International Journal of Science and Technology Volume 3 No. 5, May, 2014 IJST © 2014– IJST Publications UK. All rights reserved. 285 Calcareous Nannofossil Biostratigraphy of XY-1 Field,Offshore Western Niger Delta, Nigeria S. O. Obaje 1 , E. A. Okosun 2 , P. I. Olasehinde 2 & E. E. Udensi 3 1 Nigerian Geological Survey Agency, P.M.B. 1423, Fate, Ilorin, Kwara State, Nigeria 2 Department of Geology, Federal University of Technology, P.M.B. 65, Minna, Nigeria. 3 Department of Physics, Federal University of Technology, P.M.B. 65, Minna, Nigeria. ABSTRACT The scope and objectives of the research study involved the following: the identification of calcareous nannofossils and their abundance in the strata penetrated in the wells, calcareous nannofossil biozonation of the wells intervals, determination of the ages of the studied well depth intervals and the correlation of the five wells in the study area. The area of study is located in the offshore XY-1 Field of the offshore western Niger Delta area of Nigeria. Niger Delta lies between latitudes 4° and 6° N and longitudes 3° and 9° E in the south-south geo-political region of Nigeria. A total of 1,100 ditch cutting samples were collected from five wells at the industry standard of 9.14 m depth intervals. The pipette and smear laboratory technique was used for the calcareous nannofossils preparation. The identified calcareous nannofossil assemblages were used for biostratigraphic interpretation. Three calcareous nannofossil biozones were identified and they are Calcidiscus premacintyrei (Zone NN6-NN7), Catinaster coalitus (Zone NN8) and Discoaster hamatus (Zone NN9). The biozones were used for the correlation of the five wells. These biozones are indicative of Middle to Late Miocene age for the well depth intervals. The biostratigraphic interpretation of these wells is useful for mapping subsurface horizons in the offshore western Niger Delta area of Nigeria. Keywords: Calcareous nannofossils, biostratigraphy, XY-1 Field, offshore western Niger Delta, Nigeria 1. INTRODUCTION The study area is located in the offshore western Niger Delta area, Nigeria (Figure 1). The Niger Delta is situated in the Gulf of Guinea on the west coast of Central Africa. It lies between latitudes 4° and 6° N and longitudes 3° and 9° E in southern Nigeria [9]. The Delta is situated at the intersection of the Benue Trough and the South Atlantic Ocean where a triple junction developed during the separation of South America and Africa in the Late Jurassic [13]. The aim of the research was to identify and analyze the abundance of calcareous nannofossils in the strata penetrated by the studied wells and to use the data to zone, date, and correlate the wells. 2. GEOLOGICAL SETTING Three formations have been recognized in the subsurface of the onshore and offshore Niger Delta [6], [10], [12], [1], [7], [11]. These are the Akata, Agbada and Benin Formations. These formations were deposited in marine, transitional and continental environments, respectively; together they form a thick, overall progradational passive-margin wedge [5]. The Akata Formation, the basal unit, is composed mainly of marine shales believed to be the main source rock within the basin. The Agbada Formation is made up of alternating sandstone, siltstone and shale sequences that constitute the petroleum reservoirs of the basin and it is Eocene to Pliocene in age. The topmost unit called the Benin Formation largely consists of non-marine sands with shaly intercalations [5]. 3. METHODS OF STUDY 3.1 Calcareous Nannofossils: Pipette and Smear Method The five wells are denoted by TMB- 1, 2, 4, 5 & 6 and they have 226, 212, 222, 216 and 224 ditch cutting samples, respectively, totalling 1,100 samples that were collected at the