Diagenetic Constraints and Impact of Silicification on the Quality of Pre‐Salt Reservoirs of the Sépia Field, Santos Basin, Brazil: Insights From Oxygen Isotopes and Fluid Inclusions

Abstract

ABSTRACT The conditions and role of silicification in the giant South Atlantic pre‐salt reservoirs have been widely debated recently. Many studies link silicification with degraded reservoir quality, whereas others associate it with increased permeability. Predicting the occurrence of silicification in the pre‐salt succession remains a significant challenge. To understand the conditions and distribution of silicification, a study was conducted on two wells at different structural settings in the Sépia Field, Santos Basin, integrating seismic interpretation, log analysis, systematic petrography, fluid inclusions (FIs), and secondary ion mass spectrometry (SIMS) oxygen isotopes. Eodiagenetic silica phases (microquartz, spherulitic chalcedony, quartz rims, and megaquartz) display heavier δ 18 O values (38–40‰ SMOW ) than the mesodiagenetic phases (megaquartz, chalcedony, and flamboyant quartz). The most intense silicification occurred near the main fault at the apex of the field structure, with mesodiagenetic phases displaying lighter δ 18 O values (megaquartz: 28–34‰ SMOW ) and FIs homogenization temperatures (Th) up to 200°C. At the structural flank, away from the main fault, magnesian clays are preserved, and silicification is spatially limited and restricted to eodiagenetic replacement. Early length‐slow chalcedony indicates precipitation in a saline and magnesium‐rich environment. Petrographic and geochemical evidence suggested that rocks originally rich in Mg‐clays were percolated by hydrothermal fluids, leading to selective silicification in the faulted apex of Sépia Field structure, converting originally non‐porous facies into permeable reservoirs. These results highlight the role of diagenetic processes in controlling the quality of some pre‐salt reservoirs and the importance of their understanding for decreasing exploration risks and optimizing production.