Porosity evolution and geometry in Santos Basin Aptian presalt petrofacies

ABSTRACT The presalt deposits from offshore southeastern Brazil account for approximately 3/4 of the total hydrocarbon production of the country. Consequently, they are the subject of various studies aimed at better understanding the primary and diagenetic controls on their reservoir quality. The Barra Velha Formation (Aptian, Santos Basin) constitutes the main reservoirs of the presalt sag section, essentially composed of magnesian clays, calcite spherulites and fascicular shrubs, and intraclasts reworked from these aggregates. The pore systems of these rocks are highly complex, owing to depositional and diagenetic controls. Therefore, the origin and distribution of porosity and permeability are difficult to understand. To better understand and characterize the pore systems of the unusual presalt reservoirs, this study aimed to recognize the relationships among their porosity and permeability values and pore types within the context of the evolution and geometry of their pore systems. X-ray microtomography scanning of 251 samples from 3 wells was performed to obtain the three-dimensional (3-D) porosity distribution, and the main pore types were described in detail in 583 thin sections. Thirteen petrofacies were defined for the studied samples, and the 3-D pore network was reconstructed for characteristic samples of each petrofacies. Image segmentation was applied to quantify pore sizes and shapes, as well as to visualize the connections between them. Petrofacies with low quality or considered nonreservoirs correspond to rocks in which the magnesian clay matrix was partially replaced by dolomite or for which dolomite or silica filled interparticle pores where pores were generated by matrix dissolution, leading to poorly connected vuggy pores. High-quality reservoirs, however, correspond to in situ rocks with porosity generation occurring mainly through widespread dissolution of the Mg-clay matrix, further enhanced by the dissolution of calcite spherulites and shrubs. They also correspond to reworked rocks with high interparticle primary porosity and crystalline rocks formed by pervasive dolomite replacement followed by dissolution, creating intercrystalline porosity. Dissolution played a significant role in increasing not only porosity but also pore connectivity and pore size, as observed through pore network segmentation tools and through the Area 3-D, EqDiameter, and ShapeVA3D attributes. Increased understanding of the presalt reservoir porosity patterns is important not only for exploring for new accumulations but also for optimizing the recovery from currently producing reservoirs.