Sedimentary evolution of the Alagamar Formation in thesoutheast portion of the Potiguar Basin, Brazil, based ongamma ray spectral logs

The Alagamar Formation plays an important role in the dynamics of the petroleum systems that occur in the Potiguar Basin, Brazil, as this formation could act as reservoir, source, and/or seal. Although its importance, most of the publications focus on its biostratigraphic record and little is published about its role in hydrocarbon production. In this paper, we investigate the sedimentary evolution of the Alagamar Formation in the southeast portion of the Potiguar Basin, employing detailed well log correlation and the identification of alluvial fan stages. Using spectral gamma ray logs (SGR), the results reveal two stratigraphic sequences comprising five distinct stages, each characterized by unique lithological and geochemical signatures. The lower stages (1-3) exhibit characteristics indicative of continental depositional environments, influenced by fluctuations in sedimentary supply and tectonic activity, with a decreasing upward pattern in the Th/K log. High Th/U ratios suggest oxidizing conditions, while variations in Th concentration highlight the impact of tectonism on sedimentation dynamics, as Th is directly related to detrital and proximal sources. In contrast, stages 4 and 5 signify a transition to marine incursions, marked by lower Th/U ratios and the deposition of thin marine sediments in a more reducing environment, with increased shale content. The proposed evolutionary schematic model provides a valuable framework for understanding the sedimentary history of the Alagamar Formation in this portion of the basin, shedding light on the complex interplay between tectonic processes, sedimentation dynamics, and environmental conditions. The observed differences between stages underscore the heterogeneity of depositional environments within the study area, emphasizing the importance of integrating geological, geochemical, and tectonic data in understanding these geological processes. Additionally, these proxies allowed us to provide a correlated well-log zonation, offering valuable support for exploration and production (E&P) processes.