Evidence From Microscopy and U–Pb Geochronology as a Clue to the Influence of the Cretaceous Magmatism in the Diagenesis of Pre‐Salt Carbonate Reservoirs in the Santos Basin (Brazil)

Analyses of U–Pb isotopic geochemistry, optical, and scanning electron microscopy with mapping of chemical elements and fluid inclusions in pre‐salt limestone samples from Sururu and Jupiter Field wells in the Santos Basin, confirm that at ages 121 Ma, 119 Ma, 113 Ma, and 95 Ma there was CO 2 emplacement during the formation of U‐bearing calcites. The CO 2 would have been released from the crystallised magma, represented in the Sururu Field by tabular intrusive sills, and migrated through faults to the carbonate mounds. The mixing of the hot magmatic CO 2 fluid inclusions and colder lake aqueous fluid inclusions with evaporitic carbonates, as well as the high permeability of the limestone, provided thermal convection, an essential process for the formation of U contained in the calcites. The circulation of the magmatic fluid provided sufficient reducing agents to mix with a U‐rich fluid already existing in the lacustrine system for U mineralisation to occur. The high temperature caused by the sill intrusion will have relocated the U initially complexed with organic matter in the shales.