline basement, or within Archaean-derived sedimentary rocks (Flude et al., 2019 [17]).

The natural production of the continental H2 can be of various origins (Guélard et al., 2017 [26]). Studies in deep mines from the Witwatersrand basin (South Africa) and the Timmins basin (Ontario, Canada) have suggested a link between dissolved H2 and the radiolytic dissociation of water (Lin et al., 2005a [27]). In addition to radiolysis, hydration of ultramafic rocks coupled to H2O reduction could also be responsible for H2 generation in Precambrian shields (Goebel et al., 1984 [28]; Sherwood Lollar et al., 2014 [29]). For example, the serpentinization of the gabbroic basement has been proposed as the process responsible for H2 production in Kansas (Coveney et al., 1987 [8]).

Radiolysis and serpentinization both require specific environments, which can be identified from geophysical and mineralogical investigations. Regarding the São Francisco basin, we detail these two possible processes of H2 formation in the following subsections.

### *4.1. Production of H2 by Water Radiolysis*

Distinct Archean gneissic–granitic complexes characterize the Southern part of the São Francisco Craton basement. They constitute a medium- to high-grade metamorphic terrain that crops out from the Quadrilátero Ferrífero towards the west, and mainly comprises TTG rocks, migmatites and K-rich granitic plutons (Teixeira et al., 2017 [20]). These rocks record deformational and metamorphic Archean episodes (from 2.55 Ga to over 3.3 Ga). It is known that the crystalline basement, rich in radiogenic elements and particularly this type of old basement Precambrian rock, represents potentially fertile deep-seated sources of H2 (Parnell et al., 2017 [30]; Sherwood Lollar et al., 2014 [29]). Indeed, molecular hydrogen production from water radiolysis requires the presence of radiogenic elements such as U, Th or K, which split the water molecules by ionizing radiation to produce molecules of H2. For the São Francisco Craton, the measured concentrations of uranium (U), thorium (Th) and potassium (K) are presented in Table 2. The Bambuí Group exhibits intermediate-to-high K and Th contents, while U-levels are around 2.5 ppm (Reis et al., 2012 [31]).



1 Sighinolfi et al., 1982 [32]; 2 Donatti-Filho et al., 2013 [33].

In a coarse-grained rock like granite, beta-irradiation from K is more prone to affect inter-granular fluid than the shorter-range alpha irradiation from U. Since K is also more pervasively distributed than U in granite, it can contribute to a larger scale radiolysis process.

Given the rather consistent range of U, Th, and K concentrations reported in the São Francisco Basin, we could expect in this zone a production rate of radiolytic H2 in water ranging from 10−<sup>8</sup> to 10−<sup>7</sup> nmol·L−1·s<sup>−</sup><sup>1</sup> (Lin et al., 2005b [34]). The methodology proposed by Sherwood Lollar et al. (2014 [29]) to estimate the contribution of the Precambrian continental crust to H2 production via radiolysis may then be applied to infer the regional H2 flux. The total radiolytic H2 production rate in water-filled fractures of the Precambrian crust was estimated to range from 0.16 to 0.47 × 10<sup>11</sup> mol·yr<sup>−</sup><sup>1</sup> for a corresponding surface area of 1.06 × 10<sup>8</sup> km2. Given the surface area of the Sao Francisco Basin of 300,000 km2, this corresponds to a H2 diffusive flux of 0.45 to 1.34 × 10<sup>8</sup> mol·yr<sup>−</sup>1, i.e., 90 to 266 tons·yr<sup>−</sup>1.

### *4.2. Production of H2 by Serpentinization or Hydration*

Serpentinization occurs when meteoric or oceanic waters alter ultramafic rocks originating from the Earth's mantle, such as peridotites and volcanic rocks. These rocks undergo changes in pressure and temperature conditions, which cause them to react in the presence of water (Schlindwein and Schmid, 2016 [35]; Horning et al., 2018 [36]): They are oxidized and hydrolyzed with water into serpentine, brucite and magnetite. The anaerobic oxidation of Fe(II) by the protons of water leads to the formation of H2 (Foustoukos et al., 2008 [37]; Proskurowski et al., 2008 [38]). In Precambrian rocks, Sherwood Lollar et al. (2014 [29]) propose that for the totality of the Precambrian crust (i.e., 1.06 × 10<sup>8</sup> km2) around 0.2 to 1.8 × 10<sup>11</sup> mol·yr<sup>−</sup><sup>1</sup> of H2 are produced by hydration. Here again, rescaling these values for the São Francisco basin (300,000 km2), we obtain a H2 production rate from hydration reactions of 0.56 to 5.09 × 10<sup>8</sup> mol·yr<sup>−</sup>1, i.e., 113 to 1018 tons·yr<sup>−</sup>1.

Favorable conditions to produce H2 by a serpentinization process would imply the presence of low-silica mafic and ultramafic rocks as well as an optimum temperature.
