Petrographic Characterization and Diagenetic Evolution of a Carbonate Interval of the Barra Velha Formation, Santos Basin, Brazil

The pre-salt succession contains some of the most important petroleum discoveries from this century. The main reservoirs belong to the Aptian Barra Velha Formation (BVF) from the Santos Basin, which was formed in an alkaline lacustrine environment. Owing to the unusual depositional conditions, extreme heterogeneity and complex diagenetic history, the relationships among diagenetic processes and their impact on porosity and permeability of the BVF reservoirs remain poorly understood. This study aims to unravel how the diagenetic processes and products have affected the quality of BVF reservoirs, through a combination of detailed petrographic, petrophysical and core description, cathodoluminescence, UV epifluorescence, and QEMSCAN analyses. Based on petrographic observations, 10 distinct reservoir petrofacies were identified and characterized. Matrix dissolution played a major role in porosity generation, particularly in in situ deposits, where unstable magnesian clays were dissolved or replaced by silica and dolomite. Silicification had complex effects on reservoir quality, either increasing porosity, where associated with dissolution, or reducing it through extensive cementation, especially in the resedimented deposits. Dolomitization also affected the reservoirs, either due to enhanced permeability through intercrystalline porosity, or to porosity obliteration. Late-stage processes, probably hydrothermal origin, produced minor impact on the analysed reservoirs. Understanding the heterogeneous diagenetic processes is key to improve pre-salt reservoirs characterization and to optimize hydrocarbon exploration and production.