Micropaleontology is an important aspect
of geology. It involves the study of microfossil. 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
The scope of this study includes:
- The well is code-named well X for confidential reasons and the samples were provided by EARTH PROB ***
- The laboratory processing/preparation of the ditch-cutting samples of nanofossil, from the well under study.
- The picking of nanofossilfrom washed samples,using an alluminium tray and a picking stick.
- 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 X-Formation 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
evapo-transpiration 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.
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
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
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
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