Biostratigraphic Study of the Calcareous Nannofossils of Well 02, Shallow Offshore, Niger Delta, Nigeria

A calcareous nannofossils biostratigraphic study has been carried out on the strata penetrated of well 02 located in the shallow offshore area of the Niger Delta, Nigeria. The study is aimed at establishing the age and nannofossils biozonations of the well. The samples were prepared using smear slide technique. Forty calcareous nannofossils species were identified and used to make biostratigraphic deductions of zonations and dating of the sequence.The distribution of the calcareous nannofossils enabled the establishment of three zones of Helicosphaera ampliaperta (NN4),Sphenolithus heteromorphus (NN5) and Catinaster coalitus (NN8) belonging to early Miocene late Miocene following standard zonation schemes of previous workers. The zones were based on the first and last occurrences of marker species. Nannofossil abundance / diversity patterns calibrated with chronostratigraphical bioevents reveals four condensed sections when correlated to the Global Cycle Chart. This is associated with the 16.0 Ma, 14.8 Ma and 13.4 Ma and 10.89 Ma maximum flooding surfaces.


Introduction
This study focuses on the nannofossils Biostratigraphy of well-02, located on the shallow offshore portion of the Niger Delta. (Fig. 1) The aim of the study is to present biostratigraphic information from the nannofossils recovered from the strata penetrated.
Calcareous nannofossils are tiny (less than 30 microns) marine phytoplanktons. Tappan, 1980 considers the size of Nannoplankton to be less than 2 µm and referred to those between size range of 2-20 µm as ultramicroplankton. They are classified as coccolith and nannolith, Perch-Nielsen, 1985. Coccoliths are disc-shaped plates ,while the nannolith has various shapes including the star-shaped discoasters. They strive in normal marine environments and rarely tolerate turbidity and freshwater environment, Hay et al., 1967. Calcareous nannofossils have been studied and reported in several literatures, but not much from the Niger Delta basin. This is attributed to the unpublished works of several nannofossils studies by oil companies who guard their information jealously. Agagu,1981,Petters,1982,1983,Berggren,1960,Ozumba, 1995and Okosun and Liebau,1999  He also identified those regressive phases on the basis of Microfauna and flora in the Niger delta.
The Niger Delta stratigraphic committee recently commissioned a biostratigraphic subcommittee mandated to carry out calcareous nannofossils taxonomic project as well as harmonize the various nannofossil schemes in use by different oil companies. The result of this project is yet to be made available for public use. Gallagher,1990,Farinaciari andRio,1996 andFarinaciari et.al However, this work is to identify the calcareous nannofossils encountered, establish the nanno zones in the analysed sequence and determine the age of the strata.

Materials and Methods
One hundred and twelve ditch cutting samples obtained at 30 ft interval within the range of 6000 -11000 ft from well 02 located in the shallow offshore Niger Delta was utilized for this study. Samples collected at 30 ft interval were to provide a high resolution biostratigraphic data. The well is codenamed well 02 for confidential reasons and the sample were provided by one of the multinational oil companies operating in the Niger Delta.
Smear slides method was employed for extracting nannofossils from the cuttings because; it is simple, rapid, cheap and efficient. About 4 g of the sample broken down by soaking and swirling in distilled water, then a small quantity of sodium hexametaphosphate ( calgon) is added to help disperse the sediment and ensure even distribution of the particles. A drop of this suspension is then placed on a glass cover slip followed by a drop of distilled water to dilute it and then dried on a hot plate at 60 -70 0 C for few minutes and then mounted on a glass slides using a few drops of Norland optical adhesive mounting medium. This was then cured over ultraviolet light for 45 minutes. The prepared slides were examined for their calcareous nannofossil content using a light microscope under crossed polarizers and transmitted light at x 1000 magnification.

Results
Calcareous nannofossils recovery from this study is fairly abundant, diversified and moderately preserved (Fig. 2, Plates 1 and 2).One hundred and twelve samples from 6000 -11000 ft of well 02 have been analysed for their calcareous nannofossil content. A total of 40 nannofossils species belonging to 17 genera were recognised. The nannofossils are mainly placoliths, nannoliths and helicoliths. Of these Helicosphaera carteri is the most abundant occurring throughout the entire analysed sequence. Sphenolithus heteromorphus also occur in abundance at the upper section of the studied interval and was used to define the upper section of the well. The family Noclacihabdacea represented by the genus Reticulofenestra has some of its species differentiated on the basis of size variation R. haqii( 3-5 µm), R. minuta( <3µm ),R. pseudoumbilicus ( >7µm ), Helicosphaerid coccoliths, represented by Helicosphaera carteri, Helicospheaera stalis, Helicosphaera obliqua, Helicosphaera deflandrei, Helicosphaera scissura and Helicosphaera ampliaperta which is a very important zonal index species of the lower section (NN4) of the studied well. Three major zones were delineated for the interval (6000 -11000 ft) of well 02, based on the first and last occurence of marker species. The zones are; Helicosphaera ampliaperta, Sphenolithus heteromorphus and Catinaster coalitus. (Table  1) 3. 1

.1. Heliscosphaera ampliaperta Zone
Stratigraphic interval: 8330 -11000 ft Age-early Miocene Definition: The top of this zone is recognised in the well and is placed as the first down hole occurrence (FDO) of Helicosphaera ampliaperta at 8330 ft. The base was not seen but has been placed at the base of the studied interval. The zone is also characterised by the top occurrence of Sphenolithus dissimilis and base occurrence of Helicosphaera intermedia. The zone corresponds to the NN4 zone of Martini,1971

Conclusion
The investigated interval of well 02 showed fairly abundant, diversified and well preserved calcareous nannofossil assemblage. The distribution pattern of the biostratigraphically significant nannofossil markers within the studied interval (6000 ft -11000 ft) has been studied using first and last occurrences of marker species and assemblage characteristics.