Sequence stratigraphy of subsurface upper Eocene-lower Oligocene deposits, Dahomey Basin, Southwestern Nigeria: palynological and palynofacies approach

This paper presents a sequence-stratigraphic interpretation from the lithologic data and palynological and palynofacies analyses of four shallow boreholes (Badore-1, Epe-1, Ikoyi-1, and Olokonla-1) within the upper Eocene-lower Oligocene interval of the Dahomey Basin. Three lithofacies, namely sandstones, shaly sands, and sandy shales were delineated. Sandstones and shaly sands could probably serve as reservoir rocks, while sandy shales could probably serve as source and seal rocks. Two major broad palynological zones were identified, i.e., the Verrucatosporites usmensis and Magnastriatites howardi Pan-tropical, which correlated to P500 and P400 zones. These zones were further subdivided into the P470 and P480, and P520 and P540 subzones. First occurrences (FOs) of Achomosphaera alcicornu , Homotryblium plectilum , and Adnatosphaeridium multispinosum and last appearance datum (LAD) of Homotryblium tenuispinosum were palynostratigraphic events that defined upper Eocene and lower Oligocene boundary in the four boreholes. Two and four depositional sequences were delineated in Epe-1 and Badore-1 boreholes, respectively, and three each were recorded in Olokonla-1 and Ikoyi-1 boreholes. Seven maximum flooding surfaces (38.8Ma, 37.5Ma, 36.5Ma, 35.0Ma, 32.0Ma, 29.0Ma, and 27.5Ma) and sequence boundaries (39.5Ma, 38.0Ma, 37.0Ma, 36.0Ma, 33.0Ma, 30.0Ma, and 28.4Ma) were identified in the four boreholes. The findings in this present study will be useful in understanding the significance of strata horizons and their respective ages, and system tracts sediment packages in the petroleum system.