Caracterização tectono-estratigráfica e faciológica dos reservatórios carbonáticos do pré-sal do campo de Búzios, Bacia de Santos, com base em interpretação sísmica, perfis de poços, análise de testemunho e amostras laterais

Abstract

The exploration of pre-salt in recent decades has brought worldwide attention to Brazil, due to discoveries of giant fields. These Barremian-Aptiano-age carbonate reservoirs are associated with the Barra Velha Formation (sferulites, shrubs, laminites of the Post-Rift sequences) and the Itapema Formation (coquinas of the Upper Rift sequence). The Búzios field is a giant field, with an estimated volume of oil in place (29 billion barrels) and is the second largest production field in Brazil (758,000 boe/d). This work proposes a tectono-stratigraphic characterization for pre-salt reservoirs of the Búzios field, based on seismic interpretation, well correlation, and rock data. The seismic interpretation revealed rift configuration with horts, grabens and hemigrabens structures, extremely faulted (N30W and N30E) with a complex transfer zone. The Rift sequence can be divided into Lower and Upper (Coquina`s interval) and post-rift (Barra Velha Fm.). The Upper Rift reaches thicknesses close to 800m and presents less extensive faults and wedge geometries than the Lower Rift, indicating a decrease in subsidence due to the expansion of the rift. The Post-Rift has a thickness ranging from 200 to 400 m in the highs and a maximum thickness of 850 m in the flanks of the structures. Nine facies were defined on Itapema Fm., the two main (67%) Coquina Rudstone / Grainstone reworked or not. Three facies associations were defined based on the energy of the system. Association A (Mudstone / Packstone, Mudstone / Packstone bioclastic, Wackestone bioclastic) low energy deposits (distal), Association B (Espatito, Coquina Packstone / Floatstone reworked and Coquina Packstone / Floatstone), moderate energy environment (sublittoral) and association C (Grainstone oolitic, Coquina Rudstone / Grainstone and Coquina Rudstone / Grainstone reworked), high energy environment (littoral). The facies association based on permability and porosity properties was not possible due to an extensive diagenetic alteration. The ciclity study of the Itapema interval shows different cycles in a different frequency and shows the increase of high energy facies to the top. A total of 10 facies were defined for Barra Velha, and four facies associations were proposed based on the energy and geochemical conditions of the lake: association E (Laminito, Laminito Esferulito Wackestone / Packstone, Esferulito Wackestone) distal environment, increasing alkalinity; Association F (Spherulite Packstone / Grainstone, Spherulite Packstone / Grainstone reworked) sublittoral with high alkalinity and high Mg; Association G (Shrub Packstone / Grainstone / Rudstone reworked, Shrub Esferulito Floatstone / Rudstone, Shrub Grainstone / Rudstone), littoral with high salinity and alkalinity, high Ca, and low Mg; and association H (Chert and Nodular Limestone) palustrine. Due diagenetic alteration was not possible to group the facies according to the reservoir properties. The study of cyclicity on Barra Velha Fm. allowed us to divide cycles in different orders showing the tendency to be shallow upward and increasing the alkalinity of the lake along the sequence. Several evidences indicate that the lake was undergoing a process of become shallowest from a Fm. Itapema and continuing along the Barra Velha Fm, making the water more alkaline and due not only tectonic influence but also from the climate changes. Keywords: Búzios field. Pre-salt. Facies.