Caracterização faciológica, estratigráfica e diagenética dos reservatórios carbonáticos da Formação Barra Velha, Bacia de Santos

Abstract

The Brazilian marginal basins have extensive lacustrine deposits of apatian age, known as the Pre-Salt section, where large oil accumulations were discovered since 2006. The largest volumes occur in the Barra Velha Formation, in Santos Basin. Detailed sedimentary, stratigraphic and diagenetic studies of these reservoirs are of crucial importance, for they provide subsidies for the exploration of new areas, and enable the production development in the already delimited oil fields. In this sense, the aim of this dissertation consisted in the facies, stratigraphic and diagenetic characterization of a carbonate reservoir of the Barra Velha Formation, and the incorporation of the results in a 3D numerical model. To do so, 265 meters of core, 331 sidewall cores and 221 thin sections from four wells were studied. In addition, cathodoluminescence, scanning electron microscopy, x-ray diffractometry and electron microprobe chemical analyzes were performed in selected samples. As a result of the facies analysis, 15 sedimentary facies were identified. By observing the lithological characteristics of each facies, together with their transition pattern, it was possible to establish an ideal vertical stacking of facies, which is mainly controlled by the lake's water balance (dry and humid periods). Through this pattern, and with the analysis of seismic sections and well logs, a stratigraphic correlation was made between the four wells studied, from which an interval was selected for the diagenetic characterization. This study identified that the main diagenetic minerals are, in order of abundance, dolomite, silica minerals and calcite. These minerals are mainly eodiagenetic and occur in a great diversity of forms that represent different diagenetic phases. The carbonate minerals also have differences in chemical composition. The facies characterization and the systematic quantitative description of minerals and diagenetic processes allowed the numerical modeling of the distribution of sedimentary facies and diagenetic products. With the combination of these models, a reservoir quality numerical model was obtained, which turned to be coherent with the permeability log calculated from the nuclear magnetic resonance log. Thus, it can be concluded that the diagenetic analysis was effective in identifying intervals with better permo-porous quality in the reservoir.