A Geochemical Model for the Formation of the Pre-Salt Reservoirs, Santos Basin, Brazil: Implications for Understanding Reservoir Distribution

Abstract

An evaporitic geochemical model is presented for the Aptian Barra Velha Fm reservoirs, Santos Basin, offshore Brazil. This has to explain the paucity of “normal” carbonate precipitates, the dominance of unusual calcitic morphologies and the presence or former presence of Mg-silicate matrices in the reservoir. The occurrence of metre-scale cyclothems with repetitive mineralogical, textural, micro-textural and early diagenetic changes is explained by invoking the evolution of highly alkaline lake waters with a high dissolved silica, Mg and Ca content. Several modern lakes provide partial indicators of how the evaporation of the Barra Velha lakes would have progressed in stages. Initially Mg-silicate precipitation, as a hydrous poorly crystalline gel, triggered by evaporation, produced a “chemical divide”. This resulted in the near complete depletion of Mg, but allowed evaporative concentration of residual SiO2(aq), influencing the subsequent stages of brine development and sedimentation. The depletion of Mg limited the production of Mg-silicates but lowered the Mg/Ca favoring LMC precipitation, as is observed in some lakes such as Lake Chad. The nucleation of LMC within the gels led to the growth of spherulites. CO₂ degassing, driven by solubility decreases with evaporation, drove up pH and CO32- but coupled with low Ca2+, still favored CaCO₃ crystal growth, the kinetics of which were inhibited at very high pH. This favored an increase in crystal size over new nuclei, explaining the lack of other precipitates (e.g micrite). The spherulites grew to large size and extended beyond their gel matrix producing crystal shrub cementstones with dendritic crystals reflecting high SiO2 (aq) in high pH fluids. The shrub units have the best reservoir quality. Continued evaporation led to increased levels of SiO (aq) but pluvial events caused a drop in the pH led to the growth of interstitial silica. This model predicts that even though the more porous shrub facies represents the shallowest facies, it was not restricted to the more proximal settings but to areas where lake waters ponded during evaporation, such as depositional lows such in the higher accommodation zones of half-grabens. Variations in the initial composition of the lake waters over the South Atlantic basins due to differences in catchment geology and hydrology would have resulted in significant differences in the final assemblage of carbonates and silicates.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016