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Conventional Core as a Rosetta Stone for Presalt RQ: Insights and Observations Within Upper Sag Presalt Microbialites From Conventional Core of the 3-ESSO-4A-SPS (Guarani-1ST) Well, BM-S-22, Santos Basin, Offshore Brazil


Many significant oil discoveries have been encountered in Aptian lacustrine carbonate deposits within the Santos, Campos and Espirito Santo basins, offshore Brazil. However, most of the wells rely on reservoir characterization from rotary sidewall cores, or short segments of conventional core. In contrast, the ESSO-4A-SPS (also known as the Guarani-1ST) well from BM-S-22 recovered 142.5 M of continuous conventional core from the microbial-prone Upper Sag and Middle Sag intervals. This allowed for significant uplift in characterization of lacustrine depositional models, sequence stratigraphic trends, and diagenetic factors that most significantly affect reservoir quality. Reservoir quality is governed by a complex interplay of depositional textures and subsequent diagenetic alteration. These sediments are interpreted to represent lake margin to lake plain deposits, and are broadly representative of productive lithofacies encountered in the pre-salt. Grain-prone facies are common, with allochems ranging from intraclasts, coated grains, spherules, and lithic fragments. A variety of abiotic and biotic boundstones forms are observed, including broad domal structures with involute stromatolitic laminations, digitate/arborescent forms, clotted/thrombolytic forms, and cryptalgal laminites. Diagenetic alteration is commonly associated with early subaerial exposure and “mega-fractures.” Insights into reservoir performance and potential were gleaned from routine (e.g. porosity and permeability) and special core analyses, coupled with standard and advanced petrographic description. Integration of conventional carbonate petrography and core analyses within this well indicates that leached coated grain/microbial intraclast grainstones found within relatively shallow lake margin deposits typically exhibit the best reservoir quality, manifested by solution-enhanced interparticle and vuggy porosity. Early silica and calcite cements tend to negatively affect reservoir quality, and boundstone facies generally exhibit poor reservoir quality due to porosity occlusion by clay. Although no well test was performed in this well, lost circulation zones coincide with the mega-fractures, indicating that fractures may play an important role to overall productivity within the presalt. These observations provide additional characterization of matrix and non-matrix permeability, and provide important insight into the dynamic behavior of this enigmatic reservoir type.