Calcareous Nanofossils Biostratigraphy of Well X Deep Offshore Eastern Dahomey Basin,nigeria

Chapter One of Calcareous Nanofossils Biostratigraphy of Well X Deep Offshore Eastern Dahomey Basin,nigeria

 

INTRODUCTION

GENERAL STATEMENT

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 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 PROB ***
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 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.

 

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.