Aplicação de Rock typing na caracterização de reservatórios do pré-sal na Bacia de Santos: um estudo de caso com fácies químicas

Abstract

Driven by the discovery of large oil reserves in the pre-salt interval of the Brazilian eastern margin basins, the last few decades have been marked by significant advances in the understanding of non-marine carbonate rocks. Despite this progress, understanding the heterogeneities of these rocks remains a significant challenge. Therefore, using an integrated approach, this study incorporates: (i) lithological and mineralogical data acquired at macro- and microscopic scales; (ii) geochemical data, expressed through chemical facies defined by multivariate analysis; and (iii) porosity (%) and permeability (mD) data, all obtained from core samples from two wells (A and B) drilled in the Tupi Field, pre-salt of the Santos Basin. The study aims to integrate the rock typing classification method with petrographic and geochemical data to deepen the understanding of depositional and diagenetic controls on the permeability–porosity properties of pre-salt carbonate rocks. The results indicate that, although similar, the analyzed successions present particularities regarding the relative abundance of lithologies and primary chemical composition, especially with respect to siliciclastic contribution. Although they exhibit the same dolomite textures (blocky, mosaic, lamellar, fringing, saddle, and rhombohedral) and silica textures (microcrystalline, macrocrystalline, and chalcedony), there is a clear differentiation between the wells in terms of the intensity of depositional processes. This diversity becomes evident when establishing relationships among lithologies, chemical facies, dolomite and silica textures, and Global Hydraulic Elements (GHE), which categorize the study intervals according to flow capacity. As a general trend, Well A presents a highly heterogeneous multi-petrotype system (GHE 1 to 9), with a predominance of GHE 4 and 5. In this case, the diagenetic influence did not follow a simple inversely proportional relationship; partial dolomitization and fracturing of silicified intervals contributed to the best petrophysical performance (GHE 8 and 9), identified in the intermediate portion of the Barra Velha Formation. In contrast, Well B revealed a more homogeneous reservoir with moderate performance (GHE 1 to 8, predominantly GHE 4 and 5). Its best interval (GHE 8) is also located in the intermediate portion of the well, within the Barra Velha Formation, influenced not only by diagenesis but also by greater detrital contribution. Thus, the quality of pre-salt reservoirs results from the interaction between depositional inheritance and diagenetic processes, configuring a variable and heterogeneous permeability–porosity system and making multi-technique integration indispensable for its characterization.