Natural hydrogen system evaluation in the São Francisco Basin (Brazil)

The São Francisco Basin (SFB), a double foreland bordered by two Neoproterozoic belts, hosts a proven H2 system, demonstrated by high concentrations in boreholes and numerous surface H2 emissions, some long-term monitored. However, no detailed study has been conducted to correlate these H2 occurrences with subsurface geological features. Therefore, this study evaluated the H2 system components in the SFB, using public geophysical and well data previously acquired. The investigation revealed that wells drilled in structural traps contain natural gas and variable amounts of H2, ranging from a few percent to 41%, at depths between 300 m and nearly 4 km, particularly within Neoproterozoic units. These intervals exhibit high neutron log readings when the H2 concentration is higher than that of CH4. The H2-rich layers exhibit very low porosity and permeability and are overlapped by more impermeable zones with-in the same lithostratigraphic units. The low density of sub-circular depressions near most of H2-bearing wells could be due to effective seal, however the type of soil is likely to strongly influence the SCD for-mation, since our investigation revealed that SCDs are predominantly found in poorly compacted sediments of the Cenozoic cover of the SFB. Potential H2-generating rocks include the Archean and Paleoproterozoic Banded Iron Formation (BIF), Neoproterozoic BIFs and ultramafic bodies, Cretaceous ultramafic intrusions, and the overmature organic rich source rocks. A complementary influence of radiolysis cannot be ruled out. Significant magnetic anomalies, as indirect indicators of ferrous bodies, suggest potential H2 kitchens. Notably, the Pirapora Anomaly, in the central part of the basin, may reveal the presence of iron-rich bodies at great depth. However, most of the H2-bearing wells are not in its zone of influence, suggesting the presence of other H2 kitchens, such as those indicated by the magnetic anomalies be-neath the APIP and QF, extending to the internal part of the basin. The multiphase structural history of the SFB may allow gas migration in different ways. The relative influence of a long-distance migration, usual from the compressive front toward the foreland part of the basin and a vertical short-distance migration from magmatic intrusion can be highlighted. Additionally, denudation accelerated by the opening of the Atlantic Ocean may play a role in the H2 flow measured in this basin.