Since the discovery of giant Aptian pre-salt reservoirs in Brazilian margin basins, the study of lacustrine carbonates has drawn great attention from the scientific community. Comparatively, minor attention was given to the characterization and genesis of the Mg-silicates (e.g., stevensite, kerolite) which are commonly associated with these carbonates. A systematic petrological study was performed in the Aptian Barra Velha Formation (BVF) within distinct structural compartments of the giant Lula Field in the Santos Basin, in order to recognize the patterns of primary formation and diagenetic alteration of these Mg-silicates. Mg-silicates occur as peloids, ooids, intraclasts, and fine-grained laminated deposits, either mixed in variable proportions with other particles, such as carbonate bioclasts and volcanic rock fragments, or constituting specific intrabasinal deposits. In the BVF interval, clay peloids and laminated deposits are associated with spherulitic and fascicular calcite aggregates, as substrate and hosts for these precipitates. Ooids are interpreted as formed at the sediment–water interface by the nucleation of concentric envelopes on the surface of particles (heterogeneous nucleation), through repeated rolling under gentle wave and current action. Laminated deposits, interpreted as precipitated directly from the water column (homogeneous nucleation) in highly supersaturated and low-hydrodynamic-energy environments, constitute extensive deposits in the BVF. Peloids were probably formed in intermediate energy conditions. Some ooidal arenites show porosity from the dehydration and contraction, and/or the dissolution of ooids. In some rocks, these pores are filled with fibrous calcite, while the remaining Mg-silicates are replaced by dolomite, calcite, or silica. A similar diagenetic pattern occurs in the laminated deposits, where magnesite and dolomite fill shrinkage pores formed along their characteristic wavy laminae. Owing to their elevated solubility, most of the Mg-silicates were dissolved, or intensely replaced by calcite, dolomite, or silica. The detailed petrologic analysis indicates that the original volumes of Mg-silicates were substantially larger, and that their deposition was widespread in the basin, including on structurally high areas. The types and intensity of diagenetic alteration of the Mg-silicate deposits are distinct for each structural compartment, being more intense towards the highs and closer to the overlying evaporites, which imposed a strong influence on reservoir quality.

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