CALCAREOUS NANOFOSSILS BIOSTRATIGRAPHY OF WELL X DEEP OFFSHORE EASTERN DAHOMEY BASIN,NIGERIA
CHAPTER 1
INTRODUCTION
GENERAL STATEMENT
Micro-paleontology is an important aspect of geology. It involves the study of microfossils. Most sediments
contain microfossil, and the kind of microfossil depends largely on the original age, environment of deposition
and burial history of the sediment. Because microfossils are small and abundant they can be recorded from small
sample. Hence when a geologist wish to know the age of rock or the salinity and depth of water under which it
was laid down, it is to microfossil they will turn to for quick and reliable answer. Also geological surveys, deep
sea drilling programs, oil and mining companies working with small samples available from borehole cores and
drill cutting have all employed micropaleontologists to learn more about the rock they are handling(Cavalier
Smith,T.1987).
Biostratigraphy is the grouping of strata into unit based on their fossil content with the aim of zonation and
correlation. As such biostratigraphy is concerned primarily with the identification of taxa, tracing their lateral
and vertical extent and dividing the geological column into units defined on their fossil content. Microfossils are
among the best fossils for biostratigraphical analysis because they are extremely abundant in rock (a particular
consideration when dealing with drill cuttings). It should be noted that spore, pollen, diatoms and ostracod are in
dispensablefor the biostratigraphy of terrestrial and lacustrine successions, where microfossils can be scarce.
The Eastern Dahomey basin of the Nigeria sector, contains extensive wedge of cretaceous to Recent sediments;
up to 3000m which thickens towards the offshore. The basin has been of much geological interest as a result of
reported occurrences of bitumen ,limestone, glass sands and phosphates (Nton,2001). The area of study falls
within this basin and drill cutting from well X deep offshore is been analyzed for the work.
AIM AND OBJECTIVES
The aim of this research is to identify and analyze the abundance of calcareous nanofossil in the strata of the
studied well(well X) and to identify new species in the analyzed well(if any) therefore using the data to date and
zone the well.
The objectives of this study are:
To establish lithostratigraphic sequence of the section.
Identification of the nanofossil record.
Description, illustration and classification of the nanofossils records.
Biostratigraphic zonation of the rock sequence and relative age determination.
Quantitative (statistical) analysis of the taxa identified in determination of paleoenvironment.
THE SCOPES AND METHODS OF STUDY
This study is focused on determining the calcareous nanofossil biostratigraphy of the deep offshore Dahomey
Basin,Southwestern Nigeria;
The scope of this study includes:
1. The well is code-named well X for confidential reasons and the samples were provided by EARTH
2. The laboratory processing/preparation of the ditch-cutting samples of nanofossil, from the well under
study.
3. The picking of nanofossilfrom washed samples,using an alluminium tray and a picking stick.
4. The analysis of the picked nanofossil samples using stereo-binoculars microscope, photo album,etc
V. Interpretation of the analyzed data, and Zonation.
LOCATION AND ACCESSIBILITY
. The study area See Figure 1, falls within Nigeria Sedimentary Basin; the Dahomey Basin. It is geographically
in the South West Region of Nigeria around the Abeokuta Axis. The Study Area is denoted as X- well and XFormation
as a result of Proprietary reasons. The study area is very well accessible, and lies within Latitudes 6
41’N – 7 9’N and Longitudes 3 16’E – o 2 3 41’E covering an area extent of about 915 km . The area has elevation
ranging from 40 min the South to 154 m in the North.
TOPOGRAPHY AND DRAINAGE
The area presents a relatively flat topography typical of a sedimentary terrain. The drainage pattern is dendritic
and is characterized by the irregular branching of tributary streams in many directions at almost any angle but
usually less than 900 Such characteristics usually develop on rocks of uniform resistance to erosion and are most
likely to be found on nearly horizontal sedimentary rocks like the ones
present in the study area. The drainage density of the State is high and is characterized by many big perennial
rivers such as Ogun, Ewekoro and Berre that mostly have dendritic drainage pattern (Akanni,1992)
CLIMATE,VEGETATION AND OCCUPATION
Study area occurs within the humid tropical rain forest region of Nigeria characterized by two climatic seasons;
the rainy season of about eight months (March October) and the dry season of about four months (November
February). The State has an average annual rainfall of about 1300 mm and an annual potential evapotranspiration
of about 188 mm.The area is characterize by dense vegetation and the major occupations in the
study area is farming and fishing,this is basically due to abundant vegetation and adequate rainfall in the region.
LITERATURE REVIEW
For many decades, most geoscientific research on the lithostratigraphy and tectonic framework of the basin and
some of the hydrocarbon potential. The up-dip flanks of some marginal sag basins contain tar deposit at the
outcrop which is evidence of oil lost from down dip (offshore) structures by basin where extensive tar, such is
probably the case with the Nigerian portion of the dahomey basin where extensive tar deposits occur(Adegoke et
al 1980).
The startigraphic settings of Dahomey Basin has been described in detail in the works of;Ogbe, 1972;
Kogbe,1974; Billman, 1976;Omatsola and Adegoke,1981; Ako et al..,1980; Okosun 1990 and Adekeye et
al..,2006. These authors reported five lithostratigraphic formations covering the Cretaceous to Tertiary ages.
The formations from the oldest to youngest include: Abeokuta Group comprising of Ise, Afowo and Araromi
formations (Cretaceous),Ewekoro formation(paleocene), Akinbo formation (Late Paleocene to Early
Eocene),Oshosun formation(Eocene) and Ilaro Formation (Eocene).
The Oshosun formation is one of the major lithostratigraphic units of the Eastern Dahomey basin and comprises
of green,greenish-grey or biege clay and shale with interbeds of sand. The shale is usually thickly laminated,
calcareous and glauconitic(Okosun and Alkali,2012).
Few works have carried out on the basin on foraminiferal biostratigraphy and paleoecography. Previous works
of Reyment (1965), Adegoke et al (1980) have only discussed the stratigraphic occurrenceand the taxonomy of
foraminiferal species in the basin. Quantitative analyses of foraminiferal assemblages and modern interpretations
are still missing for the region. Several other contributions deal with various fossil groups occurring in rocks of
the Paleocene to Eocene Oshosun Formation of Southwestern Nigeria or its Lateral Equivalents.
Bankole et al..,(2006) used the occurrences of diagnostic cysts to confirm the Late Paleocene to Early Eocene
age for the newly exposed section of the Oshosun Formation in the Shagamu quarry. They further inferred
marginal marine environment based on these dinocysts, sporadic occurrences of pollen and spores and the fresh
water algae(Derbaya glyptosperma).Other works include eg, Ostracods,(Okosun,1990),Pollens and spores and
calcareous nannofossils.
Ogbe(1972) reported that the Ewekoro formation contain plaleocene planktonic foraminifera, such as
Globorotalia Pseudobulloides,G. acuta,Globigerina triloculinoides and Globorotalia velascoensis which is
diagnostic of the G. velascoensis zone . He interpreted the Akinbo formation to be between latest Thanetian and
earliest Ypresian based on the presence of such foraminifera as Globorotalia convexa, G aequa, and G.
Velascoensis (Early to late Paleocene) and some typical Eocene forms such as Globorotalia aragonensis and G.
Simulatilis.
Ako et al..,(1980) suggested the name Akinbo formation proposed by Ogbe(1972) Should be discontinued due to
the similarities between the formation and Oshosun formation .He processed the Oshosun formation for
foraminiferal assemblages and recovered both planktonic and benthic assemblages. They includes Subbotina
Linaperta, Globorotaliaincrebescens, G,erroazuensis, Eponides pampalasoi, Which provided a middle Eocene
age for the studied interval studied.
On the basis of foraminifera and ostracods, Okosun (1990) considered the age of the Oshosun formation to early
to middle Eocene. He also reported that the occurrence of the foraminifera: Eponides Pseudoelevatus,Globorotalia velascoensis and Globigerina triloculinoides in the upper part of Araromi Formation which indicates a late Paleocene age.
The integration of foraminifera paleoecology and organic geochemical studies of shale samples from shallow
onshore boreholes,and deepwater X-well in the Eastern Dahomey Basin has been used by Akande et al..,2012 to
appraise the paleoecologic and organic geochemical assessment of Cretaceous hydrocarbon source rocks in the
Gulf of Guinea. The authors concluded that their study confirmsthat of Cenomanian-Coniacian transgressive
sequences consisting of open Marine oil-prone source rocks extends through the Eastern Dahomey basin intothe
lower Benue Basin thus enhancing the chances of hydrocarbon discoveries in Cretaceous target of this Basin
with the possibility of charging the adjacent Niger Delta basin reservoir in the outboard areas.
