Impacts of diagenetic processes on petrophysical characteristics of the Aptian presalt carbonates of the Santos Basin, Brazil

Abstract

The presalt carbonate reservoirs located at the marginal basins of Brazil are one the most important hydrocarbon provinces worldwide. These reservoirs are responsible for approximately 75% of the Brazilian offshore oil production. Despite the presalt reservoirs’ present good petrophysical qualities (porosity >15% and permeability >100 md), there are still challenges related to the lack of understanding of the petrophysical controls resulting from a complex depositional and diagenetic history. To address such problems, an overall evaluation of the carbonate reservoir was provided on the Aptian Barra Velha Formation in the Sapinhoá field, Santos Basin. This research used an extensive data set of well logs, petrophysics, and x-ray diffraction, which identified facies heterogeneities, variated petrophysical distribution, and five hydraulic flow units. Overall, the best petrophysical intervals with highest porosity and permeability are found in the wells located at the structural high comprising the flow units 4 and 5 and mostly consisting of shrub and grain-supported facies followed by an intermediary flow unit 3 found in all wells. Moreover, among all units, flow units 1 and 2 presented the lowest petrophysical values, mainly found at the basinward wells. Finally, results indicate that key diagenetic features such as dissolution of clay minerals, dissolution of calcite fabric, and dolomitization processes are notable elements that commonly enhanced petrophysical properties. Additionally, the pervasive silicification process decreases the reservoir quality. These processes are commonly found in the wells located in the structural high and basinward areas of the field. Lastly, a multiscale characterization allows a broad comprehension of the key diagenetic impacts into carbonates’ petrophysical properties.