Distribution of intraclastic deposits in the Barra Velha Formation, Santos Basin pre-salt: implications for the reworking processes

The pre-salt Aptian reservoirs are responsible for close to 80% of Brazil oil production. The unusual in situ deposits, constituted by magnesian clays, calcite spherulites and fascicular shrubs, were precipitated in a huge alkaline lacustrine system before the formation of the South Atlantic. The processes that generated the associated resedimented deposits are comparatively still poorly understood, although they correspond to important reservoirs in many pre-salt fields in the Santos, Campos and Kwanza basins. Systematic core and petrographical descriptions have provided detailed textural and compositional characterization of resedimented pre-salt deposits from the Santos Basin. They are composed mainly of carbonate intraclasts eroded from the in situ deposits. The predominant massive structure, widespread spatial distribution and lack of subaerial exposure indicate that gravitational flows, waves or surface currents cannot be the main depositional processes. Internal waves produced by perturbation of the chemocline in the stratified lacustrine system are considered able to generate the observed subtle, recurrent and widespread intercalation of resedimented and in situ deposits. The construction of realistic depositional models for the significant occurrence of these deposits in the pre-salt system will help to minimize the exploration risks and optimize the hydrocarbon recovery efficiency of the producing fields.