Lithology Identification Through X-Ray Fluorescence (XRF) Analyses on Drill Cuttings While Drilling in Santos Basin

Abstract

The present research propouses a methodology using XRF analysis on drill cuttings samples, obtained during well drilling, for characterizing pre-salt reservoir formations in the Santos Basin, southeast of Brazil in order to identify composition variations in carbonates, clay zones and potential igneous rocks. In this study 16 off-set wells were analyzed where XRF analysis was performed on drill cuttings samples. Aftwerwards the results were compared with lithogeochemical and gamma-ray spectral logs, determining the consistency of the data. The lithological interpretations were based on macroscopic (sidewall core samples - SWC) and microscopic (thin section) descriptions and geochemical analyses from SWC samples. This permitted us to comprehend the variations observed in the carbonate reservoir and to identify possible igneous rocks. From the results, a pattern of responses could be established for the XRF method for each lithology within the Pre-salt section, using the major elements (Mg, Ca, Si, k, Fe and Al) and some minor elements and trace elements (Sr, Rb, Y, Zr, Ti, Nb, Ga). The variations of the major elements were best observed in radar and bar charts, that use only the major elements, which enabled the separation of the lithological section into six main sets: limestones, dolomitic carbonates, silicified carbonates, carbonates with magnesian clay, siliciclastic rocks (shale/siltstone/sandstone) and basic igneous rocks (basalt/diabase). For quality control, still during drilling, comparative analysis by three approaches was proposed: a) the systematic comparison of the proportion of elements composing the same mineral, such as Ca and Sr, K and Rb, and Al and Ga; b) the comparison of XRF data with XRD data; and c) comparison with calcimetry data in a cutting sample. Furthermore, in order to support the interpretations, four crossplots (Ca × Si; K.Rb × Al.Zr; Rb × Sr; Zr × Fe) and one crossover (Rb × Sr) were generated aiming to individualize the six lithological types described, as well as diagrams from the literature to identify the igneous rock type. The use of XRF on cuttings to determine lithologies during the drilling of petroleum wells is new in the literature, as well as the proposed quality control, being useful for the characterization of complex reservoirs such as Santos Basin pre-salt, being a methodology already used since 2018 by Petrobras.