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International Conference & Exhibition

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New Developments in the Understanding of Pre-Salt Reservoirs (Offshore Brazil)

Abstract

The pre-salt carbonate reservoirs in the offshore Campos, Santos and Espírito Santo have played a crucial role over the last years. Until 2013, 23 discoveries have been reported in the pre-salt play, with Lula field ranking number one (9,944 MMBO), followed by Libra and Franco discoveries. Despite the high exploratory success during the early stages of exploration, wells results from the last five years have shown a high spatial variability of outcomes as per reservoir presence and performance. In order to properly understand the distribution and ranges of variety of petrophysical properties of these extraordinary reservoirs a thorough evaluation of 41 wells has been conducted for the three basins. The interpretation, mainly based on Nuclear Magnetic Resonance, was done in the main reservoir lithologies: the microbial carbonates and the Coquinas. The pre-salt carbonate reservoirs present an heterogeneous variation that can range from complete absence, to thick packages of carbonate sections, interbedded sequences of carbonates and volcanics and, in some instances, completely replaced by volcanic rocks. Moreover, diagenetic processes, such as hydrothermalism, have also significantly obliterated the initial rock matrix. According with the data analyzed, an important lithological boundary has been observed between the Espírito Santo and Campos basins. The Espírito Santo basin lacks reservoir facies and on overall these are better developed in the Santos basin. Extensive extrusive volcanism also leads to replacement of carbonate facies by non-reservoir volcanic facies in the boundary between Campos and Santos. Properties wise the Coquinas lithology presents similar average porosities to the microbialites (around 10%) but with better Net to Gross averages (64% in the coquinas against 55% in the microbialites). Also, oil saturation is higher in the Coquinas than in the microbialites (90% and 75%, respectively). There is a reasonable trend for the porosity-permeability relationship although with some dispersion. A better representation of this relationship can be obtained by analyzing the effect of porosity types as well as lithological facies and diagenesis. The overall results have been applied in the definition of a depositional and reservoir model for the pre-salt, also used to assist the decision making process in portfolio management and future exploration strategies.