Late Miocene-Pleistocene Foraminiferal Biostratigraphy of WEll Eb-1 and Eb-2 offshore Depobelt,...

Int. Journal of Applied Sciences and Engineering Research, Vol. 2, No. 3, 2013 www.ijaser.com

© Copyright 2011 - Integrated Publishing Association [email protected]

Research article ISSN 2277 – 9442

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*Corresponding author (e-mail: [email protected])

Received on May 2013; Accepted on May 2013; Published on June 2013

Late Miocene- Pleistocene Foraminiferal Biostratigraphy of Well

Eb-1 and Eb-2 Offshore Depobelt, Western Niger Delta Nigeria 1O.C Adeigbe,

2 O.C Oduneye,

3I.A Yussuph,

4C.C Okpoli

1-Department of Geology, University of Ibadan, Nigeria.

2-Departmentof Earth Sciences, Ladoke Akintola University of Technology Ogbomoso.

3-Geoscience Solutions, 115, Apapa Road, Ebute -Meta Lagos

4-Adekunle Ajasin University, P.M.B 001, Akungba Akoko.

Doi: 10.6088/ijaser.020300020

Abstract: This study entails a quantitative planktic and benthic foraminifera biostratigraphy of wells EB-1

and EB-2(OPL 227) in the offshore depobelt of the Western Niger Delta Ngeria. A total of One hundred

and fifty two (152) ditch cutting samples from depth 1,100 to 2,620m at 10m interval of Eb-1 well and

eighty eighty (88) ditch cuttings from depth 1280 to 2160m intervals of Eb-2 well were analyzed.

Recovered foraminifera assemblage is fairly rich, diverse and well preserved with both planktic and

benthic foraminifera well represented in the two wells. The study revealed the interval to have penetrated

early Pliocene to Pleistocene ages and late Miocene to Middle Pliocene ages respectively. The recovered

index forms among the foraminifera assemblage were used in used in dating and zoning of the interval.

The Early Pliocene to Pleistocene correspond to N18 –N22 zone which were characterized by

Globigerinoides rubber, Globorotalia pseudopima, Neogloboquadrina Dutertrei, Globoquadrina altispira

Globigerinoides conglobatus, , Bolivina Scalprata miocenica among other forms while the Late Miocene –

Middle Pliocene ages corresponds to N18-N20 zone, and are characterized by Haplophragmoides Sp.,

Globigerinoides sp., Globoquadrina altispira. Haplophragmoides compressa, Cyclammina cf.minima,

Globorotalia exilis ,Globoquadrina dehiscens.

Key words: Niger delta, Downhole Occurrences, Biozonation, Zones, Benthics , Planktics.

1. Introduction

The Niger Delta (Figure1) is important because of its hydrocarbon resources. Larger amount of data from

several drilled wells have led to a considerable understanding of the stratigraphy and regional geology of

the delta. The Paralic Agbada Formation followed by the Continenetal Benin Formation prograde over the

marine shales of the Akata Formation (Short and Stauble 1967; Weber1971; Weber and Daukoru 1975;

Knox and Omatsola ,1989; Doust and Omatsola, 1990; Petters, 1979,1982; Chukwueke et al., 1992). In

time past, the delta was believed to produce hydrocarbon from rocks of Eocene to Pliocene ages only, but

as the delta becomes better understood with higher biostratigraphic resolution, exploration had been

observed to have shifted to rocks of Pliocene – Pleistocene section. This assertion is the basis of this study.

Although it does not dispute the fact that the Early Miocene – Pliocene section has been the most explored,

most drilled, and most hydrocarbon bearing. Hence, it portrayed the section with most readily available

data.

Late Miocene- Pleistocene Foraminiferal Biostratigraphy of Well Eb-1 and Eb-2 Offshore Depobelt, Western Niger Delta

Nigeria.

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386

Figure 1: Location map of the studied wells.

2. Stratigraphy

Three subsurface formations have been recognized in the Niger delta. These formations have been given

the following lithofacies terms; Akata Formation, Agbada Formation and BeninFormation respectivelty.

The oldest section which is the Akata formation consists predominantly of shales with some associated

sand units. This grades upward through the Agbada Formation, a transitional series composed of sandstone

and shales of roughly equal importance deposited under paralic condition .It is the hydrocarbon bearing

unit of the sequence.The upper series which caps the succession, (Benin Formation) has massive sands and

gravels deposited under continental condition. (Evamy et al., 1978)

Figure 2: Stratigraghy of Niger Delta showing the ages of the sediment.


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3. Materials and methods

The materials used for this study were provided by Geoscience solutions limited. A total of 240 ditch

samples were used for the analysis. The conventional approach for the processing of samples for

biostratigraphic study was adopted for this study. This includes the use of 30% hydrogen peroxide (H2O2)

for disagregation of the samples. Samples were later boiled with soda ash, washed with running water in a

0.63mm – mesh sieve screen and residues picked from all sized fractions. Foraminifera counts were made

from all the samples. The recovered foraminifera were studied under the microscope for proper

identification and photomicrographs were taken. The data were imputed into Strata- Bugs (Biostratigraphy

Data Management software) to prepare the foraminiferal distribution chart of the two wells studied.

4. Results and Discussion

4.1 Quantitative Biostratigaphy (Well EB-1 and EB-2)

The following bioevents have helped in carrying out a zonation of the wells;

First Downhole Occurrence (FDO) of chronostratigraphically significant planktic/benthic

foraminiferal species.

Last Downhole Occurrence (LDO) of planktic/benthic foraminiferal marker species.

Foraminiferal abundance and diversity peaks correlated with foraminiferal markers species whose

stratigraphic ranges are well established in the Niger Delta and worldwide. The zonation was

guided by the work of Blow (1969, 1979), Bolli and Saunders (1985) while the numerical ages

(Ma) were based on the work of Vail and Wornardt (1991) and Berggren (1985) (Figures 4 and 5).

4.2 Biozonation (Well EB-1)

The entire section of the well EB-1 (1110-2620m) is believed to have been deposited during the Early

Pliocene to Early Pleistocene (?5.3Ma , to 1.6Ma ) and straddling the Globorotalia margaritae zone

(N18 ) and Globorotalia truncatulinoides (N22) zone.

4.2.1 N-18 Zone (Early Pliocene)

The upper boundary of this zone was defined by the FDO of Globorotalia merotumida/plesiotumida at

2240m. The lower limit is tentatively placed at the terminal depth (2620m) (Figures 3 and 4).This zone is

diagnostically featured by the Last Downhole Occurrence of Globorotalia tumida at 2360m, Last

Downhole Occurrence of Globorotalia crassaformis at 2380m, Last Downhole Occurrence of

Globigerinoides conglobatus at 2400m, and the co-occurrence of planktic foraminifera; Neogloboquadrina

dutertrei, Globorotalia pseudopima, Globorotalia acostaensis acostaensis, Globorotalia margaritae

margaritae, Globorotalia merotumida/plesiotumida, Globoquadrina dehiscens, and Globigerinoides ruber

which confirms the Early Pliocene (N18) age assignment.

4.2.2 N-19 Zone (Early Pliocene)

The First Downhole Occurrence of Globorotalia margaritae at 1940m defined the top of this zone,while

the base was marked by the FDO of Globorotalia merotumida/plesiotumida at 2240m (Figure. 4). Benthic

foraminiferal assemblage recorded within this interval is characterized by both calcareous and arenaceous


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species. These include; Heterolepa pseudoungeriana, Heterolepa crebbsi, Uvigerina auberiana, Planulina

arimaensis, Hanzawaia mantaensis, and Eggerella scabra, Cyclammina cancalleta, Karreriella siphonella,

Karreriella subcylindrica, Haplophragmoides narivaensis, Haplophragmoides compressa,

Alveolophragmium crassum, Saccammina complanata and Cyclammina .cf. minima. The peaks of

foraminiferal abundance and diversity (Condensed Sections) observed at 1940 – 1980m and 2100 – 2170m

(Figures.3 and 4). are believed to be associated with 3.4Ma and 4.0Ma Maximum Flooding Surfaces (MFS)

respectively. FDO of Globorotalia margaritae magaritae at 1940m confirms 3.4Ma event.

4.2.3 N-20 Zone (Late Pliocene)

The base of this zone was defined by the FDO of Globorotalia margaritae at 1940m, while the top was

marked by the FDO Globoquadrina altispira (1590m) (Figure.5). The co-occurrence of planktic

foraminiferal, Sphaeroidinella dehiscens, Globigerinoides obliquus obliquus, Globigerinoides conglobatus,

Globigerinoides ruber, Globorotalia pseudopima, Neogloboquadrina dutertrei, Globorotalia miocenica

and Globoquadrina altispira (Figure. 3) within the 1590 -1940m interval confirms the assigned age of

Late- Pliocene (N20). However, Benthic foraminiferal species characterizing this interval include:

Siphouvigerina auberiana attenuata, Heterolepa crebbsi, Bulimina aculeatea, Uvigerina peregrina,

Uvigerina auberiana, Bolivina scalprata miocenica, Sigmolopsis schlumbergeri and Stilostomella monilis

(Figure.4)

4.2.4 N21-22 Zone (Late Pliocene-Pleistocene).

This zone has its lower limit defined by the FDO of Globoquadrina altispira at 1590m, while the upper

limit was placed at 1110m (top of analysed interval)(Figures 3 and 5). The continuous occurrence of

Globigerinoides extremus, up to 1110m depth (top of the studied interval) suggest an age not younger than

Early Pleistocene. Foraminiferal abundance and diversity that cluster within the interval 1450 –1570m of

this zonal interval may indicate a Condensed Section Maximum Flooding Surface (Fig. 4). This event may

be a regionally important datum and could be correlated to the 2.0Ma sea level rise. This bloom is

characterized by FDO Globorotalia exilis depressed (2. 1Ma) at 1580m.

Figure. 3: Foraminiferal Biostratigraphy summary of well EB-1


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Figure 4: Foraminiferal distribution chart for well EB-1


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Figure 5: Foraminiferal Distribution Chart for well EB 2

Figure 6: Foraminiferal Biostratigraphy Summary of well EB 2


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4.3 Biozonation of well EB-2

From the result of the analysis for well EB-2, it was deduced that the entire section (1280-2160m) was

deposited during Late Miocene- Middle Pliocene epoch (5.5-2.8Ma) which spanned Globoratalia

margaritae zone (N18) and Globorotalia altispira (N20) zone ( Figures 5 and 6).

4.3.1 N 18 Zone (Late Miocene- earliest Early Pliocene)

The lower limit of this zone was defined by the FDO of Globoquadrina dehiscens at 2160m, while its

upper limit marks the top of the FDO of Globorotalia merotumida/plesiotumida (Figures 5 and 6).

The sharp decline in arenaceous species at depth 2020m corresponds to N18 zone and it is dated late

Miocene. First Downhole Occurrence of Globoquarina dehiscens also confirms the N18 zone with an

absolute age of (5.5Ma). The ditch cuttings sample analysed at depth 2160m proved barren.

However ,the entire interval is characterized by co-existence of some planktic and benthic

foraminiferal assemblage which include; Globigenoides spp, Orbulina universa, Hastigerina

Siphonifera, Glomospiranoides bulloidens, Globiquadrina altispira, Globigerinoides rubber and

Haplophagrnoides cornpressa, Haplohisragmoides narivensis, Haterolepa pseudoungerina,

cyclamina cf. minima, Saccamina Complanata, Uvigerina auberiana, sigmolopsis Schlumberger

respectively

4.3.2 N-19 Zone (Late Miocene- part of Early Pliocene)

FDO of Globorotalia Merotumida/plesiotumida at 1990m marked the lower limit of this zone while

its upper limit corresponds to the FDO of Globorotalia margaritae at 1750m (Figures 4 and 6). The

LDO occurrence of Globorotalia crassaformis crassaformis, equally corresponds to N19 zone. The

abundance and diversity of forms that clusters within the interval 1750 -1990m shows evidence of

maximum flooding surfaces being dominant in the agglutinated calcareous benthics and fewer among

the planktic forms. A condensed section also occur within this interval between depths 1875m–1960m

(Figure. 5)

4.3.3 N-20 Zone (Late Miocene- part of early Pliocene)

This zone has its lower limit defined by the FDO of Globorotalia margaritae at 1750m, with its upper

limit marking the top of the analysed interval (1280m) (Figures 5 and 6). The First Downhole

Occurrence of Globoratalia altispira at depth 1280m dated Middle Pliocene and corresponds to N20

zone. Globigerinoides rubber and Globorotalia pseudopima has continous occurrence up to depth of

1280m(top of the studied well) which suggest an age not younger than Middle Pliocene.

4.4 Correlation of zones for well EB-1 and EB-2

Four zones N18, N19, N20 and N21-N22 were recognized in well EB-1 while three zones N18, N19

and N20 were penetrated by the EB-2 well.

At depth interval of 2620-2240m, we have the N18 zone for well EB-1 and its top correlates with the


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top of N18 zone for well EB-2 at depth 1990m. N18 zone for well EB-1 has a relatively thicker

lithologic section compared with that of N18 zone in well EB-2 (Figure. 7).

N19 zone for well EB-1 extends from 2240-1940m and correlates with N19 zone of well EB-2 at

depth range 1990-1750m. The top of N19 zones (bottom of N20 zone) for the two wells are

correlative but the top of their N20 zones are not due to the presence of faunal discontinuity in the

N20 of the well EB-2. Although faunal discontinuity may be locally or regionally correlative, and may

coincide with key strata surfaces (e.g sequence boundaries, flooding surfaces), Local geologic

complexities or the effects of sampling drill cuttings (i.e caving and proximity to casing points)

(Figure.7).

Figure 7: Correlation of Zones

5. Conclusion

Biostratigrapic analysis of the two wells EB-1 and EB-2 showed that the two wells were fairly rich and

diverse in both Planktic and Benthic foraminifera .Four zones have been deduced for well EB-1 which

include N18, N19, N20 and N21-N22 (Early Pliocene to Pleistocene ) while three zones were deduiced for

EB-2 well ranging from N18, N19, and N20 respectively with their corresponding ages spaning Late

Miocene to Middle Pliocene. Index forms among the recovered foraminfera also helped in zoning of the

wells.


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5.1 Plate 1 from Well EB-1

1

2 3

4 6 5

7 8 9


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5.2 Plate 2 from Well EB-2

1 2 3

7 8

9

4 5 6


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5.3 PLATE 1 Details PLATE 2 Details

1. Globorotalia crassaformis 1. Globigerinoides conglobatus

2. Globigerinoides rubber 2. Bolivina sp .

3. Globoquadrina dehiscens 3. Globigerina nepenthes.

4. Globorotalia acostaensis 4. Valvulina flexilis

5. Globoigrinoides immaturus 5. Bolivina isidroensis

6. Globorotalia plesiotumida 6. Bolivina scalprata miocenica.

7. Globorotalia hirsuta 7. Bulimina aculeata.

8. Globigerinoides rubber (2) 8. Haplophragmoides narivaensis.

9. Globigerinoides immaturus (2) 9. Cyclammina cf. minima

Acknowledgement

Many thanks to Mr. Wale Yussuph who painstainkingly took us through the process of interpreting

biostratigraphic data and also helped with the data acquisition. Dr O.C Adeigbe was also very cooperative

in putting this work together. Lastly we all appreciate IJASER editorial board for given this work a kind

consideration.

6. References

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chronostratigraphy . In: Smelling N.J. (ed), Geochronology and the Geologic Time Scale, Geol.

Soc. London Mem., 10, 211-260

2. Blow. W.H, 1969. Late Middle Eocene to Recent Planktonic Foraminiferal Biostratigrpahy.

Proceedings, First International Conference on Planktonic Microfossils, Geneva, 1967,

pp199-422.

3. Blow, W.H, 1979. The Canozoic Globigerinida. E.J Brill, Leiden 3, 1413.

4. Bolli. H.M, Saunders. J.B, and Perch Nielson. K, 1985. Planktonic Stratigraphy. Cambridge 1st

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5. Chukwueke. C, Thomas. G and Delfaud. J, 1992. Processus sedimentaries et flux

thermique dans la partic ditale du delta du Niger . Bull. Centres Rech Explo-Prod .Elf Aquaitaine.

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1978. Hydrocarbon Habitat of Tertiary Niger Delta AAPG Bulletin, 62, 1-39

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12. Short K.C. and Stauble A.J, 1967. Outline of Geology of Niger Delta. AAPG. Bulletin. 51(5)

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Mijnbouw, 50(3) 559-516.

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Late Miocene-Pleistocene Foraminiferal Biostratigraphy of WEll Eb-1 and Eb-2 offshore Depobelt,...

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