HIGH RESOLUTION CALCAREOUS NANNOFOSSILS BIOSTRATIGRAPHY OF THREE WELLS 1, 2 AND 3 IN NORTHWEST OFFSHORE NIGER DELTA, NIGERIA

Abstract

Over the years, various biostratigraphic approaches (foraminifera, pollen and spores) have been used for determining age and biozonation which are useful for exploration and production activities in the Niger Delta. However actual age determination and refined zonation of the deep offshore Niger Delta area have not been achieved. Hence the need to determine the litho-stratigraphic sequence, age of the strata and a sequence stratigraphic frame work for the Paleogene – Neogene section of the Niger Delta. This study was therefore designed to establish absolute age using calcareous nannofossils for the sediments penetrated by three Wells 1, 2 and 3 in Northwest offshore Niger Delta. Five hundred and ninety nine ditch cuttings (599) from three wells (1, 2, 3) offshore Niger Delta were lithologically examined and lithofacies delineated. Slides from the ditch cuttings were prepared using standard procedures and studied using light microscopy. The abundance and diversity of the markers of the calcareous nannofossils were identified using standard procedures and were used for precise zonal delineation and recognition of dated events. Greyish to brownish shale, silt and mudstone intercalated with coarse to medium and fine-grained sand beds were identified. Forty-one calcareous nannofossil species identified confirmed Early Paleocene to Early Pliocene age (NP3-NN15 zones) for the three wells. Well 1 penetrated Middle Miocene to Early Pliocene (NN7-NN15 zones); Well 2 was restricted to Early to Late Miocene age (NN3-NN11), while Well 3 encountered the Early Paleocene (NP3-NN4 zones). Established on the first and last appearances of the marker species as well as their relative abundances three major zones (NN7-NN8, NN9-NN10, NN10 and Younger) four major zones (NN3-NN4, NN7, NN8-NN10, NN11) and three major zones (NP3-NP6, NP14-NP21, NN4) were identified for well 1, 2 and 3 respectively. The acme events of Discoaster prepentaradiatus, Catinaster coalithus, Discoaster kugleri, Helicosphaera ampliaperta, Helicosphaera scissura, Pemma basquensis and Chiasmolithus daniscus were associated with the 8.80, 9.50, 12.18, 14.20, 15.60, 32.47 and 64.75 Ma maximum flooding surfaces, respectively. The correlation of the three wells showed basin-ward younging direction and two zones recognized in wells 1 and 2 (NN7 and NN8) belonging to the Late Miocene age was confirmed. The established zonation scheme for Wells 1, 2 and 3 subdivided the offshore Niger Delta Paleocene to Pliocene sequence into zones, subzones and lithostratigraphic sequences and hence the absolute age determined