Syngenetic, Diagenetic and Hydrothermal Processes in the Pre-Salt Sag Section of Santos and Campos Basins
Luiz Fernando De Ros
Abstract
Summary The Pre-salt reservoirs of the sag section from the Santos and Campos basins are the product of a vast lacustrine system, formed during the early South Atlantic opening. The in situ deposits were formed by the syngenetic deposition of stevensite and similar magnesian phyllosilicates, which were replaced, displaced and covered by calcite spherulites and fascicular shrubs during early diagenesis. Frequent, high-frequency compositional changes, which may have been related to lake waters stratification, promoted the alternation of levels with predominance of calcite shrubs, spherulites, or of stevensite matrix, as well as its early diagenetic dissolution and replacement by silica, dolomite, magnesite or calcite. The flow of burial diagenetic and hydrothermal fluids through porous layers, faults and fracture zones caused the precipitation of quartz, chalcedony, dolomite, calcite, barite/celestite, pyrite and other sulphides in some areas, but also significant dissolution and porosity enhancement, responsible for the exceptional performance of some reservoirs. Although for the recent advances in the understanding of the origin and evolution of the South Atlantic Pre-salt reservoirs, the sources for the huge volume of Ca, Mg and Si precipitated in the Pre-salt sag succession, remain to the determined.