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

Abstract

The São Francisco Basin (SFB), a double foreland bordered by two Neoproterozoic belts, hosts a proven hydrogen (H 2 ) system, demonstrated by high concentrations in boreholes and numerous surface H 2 emissions, some long-term monitored. However, no detailed study has been conducted to correlate these H 2 occurrences with subsurface geological features. Therefore, this study evaluated the H 2 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 H 2 , 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 H 2 concentration is higher than that of CH 4 . The H 2 -rich layers exhibit very low porosity and permeability and are overlapped by more impermeable zones within the same lithostratigraphic units. The low density of sub-circular depressions (SCD) near most H 2 -bearing wells could be due to effective seal, however, the type of soil is likely to strongly influence the SCD formation since our investigation revealed that SCDs are predominantly found in poorly compacted sediments of the Cenozoic cover of the SFB. Potential H 2 -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 H 2 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 H 2 -bearing wells are not in their zone of influence, suggesting the presence of other H 2 kitchens, such as those indicated by the magnetic anomalies beneath the Alto do Paranaíba Igneous Province (APIP) and Quadriláfero Ferrífero (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 long-distance migration, usually 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 H 2 flow measured in this basin.