Flow units in complex carbonate reservoirs: A study case of the Brazilian pre-salt

Flow units (FUs) have been widely used for rock typing in many reservoirs, especially where there is no direct correlation between petrophysical properties and geological characteristics. This scenario is extremely common in carbonate rocks that are likely to be shaped and modified by pedo-diagenetic processes forming heterogeneous reservoirs with geological and petrophysical properties varying in all scales. The present work aimed to test different well-established petrophysical rock-typing methods in complex carbonate reservoirs and integrate them with geological-driven rock classifications. The goal was to find a correlation between facies, diagenesis and the quality of the reservoir as well as understand how different rock-typing methods behave in complex reservoirs. This study was implemented on the carbonates of the Barra Velha Formation which are known for their complexity regarding facies, diagenetic features, and petrophysics. The study was performed in two oilfields of Santos Basin (Brazil) and employed data from 44 wells, including routine core analyses and well logs, along with core and thin section high-resolution images. As a result, 13 facies were identified and grouped into six facies associations representing different zones of a lacustrine environment: Deep Lake, Lower Shoreface, Upper Shoreface, Lower Foreshore, Upper Foreshore, and Backshore. Three rock-typing methods were tested for the flow units identification: FZI, Winland R35 and Aguilera R35. The Barra Velha Formation was subdivided into 4 flow units. All methods reveal that the Deep Lake and Lower Shoreface Facies Associations comprise the poorer reservoirs of the Barra Velha Formation and that the Lower and Upper Foreshore Facies Associations concentrate the best reservoirs. Shrubstones were the facies with the highest R35s and FZIs values followed by the grainstones. The FZI methodology was the most optimistic and Winland R35 the least. Aguilera R35 presented the highest correlation with permeability, proving to be a reliable method for application in complex carbonate reservoirs. The approach and equations obtained in the Pre-salt reservoirs can be employed for different oilfields. Therefore, for a more accurate result, we recommend the calculation of the constants using data from the studied field. •Barra Velha Formation comprises in situ, reworked, and altered facies that were subdivided into 6 facies associations.•The formation was subdivided into 4 flow units based in 3 di