Depositional and Diagenetic Controls on Reservoir Quality in Pre-Salt Carbonates

Abstract Reservoir quality in the Barra Velha Formation is controlled by the interaction between depositional facies, diagenetic processes, and mineralogical composition, resulting in marked vertical and lateral petrophysical heterogeneity. This study evaluates these controls in the production intervals of Barra Velha Formation on the Tupi Field, Santos Basin, Brazil. Flow units (FUs) were defined from effective porosity and horizontal permeability using the Flow Zone Indicator (FZI) and cumulative S-curve approach, and subsequently integrated with sedimentological, mineralogical, well-log, and seismic data. Four FUs were identified, ranging from low-quality FU1 (log(FZI) < −0.1) to highly connected FU4 (log(FZI) > 0.7). FU4 is predominantly associated with reworked, calcite-rich shrubstone facies and contains >80% calcite, whereas FU1 shows greater mineralogical variability, including dolomite, quartz, and stevensite-bearing intervals. Stevensite-rich intervals are characterized by restricted permeability and an anomalous elastic response, with relatively low acoustic impedance despite poor reservoir quality. The distribution of FUs also varies markedly among reservoir intervals: BVE100 is dominated by FU2 (57%), BVE200 by FU1 (73%), whereas BVE300 contains mainly FU2 (48%) and FU1 (40%). These relationships were propagated away from well control through probabilistic pre-stack geostatistical seismic inversion. One hundred stochastic realizations were generated for each reservoir interval, and validation at wells excluded from the inversion showed differences of less than 10% between observed and simulated FU proportions. Effective porosity was co-simulated from elastic properties and FU distributions, whereas horizontal permeability was modeled from FU-dependent porosity–permeability relationships because of its weak correlation with seismic elastic properties. The results show that reservoir quality in the Barra Velha carbonates is primarily controlled by depositional and diagenetic heterogeneity and that mineralogical effects, particularly stevensite occurrence, can decouple elastic response from hydraulic quality. Accounting for these controls is therefore required to predict the three-dimensional distribution of reservoir quality beyond well control.