Characterization of the Ubatuba formation using gamma radiometry, X-ray fluorescence and well logs: A case study from the Upper Cretaceous to the Lower Paleogene in the Campos Basin.

In the Campos Basin, the Turonian, Campanian, and the Cretaceous-Paleocene-Lower Eocene transition are characterized by the presence of rollover-type anticlines and listric faults, reflecting a pronounced tectonic influence during sediment deposition and reorganization. This tectonic activity resulted in distinct depositional patterns and abrupt environmental changes, observable at multiple scales through geophysical methods. These variations span from abyssal to neritic environments, as well as the interfaces between bathyal and neritic domains. The aim of this study was to define the main characteristics of these successions and establish an analytical relationship among the channel fragments (drilling cuttings) to gain a better understanding of the sedimentary processes that controlled the geological evolution and filling of the Campos Basin. Radiometric analyses performed using gamma spectrometry with an HPGe detector, combined with X-ray fluorescence (XRF) data, enabled the characterization and differentiation of system tracts, as well as the interpretation of key stratigraphic boundaries. This integration contributed to a broader understanding of paleodepositional and paleoenvironmental conditions and facilitated the characterization of the parasequences identified in well 1-DEV-16-RJS. The analyses involved integrating well-log information with laboratory data, focusing on the interpretation of five stratigraphic surfaces based on the concentrations of calcium (Ca), silicon (Si), aluminum (Al), titanium (Ti), potassium (K), uranium (U), and thorium (Th) obtained via XRF, together with density and neutron log responses. The results, consistent with the HPGe gamma spectrometry analyses, indicate that a marine transgression began in the Lower Turonian and extended through the Coniacian. This phase was followed by a regression that persisted into the Paleogene, with a maximum flooding event recorded in the Campanian, each stage accompanied by characteristic sedimentary and geochemical variations.