Biostratigraphy, stratigraphy and paleoenvironments of middle Eocene – middle Oligocene sediments from Onshore Niger Delta, Nigeria: Insights from foraminifera and palynomorphs

Abstract

• Integrated biostratigraphy using foraminifera and palynomorphs. • Four maximum flooding surfaces were dated and correlated. • Paleoenviromentally significant foraminifera and palynomorphs species were utilized. • Depositional environment ranged from Coastal Deltaic and Middle Neritic. In recent times, biostratigraphy, apart from its primary roles in age dating, paleoenvironmental reconstruction and correlation now constitutes a significant aspect of sequence stratigraphy applicable in exploration and production activities. Unfortunately, published biostratigraphic and biochronostratigraphic records of Niger Delta Eocene –Oligocene sediments are scanty. This integrated study of the H1-P well (1250–3931 m) from the Central Niger Delta revealed two lithostratigraphic units, the continental Benin Formation (1250–2384 m) and the Upper Agbada Formation (2384–3931 m) composed of alternating sand and shale intervals. The uppermost section (1250–2384 m), which is within the continental Benin Formation was barren of foraminifera. However, it was dated Early to Middle Oligocene (P520-P560) based on diagnostic palynomorphs. Conversely, the lowermost part (3931–2384 m), consisting moderate numbers of foraminifera species, was dated Late Eocene (P480). The paleoenvironments, which fluctuated between Coastal Deltaic to Middle Neritic, were subdivided into four highstand and four transgressive systems tracts. The 39.4 Ma, 38.0Ma, 36.8 Ma and 35.9 Ma maximum flooding surfaces, and three sequence boundaries were inferred. The youngest maximum flooding surface dated 35.9 Ma at 2385 m, was defined by the FDO Nonion oyae , FDO Brizalina ihuoensis (2402 m), presence of Globigerina yeguaensis and FDO Uvigerinella sparsicostata at 2713 m, the FDO of the diagnostic palynomorphs Grimsdalea polygonalis, Doualaidites laevigatus, at 2414 m / 2792 m respectively indicated the penetration of Late Eocene. The FDO Uvigerina hourqi delineated the Middle/Late Eocene boundary at 3121 m. The Middle Eocene is further characterized by Acme Uvigerinella sparsicostata and occurrence of Chiloguembelina martini . The sequence stratigraphic interpretation permitted age dating of the maximum flooding surfaces and the sequence boundaries within the well section.