Mechanisms and controls of silicification in continental carbonates of the Barra Velha and Itapema formations (Aptian of Santos Basin, Brazil)

Silicification is a diagenetic process that affects the porosity and permeability of carbonate rocks, such as those in the Itapema and Barra Velha formations, Pre-salt of Santos Basin. This contribution integrates macroscopic and microscopic descriptions of silicified intervals, identifying controls and mechanisms of dissolution, replacement, and precipitation, relating them within the diagenetic history of the formations. The pre-compaction eodiagenetic silicification comprises tabular and nodular geometries, reflecting fine-grained laminated and coarser-grained massive lithologies. The post-compaction mesodiagenetic silicification comprises irregular silica disseminated throughout carbonates. Silica bodies consist of replacive (cryptocrystalline silica, mosaic microcrystalline quartz and spherulitic chalcedony) and pore-filling textures (chalcedony and macrocrystalline quartz). The highly selective nature of the replacement suggests that carbonate dissolution initiates silica precipitation as a fluid-mediated process that transfers crystallographic information. The successive zoning with decreasing crystallinity in filling/cement textures indicates a decrease in the fluid supersaturation as the precipitation advances. These features indicate that textures and silicification mechanisms reflect precipitation physicochemical conditions. The first silica event follows the dolomitization stage that replaces primary clays and carbonates, while the second event occurs after mechanical compaction and is later followed by replacive/pore-filling barite textures. Late macrocrystalline quartz veins host primary hydrocarbon fluid inclusions, linking the second silicification event with oil migration.