Análise multivariada aplicada à caracterização química de rochas carbonáticas do marco Lula, pré-sal da Bacia de Santos

Abstract

Since the discovery of the pre-salt reserves in the Santos Basin, significant progress has been made in understanding non-marine carbonate reservoirs, although debates persist regarding depositional processes and spatial distribution. In this context, the Lula’s Fingers stands out, located at the top of the Barra Velha Formation, near the evaporitic succession of the Ariri Formation. This interval is characterized by total and spectral gamma-ray anomalies, particularly uranium, and gathers consistent evidence of microbial activity and subaerial exposure, contrasting with the classical abiotic precipitation model. Given the relevance of the Lula’s Fingers, this study aims to identify and describe the textures, sedimentary structures, and diagnostic chemical signatures of the interval, as well as to compare results from wells drilled in the Tupi and Iracema Fields. To achieve this, a multi-technical and multi-scale analysis was proposed, involving drill cores, petrographic thin sections, and chemical analyses via portable X-ray fluorescence (p-XRF) and SEM-EDS from three wells (A, B, and C). Quantitative p­XRF chemical data were processed using an unsupervised machine learning algorithm to define chemical facies, while SEM-EDS point data were used to refine these facies and improve the understanding of compound distribution. Results show that the Lula’s Fingers is essentially composed of in situ rocks, calcilutites (CLU), spherulestones (SPH), and shrubstones (SHB), and reworked rocks, such as calcarenites (CRE) and calcirrudites (CRU), with a notable recurrence of CLU featuring irregular or crenulated laminations. The proportion and stacking patterns of these lithologies vary significantly between wells, with distinct features in Well C (Iracema Field) suggesting a different tectonic pattern compared to the Tupi High. Multivariate cluster analysis identified four common chemical facies (FQ1, FQ2, FQ3, and FQ4) across all three wells, with a localized occurrence of FQ5 in Well C. These facies record distinct diagenetic and depositional processes, including dolomitization, silicification, increased detrital input, high evaporation rates, and hydrothermal influence. Probability analysis indicates the establishment of microbial plains associated with the shallow lacustrine pre-salt system, with evidence of marine incursions. This integrated approach not only advances data processing methodologies but also provides a better understanding of the paleoenvironmental evolution of the Lula’s Fingers, reinforcing the importance of integrating petrographic and chemical data in characterizing complex depositional systems such as the pre-salt of the Santos Basin.