Multi-stage silicification in the Aptian pre-salt reservoirs of the Santos Basin, Brazil: lacustrine recirculation and hydrothermal processes of silica precipitation in continental carbonates

ABSTRACT Silicification is typically regarded as a minor process in carbonate sequences, yet it can significantly alter the fabric, preserve or erase depositional characteristics, and impact reservoir quality. The giant Aptian pre-salt lacustrine carbonate reservoirs in the Campos and Santos basins have been the subject of extensive discussion regarding their formation and diagenetic evolution. In the pre-salt section of the Santos Basin, as in other South Atlantic basins, various forms of silica commonly replace primary and diagenetic constituents or occur as pore-filling cements. Overall, the controls, conditions, and distribution of silicification in the pre-salt are poorly understood. Additionally, hydrothermal alterations superimposed on prior diagenetic changes have complicated the study of primary depositional and geochemical characteristics, also impacting reservoir quality. Here, we present an integrated study using core descriptions, detailed petrography, oxygen-isotope, and fluid-inclusion data to reveal the complex silicification patterns in the Santos Basin pre-salt deposits. A paragenetic sequence of silica species precipitation is defined, and three main silicification patterns are identified, including early-stage and late-stage silicification. Stratabound eodiagenetic silicification, mainly by fine crystalline silica phases, resulted from chemical changes in lacustrine fluids, leading to the replacement of the magnesian-clay matrix. Fault-focused, low-temperature silicification likely involved the recirculation of Si-enriched lacustrine waters due to extensive eodiagenetic dissolution of magnesian clay minerals. Mesodiagenetic quartz precipitated from evolved brines, and late-stage hydrothermal silica precipitation occurred mainly along faults and in permeable layers, with the precipitation of coarse and prismatic quartz. Oxygen isotope data indicate that the fluids were strongly influenced by evaporation, while fluid-inclusion analysis suggests precipitation at low temperatures from high-salinity fluids, with subsequent overprinting by high-temperature fluids of intermediate to low salinity, influenced by juvenile fluids. A schematic model of the interpreted silicification processes is presented, along with their positioning in the burial and thermal evolution of the Santos Basin, revealing associations with key tectono-magmatic events. These findings provide new insights into the complex silicification processes in the Aptian pre-salt carbonates of the Santos Basin. The results are expected to improve the modeling of depositional and diagenetic processes not only in the pre-salt system but also in other continental lacustrine successions globally.