Caracterização sedimentológica e estratigráfica dos esferulitos da Formação Barra Velha da Bacia de Santos, Brasil

Abstract

The Brazilian marginal basins became prominent worldwide in 2006 when extensive hydrocarbon accumulations were discovered in the Pre-Salt section of Santos Basin. It is characterized by deposits of carbonate rocks deposited during the Aptian. The largest hydrocarbon volume occurs in the Barra Velha Formation (BVF), which is composed mainly by laminites, spherulitites, shrub limestone, intraclastic grainstone and rudstone. Spherulitites has increasingly gained importance due to their expressive occurrence in the BVF (although uncommon in carbonates wordwide), sometimes as important reservoir rock, and for this reason they were studied in detail in this work. Two wells located in different structural contexts in an oil field on Santos Basin were chosen for this study. Variations in texture, morphology, mineralogy, diagenesis and porosity in spherulitic facies were observed through macroscopic description of cores, petrography and the following complementary analysis: X-ray diffratometry, mineralogical map using scanning electron microscopy, punctual chemical analysis using electron microprobe, cathodoluminescence, capillary pressure by mercury injection, and porosity and permeability measurements. Different types of arrangement, morphology, and internal texture of calcitic sherulites were determined. The BVF spherulites present maltese cross extinction pattern or an irregular one, and they can have inclusions (siliciclastic grains, peloids, fossils) or not. Regarding to the size, spherulites were classified from very small to very large. Concerning to the external morphology they were classified as: round and concentric, subrounded or ovoid, polygonal, composite or coalescent, and other morphologies. The spherulites can have different spatial arrangements, from open to very tight. The material between the spherulites most commonly are dolomite, silica and syngenetic Mg-clay minerals. Chemical analyzes in the spherulites showed an increase in Mg content toward the top of the BVF, probably due to the progressive concentration of this element in an increasingly evaporitic environment. A paragenetic sequence of the spherulitites in the BVF was also determine, being the Mg-clay mineral the first phase to form followed by the calcitic spherulite. Dolomite, silica, dawsonite and other mineral phases occurred during eodiagenesis. Dissolution of the Mg-clay mineral between the spherulites during eodiagenesis would be responsible for the generation of secondary porosity. Spherulities very often presents porosity, mainly interspherulite and intercrystalline. Porosity values of most samples are between 5 and 15%, and permeability values are between 0.001 and 500 mD, but can reach values higher than 1000 mD. Three groups of spherulitites were determined based on its petrophysical characteristics: low, intermediate and high reservoir quality. Comparing the two wells, it was noticed that in the one located in the higher structure the spherulitites presents tighter arrangement, bigger spherulites with composite or coalescent morphology, and lack of Mg-clay minerals. Vertical facies stacking show less transitional pattern than the one in the well located in the structural low. Concerning to reservoir quality, the spherulitites in the structural high tend to be better due to the absence of clay minerals