FORAMINIFERAL BIOSTRATIGRAPHY OF Y-1 AND Z-1 WELLS, WESTERN SECTOR OF NIGER DELTA, NIGERIA

Abstract

A foraminiferal biostratigraphic and paleoenvironmental study was carried out on 251 ditch-cutting samples from wells Y-1 (3220–1920 m) and Z-1 (5110–3680 m) in the coastal swamp depobelt (western sector) of the Niger Delta Basin. The study aimed to establish stratigraphic age, reconstruct depositional paleoenvironments, define a sequence-stratigraphic framework, and assess preliminary petroleum potential. Lithostratigraphic analysis delineated three sub-lithofacies units (Y1–Y3) in Y-1 and four units (Z1–Z4) in Z-1, characterized by rhythmic sand–shale alternations typical of the Agbada Formation. Foraminiferal assemblages show moderate to low abundance and diversity, dominated by benthic forms (calcareous and agglutinated/arenaceous); planktonic taxa occur in Y-1 but are absent in Z-1. In Y-1, two planktonic zones were identified: Globorotalia humerosa Zone (N17) and Globorotalia margaritae Zone (N18), whereas Z-1 yielded a composite benthic assemblage correlatable with the Globigerinoides primordius–Catapsydrax dissimilis interval (N4–N5). Benthic zonation indicates the Cyclammina minima Zone (NNDF 05) in Y-1 and the Hanzawaia concentrica Zone (NNDF 16) in Z-1, consistent with the Niger Delta chronostratigraphic framework. The studied intervals are dated Late Miocene–earliest Early Pliocene (Y-1) and Early Miocene (Z-1). Integrated biofacies and lithofacies analyses define four paleobathymetric settings: coastal delta plain, inner neritic, middle neritic, and outer neritic environments. Sequence-stratigraphic interpretation, constrained by the absence of wireline logs and seismic data, identified two major maximum flooding surfaces: MFS-Y at 3080 m (~6.0 Ma) in Y-1 and MFS-Z at 4290 m (~23.2 Ma) in Z-1, separating transgressive and highstand systems tracts. The Agbada intervals show promising petroleum potential, with reservoir-quality sands, effective marine shale source–seal pairs, and favorable trapping conditions.