Identification of the Pre-Alagoas Unconformity through real-time fluorescence and X-ray diffraction analyses of drilling cuttings
DOI:
https://doi.org/10.70369/6rnx5120Keywords:
Pre-salt, Santos Basin, Barra Velha Formation, Itapema Formation, geochemical stratigraphyAbstract
Oil well drilling requires rapid interpretation and timely decision-making. The success of these decisions largely depends on the effective use of conventional tools, such as drilling parameters, gas analysis, well logs, and cuttings descriptions. However, in certain situations, the integration of additional analytical techniques becomes necessary, particularly X-ray fluorescence (XRF) and X-ray diffraction (XRD). The application of XRF and XRD techniques at the wellsite during drilling operations in the Brazilian Pre-Salt has proven essential not only for the identification of igneous rocks, but also for providing valuable supplementary stratigraphic information across drilled intervals. Based on stratigraphic markers established by other methods, XRF analyses of cuttings samples have revealed increased concentrations of Fe, Al, and Mg in the clay-rich zones of the Barra Velha Formation, and elevated levels of Fe, Al, and K in the clay-rich zones of the Itapema Formation. The combined use of XRD has enabled mineralogical control over the geochemical data obtained. In wells with minimal barite contamination and limited dolomitization or silicification, it is possible to detect variations in the Ca/Sr ratio across the Pre-Alagoas unconformity, which marks the boundary between the Itapema and Barra Velha formations. In the carbonates of the Barra Velha Formation, a more pronounced substitution of Ca by Sr in calcite is observed, suggesting that the depositional environments of this unit were characterized by higher salinity compared to those of the Itapema Formation.
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