Controls of clay and carbonate syngenesis and diagenesis on the porosity evolution of Santos Basin Aptian pre-salt reservoirs

The Aptian Barra Velha Formation lacustrine deposits from the Santos Basin answer for almost 80% of Brazil's total hydrocarbon production. Despite recent advances, models for their origin and evolution are still incipient, and previously proposed depositional systems fail to explain their characteristic high frequency lithotype intercalation. A detailed study of 7 wells from 3 Santos Basin oilfields integrating core and petrographic descriptions with XRD, RCAL, and micro-CT analyses allowed to understand the origin and evolution of in situ rocks, their pore systems, and the main controls on pore distribution and geometry. Chemocline oscillations in a stratified alkaline lake are evoked to explain the high-frequency lithotype intercalations. Periods of shallower chemocline favoured Mg-clays deposition, while deeper chemocline favoured carbonate precipitation. Twenty-two in situ reservoir petrofacies were defined, showing that magnesian clays matrix dissolution was their main porosity generating process . Consequently, matrix replacement by less soluble dolomite or silica reduced the potential for porosity generation. Early congruent matrix dissolution created excellent reservoirs, even in matrix-rich rocks. Shrub-dominated rocks with large pores and greater vertical connectivity constitute the best reservoirs. Reservoir petrofacies analysis showed limited control of dolomite or silica amount on reservoir quality, which also depends on calcite aggregate size, type, distribution, and coalescence degree. Therefore, in situ pre-salt deposits reservoir quality is mostly controlled by calcite framework distribution, modification type and degree, and matrix dissolution timing. Understanding the depositional and diagenetic processes is crucial for reducing exploration risks and optimizing production of these reservoirs.