*4.4. Temperature Ranges at Depth*

The potential presence of ultramafic rocks within the area of gas seepages, could sugges<sup>t</sup> a serpentinization process, but to be active, this process would require a favorable range of temperatures. Crustal thermal models have been developed to examine the implications of the observed intra-cratonic

variations in heat flow across the São Francisco Basin (Alexandrino et al., 2008 [48]). The thermal models take into consideration the variation of thermal conductivity with temperature. It is thus possible to ge<sup>t</sup> the temperature distribution calculated along a large profile that crosses the São Francisco Basin. It turns out that our zone of interest in the São Francisco Basin, exhibits an abnormally high heat flow value for a craton (Figure 6). In the gas seepage zone (green ellipse in Figure 5), the temperature gradient is about 25 ◦C/km (Alexandrino et al., 2008 [48]). The optimum temperature for serpentinization was found to be around 250–300 ◦C with a maximum production of magnetite (Klein et al., 2013 [49]). However, some experimental data sugges<sup>t</sup> that below 150 ◦C, H2 can still be produced through the formation of Fe3<sup>+</sup> oxi-hydroxides (Mayhew et al., 2013 [50]; Miller et al., 2017 [51]), with the formation of H2 being possibly catalyzed by the surface of spinel-structure minerals occurring in ultramafic rocks. In such thermal conditions, H2 could still be produced at a low rate, at a depth lower than 6 km, near the gas seepage zone, and the optimum depth for its production would be at 10–12 km.

**Figure 5.** Bouguer anomaly map in the São Francisco Basin within the studied zone (modified from Oliveira and Andrade, 2014 [44]).
