Diagenetic evolution and reservoir quality of Aptian lacustrine reworked carbonates from the Santos Basin, Brazilian pre-salt

Pre-salt carbonate reservoirs of the Santos Basin, eastern Brazilian margin, contain giant hydrocarbon accumulations, with reworked deposits constituting the highest-quality reservoirs across several oilfields. Despite their importance, the diagenetic evolution of the lacustrine reworked deposits remains poorly understood. This study investigates the diagenetic patterns and controls on reservoir quality of the reworked deposits of the Aptian Barra Velha Formation through integrated petrography, fluid inclusions, petrophysical, and micro-computed tomography (µCT) analyses. Reservoir petrofacies were characterized according to the main controls on porosity evolution, which comprised: dolomitization, silicification, and compaction. Dolomite and silica precipitation took place in a multi-phase scheme, as evidenced by paragenetic relations, and by fluid inclusions data showing heterogeneous entrapment with wide homogenization temperature ranges. Blocky dolomite precipitated both before and after compaction, while late saddle dolomite formed during deeper burial. At least two eodiagenetic silicification phases occurred, followed by late diagenetic coarse quartz precipitation. The relative timing between compaction and eodiagenetic dolomite, silica and calcite precipitation fundamentally controls reservoir quality in the reworked rocks. Mesodiagenetic and hydrothermal processes had subordinate influence on porosity evolution. Reworked rocks show greater susceptibility to compaction than in situ rocks, owing to the more stable crystalline framework of the latter. This study provided essential insights to improve the reservoir characterization and optimize the development strategies for the pre-salt accumulations, as well as for understanding the diagenetic evolution of intraclastic lacustrine carbonates worldwide.