RESERVOIR QUALITY ASSESSMENT AND PETROFACIES OF THE LOWER CRETACEOUS SILICICLASTIC, CARBONATE AND HYBRID ARENITES FROM THE JEQUITINHONHA BASIN, EASTERN BRAZIL

A study of the rift to early drift phase, fluvial, deltaic and shallow-marine Lower Cretaceous (Upper Aptian to Lower Albian) siliciclastic sandstones, hybrid arenites and calcarenites from the Jequitinhonha Basin, eastern Brazilian margin, has shown that the distribution of diagenetic alterations and of related reservoir quality evolution can be constrained within a sequence stratigraphic framework. Description of cores, wireline logs, thin sections and petrophysical (porosity and permeability) analyses were integrated in order to investigate the controls on patterns of diagenesis in these rocks, and hence of reservoir quality distribution. The results of this study will be relevant to the exploration of rift and early drift successions in Atlantic-type passive margin settings. The study suggests that depositional facies and detrital composition impact the diagenetic evolution and the characterization and prediction of reservoir quality. Reservoir quality assessment was based on the concept of reservoir petrofacies, which is defined in terms of the main attributes affecting reservoir quality including depositional structures, textures, primary composition, diagenetic processes and products, and pore types. This paper demonstrates the practical use of the reservoir petrofacies concept in the evaluation of the controls on porosity and permeability, as well as on seismic and log signatures.