Drill Cutting Samples Assessment for the Palynofacies Analysis and Paleoenvironmental Reconstruction: Codó Formation, São Luís-Grajaú Basin (Brazil)

The Codó Formation has 170.000 km² of extension and represents part of the Cretaceous Supersequence of the São Luís-Grajaú Basin, located in the northeastern part of Brazil. This formation is composed of shales, siltstones, limestones, sandstones, and some anhydrite levels. The marine influence in the Codó Formation is still being discussed in terms its geographic extension and time interval. Previous palynological studies on the 2-ANP-5-MA well and other sections have shown the presence of dinoflagellate cyst (Subtilisphaera genus) and fresh-water algae (Scenedesmus, Pediastrum and Botryococcus). This research aims to classify the sedimentary organic matter in drill cuttings of the 2-ANP-5-MA well. The drill cutting samples are from 2469–2559 m depth, and 25 slides were made with 3 m intervals between them. We applied the usual palynological method of preparation to identify the sedimentary organic matter (SOM) groups. Through the distribution analysis of the main groups and subgroups of the SOM, and application of the APP-diagram, four palynofacies were defined, as well as the redox conditions, the principal kerogen type generated and the depositional paleoenvironment. In addition, a statistical treatment with multivariate techniques (PCA and K-means clustering) and Fuzzy logic was used, aiming a better application of the drill cutting samples to paleoenvironmental reconstructions. In this stratigraphic section, the Codó Formation record is interpreted as a closed lagoonal system to a fluvial system passing through a marine incursion. The statistical and Fuzzy logic approaches appear as automated tools for grouping drill cutting samples, with a proposition of the PARE (Paleoenvironmental Reconstruction Ratio) that shows a good relationship with SOM data and previous works done in this stratigraphic section.