Análise estratigráfica de alta resolução aplicada aos depósitos da Formação Barra Velha, Bacia de Santos: identificação, correlação e mecanismos de controle de ciclos sedimentares

Abstract

The Santos Basin, located in the continental margin next to the Southeastern Region of Brazil, evolved from the break-up of Gondwana supercontinent, initiated on the Early Cretaceous. It s sedimentary record can be divided in three supersequences, associated with the main tectonic phases of the basin: Rift, Post-Rift and Drift. In the last decade, the discoveries of gigantic hydrocarbon accumulations in the so-called "pre-salt" section of this basin, corresponding to its Rift and Post-Rift sequences, promoted the development of academic studies aiming the understanding of the genesis of this stratigraphic interval. In this context, the Barra Velha Formation, Aptian in age, constitutes its main reservoir, and was deposited under a tectonic regime of limited fault activity, with a dominant thermal subsidence (Post-Rift phase). Their rocks were deposited in high saline and alkaline lacustrine settings, under the influence of volcanic terrains, which allowed the development of syngenetic chemio and biogenic elements such as magnesian clays, calcitic spherulites and calcitic shrubs. The lacustrine dynamics, conditioned by climatic oscillations associated to astronomical cycles, controlled the organization of these deposits. Facies rich in calcitic shrubs are related to dry periods where the progressive shallowing and the increase in the alkalinity of the lake were expressive. Through the application of high resolution stratigraphic analysis, elementary sequences, identified with the support of quantitative approach of Markov Chain analysis, were grouped in higher hierarchy small, medium and large scale sequences and had their mappabilitty evaluated. Fischer-plots were used to support this analysis. The elementary sequences, given their very high frequence nature and decimetric scale, have limited and non-operational mappability and are affected by autocyclic factors. Small scale sequences, usually metric, were correlated using rock data over tens of kilometers in the studied area. Medium-scale, usually decametric, and large-scale sequences, with more than a hundred meters, have operational mappability, defined also by means of electric logs, throughout the whole studied area. Concluding, the application of high resolution stratigraphy, combined with quantitative techniques, allowed the understanding of the depositional controls and the evolution of this relevant stratigraphic interval, which makes it a powerful tool for faciological and diagenetic prone surfaces prediction, with immediate applicability for the characterization of the "pre-salt" reservoirs.