Bruno M. Jesus

Silicification is a major diagenetic process in carbonate reservoirs, profoundly influencing mineralogy, pore-system architecture, fluid-flow pathways, and reservoir quality. Although chert is widespread in the Aptian Barra Velha Formation of the Santos Basin pre-salt province, its origin and relationship with fluid circulation, karstification, and reservoir evolution remain poorly constrained. This study investigates cherts within a 145.65 m cored interval distributed across five cores using an integrated multiscale workflow combining core description, petrography, UV epifluorescence, SEM-EDS, portable and micro-XRF, computed tomography (CT), micro-CT, fluid inclusion petrography, and coupled silicon and sulfur stable isotope analyses. Four chert geometries were identified: tabular, nodular, domal, and irregular. Tabular and nodular cherts preserve primary carbonate fabrics and record replacement-dominated silicification followed by progressive recrystallization from opal and chalcedony to microcrystalline and macrocrystalline quartz. Domal cherts occur exclusively within microbialite-bearing facies and preserve filamentous and reticulated fabrics within microcrystalline silica, suggesting localized early silica precipitation associated with microbial substrates. In contrast, irregular cherts are concentrated within fractured, brecciated, and karstified intervals and are characterized by diffuse silicification, macrocrystalline quartz cementation, sulfate mineralization, and strong structural control. Five karstified intervals, representing approximately 37% of the studied succession, contain the most interconnected pore systems identified by CT and micro-CT, with porosity increasing from approximately 3.1% in continuous cherts to 26.7% in brecciated intervals. These zones coincide with irregular cherts, sulfate-rich mineral assemblages, and fault-controlled permeability pathways, locally producing bulk-density values up to 3.37 g/cm³. Silicon isotope compositions range from δ³⁰Si = −0.116‰ to +1.476‰, with tabular cherts consistently exhibiting the heaviest values (up to +1.476‰) and irregular cherts the lightest (down to −0.116‰), whereas sulfur isotope compositions (δ³⁴S = 8.0–33.4‰) reveal isotopically heterogeneous sulfate-bearing fluids. Associated mineral assemblages—including barite, celestine, fluorite, titanite, goyazite, dawsonite, and pyrite—record the circulation of chemically evolved mixed fluids through structurally enhanced pathways. The integrated dataset supports a multistage fluid-flow model involving early silica precipitation, replacement silicification, karstification, structurally focused fluid migration, sulfate mineralization, and late hydrocarbon charging. Chert formation is interpreted as the product of a long-lived diagenetic system in which fluid mixing, structural permeability, progressive silica recrystallization, and fault-controlled fluid flow collectively governed silicification and the development of reservoir heterogeneity in the Barra Velha Formation.