An operational classification system for the South Atlantic pre-salt rocks

Abstract

Abstract The giant Aptian reservoirs and associated lacustrine rocks of the South Atlantic pre-salt section present a series of unique characteristics, which make the systems created for marine carbonate rocks quite inadequate for their classification. Based on our experience with the characterization of thousands of samples of the pre-salt reservoirs and associated deposits from the Brazilian basins, and on a wide literature evaluation, we propose an objective and operational system for the classification of the unusual yet extremely important pre-salt lacustrine rocks. The system allows the coherent record of structure, fabric, primary texture and composition, and main diagenetic modifications of the in situ and resedimented rock types. The in situ rocks are directly classified according to the original proportion among calcite spherulites, fascicular shrubs, and mud matrix. The resedimented rocks are classified according to the original volume of > 2 mm particles and the proportion between sand and mud, using calcirudite, calcarenite, and calcilutite as unbiased names. Such an approach can also be applied to the classification of other particulate carbonate rocks, avoiding the conceptual problems and dubious interpretation of depositional environment and reservoir quality of currently used classification systems. The direct, objective, and intuitive proposed classification system shall contribute to the understanding, exploration, and production of the extraordinary South Atlantic pre-salt petroleum province.