Calcareous nannofossil biostratigraphy of well AMKP 2-13, shallow offshore Niger Delta Basin, Nigeria

Abstract

Calcareous nannofossil studies were carried out on 155 ditch-cutting samples from Well AMKP 2 -13 (interval 2840m to 4360m), offshore Niger Delta basin using the short centrfuging technique. The aim was to identify key bioevents, determine the nannozones and age of the study section. Samples were generally dark to light grey, non-calcareous to mildly calcareous shale and penetrated the shale unit of the Agbada Formation. Biostratigraphic result yielded 400 calcareous nannofossil count, 12 calcareous nannofossil taxa:Helicosphaera cateri, Discoaster pentaradiatus, Ceratolithus spp, Calcidiscus laptoporus, Sphenolithus abies, Reticulofenestra pseudoumbilicus, Discoaster brouweri, Braarudosphaera biggelowii, Coccolithus pelagicus and discoaster spp, including 2 index fossils- Discoaster quinqueramus and Discoaster beggrenni. Two nannozones were identified: an indeterminate zone characterised by the absence of calcareous nannofossils at 2840m -3530m and an NN11 zone of Martini (1971) at 3530m - 4360m determined by an influx of calcareous nannofossils (ACME) at 3550m dated late Miocene (Messinian) due to the co-occurrence of Discoaster quinqueramus and Discoaster berggreni at 3560m. The base of the zone was not encountered at total depth (TD) due to non-recovery of fossils from intervals 4135m – 4360m (TD) and probably terminated within the Late Miocene NN11 Zone and? older ages of Martini (1971) at TD.