The role of compaction in the diagenetic evolution of Pre-Salt Aptian deposits of Santos Basin, Brazil

Abstract

The discovery of huge oil accumulations in the South Atlantic Pre-Salt carbonate lacustrine deposits has attracted much exploration and research interest on these challenging reservoirs, but no work has focused on the compaction processes. A petrological and geochemical study was therefore performed on two cored wells in Santos Basin to identify the main syngenetic and diagenetic constituents, and to identify the compaction features that affected these deposits. They are mainly composed of Mg-clays, calcite spherulites and fascicular shrubs, which have undergone a heterogeneous diagenetic evolution. Mechanical compaction occurred during shallow burial, promoting a closer packing of the constituents, reorientation and fracturing of calcite spherulites and bioclasts, and deformation of the clay matrix. Pressure dissolution occurred under increasing pressure and temperature during burial, developing inter-aggregate sutured contacts, dissolution seams and stylolites, as consequence of overburden. Macrocrystalline quartz and calcite, drusy quartz, and saddle dolomite precipitated during burial diagenesis, related to basinal fluids and probably to pressure dissolution, as source for the late carbonate phases. Understanding the role of compaction is essential to evaluate trends of porosity destruction, thickness modification and mass transfer, which directly impacted the quality and the dynamic evolution of fluids in the deep Pre-Salt reservoirs.