Biostratigraphic framework

This study presents a detailed palynological analysis of subsurface well samples from the Araromi-2 well in the eastern Dahomey Basin, Southwestern Nigeria, to establish a biostratigraphic framework and interpret paleodepositional environments. 33 samples were recovered and ten (10) ditch cutting samples from depths of 830 ft to 1880 ft were processed using standard palynological techniques involving hydrochloric (HCl) and hydrofluoric (HF) acid digestion, heavy liquid separation, and microscopic examination. The analysis yielded a rich and diverse assemblage of palynomorphs, including spores, pollen, and dinoflagellate cysts. The stratigraphic distribution of key taxa such as Proxapertites operculatus, Retidiporites magdalenensis, Palaeocystodinium australinium, and Cyathidites minor enabled the recognition of the Palaeocystodinium australinium - Cyathidites minor Assemblage Zone. This zone indicates a Late Maastrichtian to Early Paleocene (Danian) age for the studied interval, correlating with the upper Araromi Formation. Palynofacies analysis revealed a vertical transition from amorphous organic matter (AOM)-dominated assemblages in the lower sections to phytoclast-rich assemblages upwards. This trend indicates a regressive sequence, transitioning from a shallow, anoxic marine environment to a marginal marine and finally a deltaic/coastal plain setting. The study concludes that the Araromi Formation records a critical phase of the basin's evolution across the Cretaceous-Paleogene boundary. The findings refine the biostratigraphic zonation of the basin and underscore the value of integrated palynological and palynofacies studies in petroleum exploration and paleoenvironmental reconstruction.