Calcareous Nannofossil Biostratigraphy of A, B, C, D Wells, Offshore Niger Delta, Nigeria

Abstract

The calcareous nannofossil biostratigraphy of four deep water wells, A, B, C, D, offshore Niger delta has been studied in order to document their biostratigraphic distribution, establish biozonation and stratigraphic correlation. Ditch cutting samples were collected at 30 ft intervals. They were processed according to the standard calcareous nannofossil sample preparation technique. A total of forty-two nannofossil species were identified in the four wells. Most parts of the wells are characterized by abundant and diverse nannofossil assemblages which permitted the subdivision of the sections into zones. A total of five zones and two subzones were identified from this study based on diagnostic marker species and notable nannofossil events. The zones are Catinaster coalitus zone (NN8), Discoaster hamatus (NN9), Discoaster loeblichi which occupy the base of Discoaster berggrenii and the top of Discoaster bollii was used to identify the NN10 zone. The main marker for this zone which is Discoaster calcaris is missing in the study wells; this may be due to dissolution of the calcitic wall during sedimentation process. Other zones encountered in this study are Discoaster berggrenii subzone (NN11a), Discoaster quinqueramus subzone (NN11b), Amaurolithus tricorniculatus zone (NN12) and Ceratolithus rugosus zone (NN13). The age of the strata ranges from the middle Miocene to the early Pliocene. The zones were based on the first and last appearances of maker species as well as their relative abundances. The occurrence of Amaurolithus delicatus in wells C and B implies that the well intervals containing the species cannot be younger than early Pliocene age. The four wells show good correlation with each other based on the identified zones. The biozones identified in this study will be useful for subdivision and correlation work in the deep offshore Niger delta Neogene sequence.