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We discuss the petrophysical challenges for interpretation of presalt carbonate rocks that constitute the main reservoirs in the recently discovered hydrocarbon accumulations in the Santos Basin. These rocks are described as microbialites and have become one of the main exploratory targets in several basins worldwide, resulting in giant discoveries in the South Atlantic (Brazil, Angola) and in the Caspian Sea (Kazachstan). The origin of these rocks is still controversial, and several carbonate rock outcrops have been proposed as possible analogs for the presalt microbialites, including stromatolites, travertine deposits, and carbonate tufas. The distribution of porosity and permeability in the microbialite reservoirs are important parameters for building up reservoir models and optimization of the hydrocarbon production strategy. Petrophysical parameters may also be obtained by reconstruction of microtomography binary images obtained from reservoirs plugs. The quantitative analysis is based on an innovative numerical simulation software that uses a pore network algorithm. This tool is under development and aims at providing a 3-D visualization of the rock framework and yielding reliable evaluations of porosity and permeability factors that can be compared with traditional laboratory measurements. Introduction In the 2000’s the largest petroleum accumulations discovered in the world are concentrated in the region north of the Arabian Peninsula, particularly in the northern Caspian Sea, and in the salt basins of the South Atlantic (Brazil and Angola). The Kashagan Complex in Kazachstan and the presalt oil fields in the Santos Basin (Lula, Libra, Franco, etc.) and Kwanza Basin (Caimea, Lontra, Bicuar) are all characterized by carbonate reservoirs with evaporites as the top seal (Mohriak, 2015). The productivity of some of these wells is extremely high, reaching more than 20,000 bbl/d per well. However, the production forecast for this type of reservoir is challenging because of the complex and heterogeneous network of pores and flow regimes. The permeability distribution in these carbonate reservoirs is extremely variable depending on the different lithologic facies, which includes grainstones, boundstones, calcilutites, carbonate breccias, microbialites, coquinas, etc. (Mohriak, 2015). The characterization of the petrophysical properties of the presalt carbonate reservoirs in the Santos Basin and other areas with similar lithotypes is thus critical for the optimization of the hydrocarbon production strategy, for the selection of best methods for enhancing primary and secondary recovery, and for the performance prediction of individual wells in the production area. The presalt carbonate lithofacies variability results in extremely heterogeneous reservoir properties varying both in the vertical and horizontal directions. Consequently, the 3D distribution of the reservoir rocks is difficult to simulate by interpretation of seismic data. Usually the petrophysical parameters are also constrained by well log interpretation and by petrographic analysis of cores and plugs. Samples of the presalt carbonate rocks are rather difficult to obtain in exploratory wells and even when available they may not be representative of the overall properties of the main reservoir rocks that contribute to the oil production. Several companies and research groups thus make use of carbonate rock analogues obtained from field outcrops to investigate the possible sedimentological correlations with the reservoir rocks in the subsurface. In the case of the presalt microbialites in the Santos Basin, several investigations carried out so far have indicated that carbonate rocks such as stromatolites, travertines and tufas might correspond to end-members of the reservoir lithotypes. The reservoir characterization workflow often includes measuring the porosity and permeability by conventional methods in petrophysical laboratories. In this work we discuss some of the similarities and differences between the carbonate rock analogues and the presalt microbialites. We also show preliminary results of the Fourteenth International Congress of the Brazilian Geophysical Society SBGf 2015 |
