Introduction

 Increase in exploration and exploitation of hydrocarbons in the Niger Delta has made the search for oil and gas increasingly difficult.  It has become important to acquire knowledge and expertise, in order to improve geological research and technology to field development and production of this vast hydrocarbon resource in the Niger Delta.  This has necessitated the use of calcareous nannofossils in the exploration and exploitation of hydrocarbons.

The study of temporal and spatial distribution of fossil organisms under the field of biostratigraphy remains indispensable in all aspects of geosciences not to mention the relevance of this field to oil exploration and production.  More often than not, the focus is on three major fossils groups: foraminifera, pollen and spores, and nannofossils.  The three have proved very useful and complementary to each other.  However, the relevance of nannofossils is becoming increasingly important not only because of the advantage of its size, the limited stratigraphic range of many of its species, with resolution to some thousands of years, but also its speedy processing technique that yields rapid result especially where real time age determination is required. 

Calcareous nannofossils are tiny (30 micron) remains of golden brown calcareous algae. They are subdivided into coccolith and nannolith (Perch-Nielsen 1985). Coccoliths are disc-spherical shaped plates from the organism and thought to perform a protective role as well as concentrate sun rays for food synthesis, while the nannolith has various shapes including the star-shaped discoasters which constitute a major group used mainly for the subdivision of the Neogene nannofossil zones. Nannofossils are largely restricted to normal marine environments and have little tolerance for either turbidity or freshwater diluted environments (Hay et. al. 1967).

The aims of this study are:

v  To identify the nannofossils and their abundance of each strata penetrated by the two wells.

v  To attempt a biostratigraphic zonation of the strata.

v  To determine the age of the studied intervals.

v  To attempt a correlation of the two wells based on available information.

Geological Setting

The territory Niger Delta is situated in the Gulf of Guinea on the west coast of Central Africa. It lies between latitude 4° and 6° N and longitude 3° and 9° W in the southern part of Nigeria. Three main formation names have been assigned to correspond to the tripartite sequence of the Niger Delta. These are the Akata, Agbada and Benin formations in an ascending order (Short and Stauble, 1967; Avborbo, 1978).

The real names and location of the two wells were not made available for proprietary reasons. However, the Nigeria deepwater region is believed to lie roughly between depths of 600 meters on the inboard side and 2000 meters in the outboard side for an area of approximately 48,500 square kilometers (km²) (Sawyer et al., 2002).

Method of Study

•         The supplied ditch cuttings were logged and composited at 3.02 to 9.1 meter interval.

•         About 5g of crush sample was gently dissolved in distilled water.

•         The resulting suspension was pipette onto a cover slip, place on a hot plate and gently dried.

•         Cover slip was then mounted on a labeled glass slide using two drops of the Norland optical adhesive.

•         Slides were later cured under the ultra violet light.

•         Prepared slides were examined under transmitted light microscope at x1500 magnification.

•         Detailed identification (to species level where possible) was made, of all encountered nannofossils in 8 horizontal sweeps.

•         Fossils were recorded in the analysis sheet with other relevant information.

•         Nannofossil distribution plots were made on the scale of 1:5000 with depth on Y-axis and observed taxa on X-axis for each well.

•         The plotted charts were studied and interpretations were inferred from each of the charts, based on the first and last occurrence of diagnostic taxa, the assemblages, ratio of taxa occurrence, taxon quantitative distribution within stratigraphic interval.

Result and Discussion

A total of fifty-eight nannofossil species were identified in the two wells. Most parts of the studied section are characterized by abundant and diverse nannofossils assemblages which permitted easy zonal subdivision of the studied section into zone. Figure 1 and Figure 2 are the nannofossil distribution chart for BDX-1 and BDX-2 wells, respectively. The intervals in each well were zoned using the standard nannofossil zonation schemes. These are the NN (Neogene nannofossil) zones of Martini (1971) and CN (Calcareous Nannofossil) zones of Okada and Bukry (1980). The work of Berggren et al. (1995) was used to assign absolute ages to important bioevents. The zones encountered in this study ranged from Middle Miocene NN6 (CN5a) to Early Pliocene NN13 (CN10c). Well BDX-1 is within the Miocene Figure 3, while well BDX-2 penetrated up to Early Pliocene Figure 4 .

The zones for this study are: Cyclicargolithus floridanus zone NN6 (CN5a) occurs once in both wells, the interval represents Middle Miocene which usually has paucity of nannofossils. Discoaster bollii zone was assigned to NN7-NN8 (CN5b-CN6) because there were no marker species for NN7 within the interval, which makes it difficult to determine the boundary between the two zones, stratigraphic positioning was therefore employed to mark the top and the base of NN7 and NN8. Discoaster hamatus zone NN9 (CN7a-CN7b) occurs in a short interval in both wells. Minylitha convalis zone NN10 (CN8a-CN8b) is conspicuously representing the interval between the D. hamatus and D. berggrenii zone. Discaoster berggrenii zone NN11a (CN9a) is characterized by the dominance of D. berggrenii over D. quinqueramus. NN11b (CN9b) is D. quinqueramus zone; here D. quinqueramus dominate over D. berggrenii. In actual sense, D. berggrenii rarely get to the top of this zone. Ceratolithus spp. zone NN12 (CN10a-CN10b) is the boundary of Miocene Pliocene. The last zone encountered is Gephyrocapsa spp. (small) zone. Only the base of this zone can be determined, it is then assumed to be NN13 and younger (CN10c). The intervals studied in the two wells contain essentially similar species of nannofossils, which permit accurate zonation and correlation. The correlation table is shown in Figure 5.

Conclusion and Recommendation

This study has revealed that certain variations occur in the stratigraphic ranges of some marker taxa in the Deep Offshore Niger Delta compared to those in the globally used zonation schemes of Martini (1971) and Okada and Bukry (1980). The two wells studied ranged from Middle Miocene (NN6) to Early Pliocene (NN13).

This study has further broadened our knowledge of marine biostratigraphy because it gives absolute ages and refined zonation in the Neogene times. This will also help a great deal in oil exploration activities where well-site biostratigraphers are employed while drilling.

It is recommended that the study of nannofossil should be encouraged because of its usefulness in exploration, as well as erecting Calcareous Nannofossil Chronostratigraphic Zonation Scheme for the Niger Delta.

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