High-Resolution Biostratigraphic, and Paleoenvironmental Analysis of AJAYI-01 Well, Niger Delta, Nigeria

Abstract

A high-resolution biostratigraphy study and paleoenvironment analysis were carried out on composited fifteen (15) ditch-cutting samples retrieved from AJAYI-01 well, offshore Niger Delta, Nigeria. The studied intervals range from 4060feet – 4960feet and were subjected to foraminifera, calcareous nannofossil and palynological analysis to determine their age, biozonation and environment of deposition. The standard laboratory preparation techniques of wet sieving, simple smear method, and Hydrofluoric (HF) and Hydrochloric (HCl) acids maceration techniques respectively. A sedimentological description of the ditch-cutting samples was carried out.
 Three major foraminifera zones of Turborotalia peripheroacuta/Bolivina mandoroveensis Zone, Praeorbulina glomerosa/Nonion centrosulcatum Zone and Catapsydrax dissimilis/Buliminella subfusiformis Zone and the important marker Heterostegina sp. was recognized in the well based on the critical evaluation of bioevents and correlatable datums. The nannofossil zones of Helicosphaera ampliaperta and Sphenolithus heteromorphus were erected for this well based on the index taxa and fossil assemblage recorded. The palynological data consists of highly diversified assemblages dominated by terrestrial forms such as Zonocostites ramonae, Monoporites annulatus, Laevigatosporites sp., Verrucatosporites sp., Retitricolporites sp., Acrostichum aureum. The important marker recorded was Crassoretitriletes vanraadshooveni (FDO) at 4420feet. The well is dated early to middle Miocene (NN4 –NN5) based on the occurrence of marker species Helicosphaera ampliaperta and Sphenolithus heteromophus which corresponds to Crassoretitriletes vanraadshooveni - Echitricolporites spinosus Zone (P720-P680) subzones.
 A tentative sequence stratigraphic framework for the studied sequences was established. A condensed section that is associated with the Maximum Flooding Surface (MFS) was dated 15.20Ma at 4480 feet based on peak faunal abundance and diversity. The environment of deposition has sediments deposited in the marine environment fluctuating between middle neritic and inner neritic to the littoral, including coastal deltaic settings based on deductions from foraminiferal, palynological and sedimentological data.