Facies and Compositional Characterization of the Basaltic Volcanic Sequence in the Presalt Discovery Well, Brazil

Summary Volcanic rocks can pose significant drilling challenges, including wellbore instability and circulation losses. In the Pre-Salt section of the Santos Basin, magmatic rocks frequently intersect wells, requiring detailed lithological understanding to ensure safe and efficient drilling. This study presents a facies and compositional characterization of the basaltic volcanic sequence in well 1-BRSA-369A-RJS, the Pre-Salt discovery well. The dataset includes drilling cuttings at 3 by 3 meter intervals, sidewall core samples with thin sections, photographs, and ICP-AES data, along with wireline logs such as gamma ray, resistivity, sonic, density, photoelectric factor, neutron porosity, and caliper. Cuttings were re-described and classified into four lithofacies, correlated with four electrofacies as defined in recent studies. Samples were cleaned and analyzed using X-ray fluorescence with a Malvern Panalytical Epsilon 1 instrument and X-ray diffraction with a Bruker D2 Phaser. Geochemical and petrographic data confirm a basaltic composition, with pyroxene and plagioclase identified throughout. Log responses suggest the presence of pillow lavas and brecciated hyaloclastites. A high gamma ray zone indicates hydrothermal alteration, with enrichment in potassium feldspar and clay minerals, reduced resistivity, and uranium remobilization. Properly prepared cuttings, supported by XRF and XRD analyses, offer a powerful tool for volcanic facies identification.