Evolution of diagenesis and quality of pre-salt resedimented deposits from the Búzios Field, Santos Basin, Brazil

ABSTRACT The South Atlantic Aptian pre-salt carbonates are the most productive lacustrine reservoirs in the world because of their great thickness, extensive distribution, and excellent quality, especially in the Búzios Field of the Santos Basin. Pre-salt resedimented deposits were formed by the erosion and redeposition of in situ lacustrine carbonate deposits, composed of fascicular and spherulitic calcite aggregates, magnesian clay, ooids, and peloids. The results show that compaction is the dominant process reducing primary porosity. Diagenetic modification of the pore network is controlled mainly by dolomite and silica cementation. Blocky dolomite commonly replaces calcite allochems and fills interparticle pores, whereas saddle dolomite preferentially fills vuggy pore space. Silica is present first as chalcedony rims and replacive fabrics and later as macrocrystalline quartz filling the remaining pore volume. The best-preserved reservoir intervals are those less affected by compaction and cementation, where porosity reaches values up to 30%, and NMR data indicate laterally continuous and well-connected pore systems. Among all diagenetic precipitates, silica cementation has the most severe impact on pore connectivity and reservoir quality, followed by blocky dolomite. These findings demonstrate that understanding the controls, distribution, and products of diagenesis in resedimented pre-salt carbonates is essential for reservoir-quality prediction in Búzios and provides a basis for interpreting similar alkaline lacustrine carbonate systems.