Isotopic evidence for predominantly abiotic carbonate precipitation in the pre-salt Barra Velha Formation, Santos Basin, Brazil

This study provides an integrated isotopic characterization and interpretation of carbonate phases in the Barra Velha Formation (BVF) of the Santos Basin, a key interval within the Brazilian pre-salt reservoirs. By combining petrography, stable and radiogenic isotope analyses (δ 13 C, δ 18 O, 87 Sr/ 86 Sr), we investigated carbonates from seven wells across three oil fields. The data reveal that various calcite and dolomite phases in the BVF exhibit carbon isotopic compositions (δ 13 C V-PDB) between -1.15‰ and +4.65‰, supporting an abiotic origin for carbonate precipitation, likely driven by CO 2 degassing in a hyper-alkaline lacustrine environment, intrinsically associated to rift-related processes associated with the opening of the South Atlantic. Strontium isotope ratios ( 87 Sr/ 86 Sr) ranging from 0.711 to 0.714 suggest significant crustal input, without marine contribution. The isotopic compositions are consistent across carbonate phases and oilfields, indicating stable geochemical conditions in the South Atlantic pre-salt lacustrine system. These findings reinforce the interpretation of predominantly abiotic carbonate precipitation, ruling out a dominant microbial control, and support deposition within a large, hydrologically interconnected lacustrine system, providing critical constraints for reservoir characterization and exploration models in the Brazilian offshore pre-salt province. • Isotopic data reveal homogeneous δ 13 C–δ 18 O– 87 Sr/ 86 Sr across Santos Basin wells. • Radiogenic 87 Sr/ 86 Sr (∼0.713) denotes crustal–hydrothermal sourcing, no marine input. • Calcite and dolomite precipitated under stable alkaline lacustrine–rift settings. • Isotopic uniformity indicates a hydrologically connected pre-salt lake system. • Evidence supports abiotic carbonate formation in Aptian pre-salt lacustrine systems.