Calcareous Nannofossils and Foraminifera Studies: An Integrated Approach to the Depositional Environmental Study and Biostratigraphy of Deb-1 well offshore Eastern Dahomey Basin, Southwestern Nigeria

Abstract

High resolution biostratigraphic study of Deb-1 well located in Dahomey Embayment, southwestern Nigeria was carried out on one hundred and eighteen (118) ditch cutting samples for nannofossils and fifty eight (58) samples for foraminifera studies with the view towards identifying the biostratigraphic zones, determine the age and paleoenvironmental reconstruction of the sediments. Laboratory preparation of both nannofossils and foramaminefa involve slide preparation and identification of forms present. Six biozonations were recognised for the nannofossil which include NN11 (Discoaster quinqueramus zone), NN8-NN10 (Catinaster coalithus zone), NN5-NN7 (Sphenolithus heteromorphus zone), NN4 (Helicosphaera ampliaperta), CC13-CC22 (Eiffellithus eximius zone, Micula decussata zone) and CC12 (Eiffellithus eximius zone) which belong to Late, Middle, Early Miocene, Coniacian-Campanian and Turonian ages respectively. However, the planktonic foraminifera zones identified are Heterohelix reussi zone (Upper Santonian), Dicarinella primitive zone (Lower Santonian), Archeoglobigerina bosquensis (Lowermost Santonian), Hedbergella planispira zone (Coniacian) and Heterohelix reussi zone (Turonian). The paleoenvironment of deposition based on Fisher’s diversity index shows a value that is less than 5.52 which is suggestive of environment not deeper that outer neritic. This is further corroborated with p/b ratio which indicates that the base of the section analysed is within the deep marine, the middle section is shallowing while the upper part is characterised by prograding depositional system (deltaic in nature). This is suggestive of a marginal marine to open marine system.