INTERNATIONAL JOURNAL OF SCIENTIFIC & TECHNOLOGY … · 2013. 1. 29. · by Cretaceous and Tertiary sediments. Seven lithostratigraphic sequences were recognized in the basin which

INTERNATIONAL JOURNAL OF SCIENTIFIC & TECHNOLOGY RESEARCH VOLUME 2, ISSUE 1, JANUARY 2013 ISSN 2277-8616

58 IJSTR©2013

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Palynological Characterization Of The Tertiary Offshore Emi-1 Well, Dahomey Basin,

Southwestern Nigeria.

Adeigbe O.C, Ola-Buraimo A.O, Moronhunkola A.O.

ABSTRACT:- Palynological investigation was carried out on a sedimentary sequence of Emi- 1 well ranging in depth from 900-8000ft (247-2440m) with a view towards determining the lithological sequence, palynological zones, relative age and paleoenvironment of deposition.Three main palynozones were identified, these include Magnastriatites howardii/P680 zone characterized by quantitative base occurrence of Pachydermites diederixi and co-

occurrence of Borteria sp, Magnastritites howardii and Racemonocolpites hians; dated Early Miocene; Crassoretitriletes vanraadshooveni/P700 zone 2 characterized by assemblage of Crassoretitriletes vanraadshooveni, Belskipollis elegans, Verrutricolporites rotundiporus, and Psilatricolporites crassus, dated Middle Miocene and Echitricolporites spinosus/P820 and Younger zone 3, characterized by co-occurrence of Multiareolites formosus, Anthocerus

sp and Nymphae lotus, dated Late Miocene age. Paleoenvironmental deductions were based on the relative abundance of mangrove pollen to grass pollen, presence of environmentally diagnostic dinoflagellates and incursion of freshwater algae into the setting. The paleoenvironment varies from fluvial, swampy through marginal marine to open marine setting.

Key words: Palynological zones, Paleoenvironment, Freshwater, Dinoflagellate and Swampy ————————————————————

INTRODUCTION Palynological investigation of the Tertiary Offshore Dahomey Basin was undertaken from Emi- 1 well located in the offshore of the basin. The study becomes imperative because there is no documentation on the Tertiary sediments of the Basin located in the offshore part. Nearly all the research works on the southwest Dahomey Basin of Nigeria are on the Cretaceous sediment studies. Therefore, this study becomes interesting, by looking at the stratigraphy from palynological perspective and by giving detail account of the chronostratigraphy of the younger sediments (Tertiary) in the Dahomey Basin. The sedimentation style of the basin is progradational and younging offshore like the adjacent Niger Delta, but different in term of structures because while the Dahomey is characterized by deep seated structures such as horst and graben Niger delta is noted for growth faults and associated roll-over anticline (See Figures 1 and 2). It is hoped that this study will generate chronostratigraphic framework that will be useful for correlation of wells, solve stratigraphic problems such as unconformity, bounding surfaces for sequence stratigraphy and structures which will aid in optimizing hydrocarbon exploration and exploitation; thereby reducing drilling and overhead cost.

Notable early research works on this basin include [1], [2 et al], [3], [4], [5], [6], [7], [8], [9], [10], [11], and [12]. Other recent works include [13], [14], [15], [16], [17 et al],[18], [19 et al], and [20].This study aimed at establishing palynological zones, dating the sedimentary sequences penetrated by the well, deducing the paleoenvironment of deposition of the sediments based on the palynomorph frequency percentage distribution and occurrence of the abundance and diversities of the forms encountered.

Study area Fig 1: Location Map of the studied area.

_________________________________

Adeigbe O. C. Ph. D, Department of Geology, University of Ibadan, Nigeria PH- +2348028218932; E-mail: [email protected]

Ola-Buraimo A. O. Ph. D in view at University of Ibadan; Lectures at Department of Chemical and Geological Sciences, Al-Hikmah University Ilorin, Nigeria. PH- +2348033714079; E-mail: [email protected]

Moronhunkola A. O. B.sc, Graduated from University of Ibadan, Nigeria

mailto:[email protected]

mailto:[email protected]


59 IJSTR©2013

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Figure 2: East-West geological section showing

position, extent and thickness variation in the onshore Dahomey basin and upper part of the Niger Delta (After

Whiteman, A. J, 1982)

GEOLOGICAL SETTING AND STARTIGRAPHY Detailed geologic, tectonic evolution, sedimentologic and biostratigraphic studies of the different parts of the basin have been adequately reported in the literature such as the works of [1], [22], [23], [2 et al], [3], [4], [5], [24], [7], [25], [12], and [13]. Other important works include [11], [14 et al], [15], and [16]. Palynological studies of the basin documented in literature is few; these include [8], [9], [18], [19 et al], and [20]. Dahomey Basin, southwestern Nigeria is a wrench modified sedimentary basin containing rocks ranging in age from Cretaceous-Recent [14]. The basin extends from Southeastern Ghana (Volta Delta) in the west, to the western flank of the Niger Delta in the east {[1], [4], [7], [25]}. The basin is bounded by the Okitipupa Ridge to east (Figure 2). It is a marginal pull-apart basin or marginal sag basin initiated in the Late Jurassic to Early Cretaceous {[26], [7], and [27]}. It is characterized by both block and transform faulting superimposed across an extensive Paleozoic basin during the breakup of the African and South American continents [28]. This led to the formation of continental margin and coastal margin which was filled up by Cretaceous and Tertiary sediments. Seven lithostratigraphic sequences were recognized in the basin which include Abeokuta Group, comprising of Ise, Afowo and Araromi Formations; Ewekoro Formation, Akinbo Shale, Oshosun Formation, Ilaro Formation and the Recent Alluvium deposits referred to as Benin Formation [7].

METHODOLOGY Sixty four ditch cutting samples ranging from interval 900-8000ft were used for both palynological and sedimentological studies. Lithologic description of the samples was done by examining them under the binocular microscope by noting the textural characteristics such as colour, grain size, shape (roundness), sorting, effect of ferruginization, presence of accessory minerals and fossil content in terms of plant remains and fossil fragments. Palynological slides were prepared by subjecting the samples to initial digestion by adding dilute hydrochloric acid into them in order to remove calcium carbonate (CaCO3) that might be present. This is followed by hydrofluoric acid (HF) digestion overnight for proper liberation of the organic macerals present in the samples. Recovered macerals from sieving with nylon sheet of 10µm in order to remove clay particles present is followed by oxidation, heavy liquid separation and mounting of the residue on glass slides with D. P. X. mountant, ready for palynological analysis. The method of preparation conforms to international standard. Taxa counts were made to determine the relative frequency of each species in each sample, after which the diagnostic species photographs were taken using Koolpix camera 6000 model.

RESULTS AND DISCUSSION Sedimentology The litho-description follows the standard method of describing samples as described in the methodology. Four informal sedimentary units were deduced from the analysis of the Emi-1 well. These lithofacies units are unit 1- sandy shale (900-1400ft, 1800-2000ft), unit 2- silty shale (1400-1500ft), unit 3- shale (1500-1800ft, 2000-2100ft, 2900-3000ft, 3600-3800ft, and4700-6500ft), and unit 4- clayey shale (2100-2900ft, 3000-3600ft, 3800-4700ft, and 6500-8000ft) [Figure 3]. The sandy shale unit 1 is dark grey in colour, blocky to fissile in nature, fairly ferruginized and shows presence of macrofossils in some samples; micaceous (muscovite); evidence of plant remains in some horizons and contains carbonate grains that show effervescence on acid test with dilute HCl. The sand grains vary from fine to pebble in size, angular to rounded and poorly sorted. Silty shale unit 2 is dark grey in colour and fissile in nature; no evidence of fossil remains in term of plant and animal but micaceous (muscovite). The sand is silty in nature and very well sorted. The shale unit 3 is dark grey in colour, rarely blocky but mostly fissile at some intervals; few sand grains are present and fairly ferruginized (Haematite). The clayey shale unit 4 is grey to dark grey in colour, fissile with clay interbeds; non-fossiliferous, and fairly ferruginized (Haematite). The lithologic sequence (900-8000ft) described above when dated palynologically shows equivalence to Ilaro Formation.


60 IJSTR©2013

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Legend

Sandy shale Silty shale Shale with claystone Shale Blank

Figure 3: Lithological descrpition of interval 900-8000ft, EMI-1 well, offshore Dahomey Basin, Nigeria.

Palynology Palynomorphs that are stratigraphically significant and environmentally necessary, recovered in the analyzed samples were plotted in order to interpret for the chronostratigraphy and zonal age dating of the interval (900-8000ft).The pollen and spores are relatively moderate in abundance and diversity, moderately rich dinoflagellates cysts, few freshwater algae (Botryococcus brunii), foraminiferal wall linings and numerous diatoms frutules. Three palynological zones were identified and established after the works of [29], and [30 et al]. The basis for

characterization of the palynozones established is given below. Zone 1: Magnastriatites howardii/P680 Interval: 6500-8000ft Age: Early Miocene

Characteristics: The base of the zone is placed at the depth (8000ft) where the analysis ended. The basal part of the interval is characterized by quantitative occurrence of Verrutricolporites rotundiporus; also strongly associated


61 IJSTR©2013

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with other important taxa such as Magnastriatites howardii, Retibrevitricolporites obodoensis, R. protrudens, Retitricolporites irregularis, Psilatricolporites crassus, Sapotacaee sp, Zonocostites ramonae, Monoporites annulatus, Acrostichum aureum and Polypodiaceoiporites sp. The upper part of the interval shows continuous occurrence of some of the forms that appeared bellow, but there is reduction in abundance of the marker forms such as Verrutricolporites rotundiporus. The top of the interval is

marked by the first uphole appearance of Crassoretitriletes vanraadshooveni. The interval is further characterized by quantitative base occurrence of Pachydermites diederixi; co-ocurrence of Borteria sp and Racemonocolpites hians. This assemblage is similar to those reported by [30 et el], for the adjacent Niger Delta, Nigeria and stratigraphically equivalent to zone P600 of subzone P680, dated Uppermost Early Miocene age (Figures 4 and 5).

Figure 4: Palynozones recognized in the studied EMI-1 well, offshore, Dahomey Basin, Nigeria


62 IJSTR©2013

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Zone 2: Crassoretitriletes vanraadshooveni? P700 Subzone: P720-P780 Interval: 2600-6500ft Age: Middle Miocene

Characteristics: The base of this interval coincides with the top of the underlying zone, marked by the first uphole appearance of Crassoretirriletes vanraadshooveni. This marker fossil shows continuous appearance throughout the interval (Figure 5). Some of the miospores that occurred in the lower and older interval continued into this zone; they are Verrutricolporites rotundiporus, Psilatricolporites crassus, Retibrevitricolporites protrudens, P. obodoensis, Zonocostites ramonae, Monoporites annulatus, Racemonocolpites hians, Retitricolporites irregularis, and Acrostuchum aureum. However, new forms of assemblage that occurred include Peregrinipollis nigericus,

Verrucatosporites sp, Laevigatosporites sp, and Belskipollis elegans. The interval is particularly characterized by the presence of organic wall microplankton such as Tuberculodinium vancampoae (6500ft), Polysphaeridium zoharyi, Operculodinium centrocarpum, Selenopemphix sp, Leiosphaeridia sp, and Histrichokolpoma rigaudiae. Other important dinoflagellate cysts present are Lejeunecysta sp, Spiniferites pseudofurcatus, Brigantedinium sp and Homotryblium sp. This assemblage is similar to those reported for Neogene sediments of Anambra Basin, Nigeria. [31] The top of the zone is marked by the first uphole co-occurrence of Multiareolites formosus and Anthocerus sp at 2600ft. The characteristic assemblage of palynomorphs in this interval is similar to that defined by [29], [30 et, al] and [31] for Tropical regions, Niger Delta, and Anambra Basin Nigeria respectively. Thus, the interval is conveniently dated Middle Miocene age (See Figures 4 and 5).

Figure 5: Palynomorph distribution chart of interval 900-800ft of Emi-1 well, offshore Dahomey Basin, southwestern Nigeria

Zone 3: Echitricolporites spinosus/P800 Interval: 900-2600ft Age: Late Miocene

Characteristics: The base of the interval is marked by the occurrence of diagnostic forms such as Multiareolites formosus and Anthocerus sp. The interval is rich in pollen, spores, and dinoflagellates. The microflora assemblage recovered are Perigrinipollis nigericus, Retitricolporites irregularis, Retibrevitrocolporites protrudens, R. obodoensis, Verrucatosporites usmensis, Magnastriatites howardii,

Zonocostites ramonae, Monoporites annulatus and Verrutricolporites rotundiporus. Dinoflagellate cysts that characterize the interval include Lejeunecysta fallax, Operculodinium centrocarpum, Spiniferites ramosus, and Tuberculodinium vancampoae The top of the interval is defined by occurrence of Perigrinipollis nigericus. However, the top of the zone is tentatively placed at 900ft where the first analyzed sample commenced. Some of the pollen and spores reported in this interval are those that define Late Miocene sediments as established in the works of [29], [30 et al], and [31]. Therefore, the interval is conveniently dated Late Miocene age (See Figures 4 and 5).

Dep

th (f

t)

1000'

1250'

1500'

1750'

2000'

2250'

2500'

2750'

3000'

3250'

3500'

3750'

4000'

4250'

4500'

4750'

5000'

5250'

5500'

5750'

6000'

6250'

6500'

6750'

7000'

7250'

7500'

7750'

TD

Lith

olo

gy

Ch

ron

ost

ratig

rap

hy

1000

Mid

dle

- Lat

e M

ioce

ne

6500

8000

Ear

ly M

ioce

neP

erio

d/E

po

ch

Eva

my

et a

l (19

78)

1000

P820

2600

P780 -P720

6500

8000

P680

Zo

ne

Ger

mer

aad

et a

l (19

68)

1000

Ech

itric

olpo

rites

spi

nosu

s zo

ne

6500

8000

Mag

nast

riatit

es h

owar

dii z

one

Zo

ne

*1

Mon

opor

ites

annu

latu

s

Zono

cost

atite

s ra

mon

ae

2 5

10 15

1 4

12

7

1

1

2 3

2

7

10

1 1

4 15

6 19

9 83

6 56

11 80

1 1

1 40

7 183

3

1 35

5

2

3

1 50

7

4 3

4 1

4 10

1 2

1

3 4

2 5

1

2 4

4 4

4 2

3?

4

2

9

7

3 6

4 5

PollenAbsolute abundance (10mm=3 counts)

Cyp

erac

eaep

ollis

spp

.P

ereg

rinip

oilis

nig

eric

usP

olyp

odia

ceoi

spor

ites

spp.

Rau

wol

fia v

omito

riaM

ultia

reol

ites

form

osus

Gem

mat

e po

llen

Ret

imon

ocol

pite

s sp

p.

Het

eroc

olpi

tes

laev

igat

us

Ret

itric

olpo

rites

spp

.

Tric

olpo

ropo

lleni

tes

spp.

Ver

rutri

colp

orite

s ro

tund

ipor

is

Cte

nolo

phon

idite

s co

stat

usR

etitr

icol

pite

s sp

p.

Ret

itrip

orite

s sp

p.

Ech

itrip

orite

s tri

angu

lifor

mis

Ret

ibre

vitri

colp

orite

s tri

angu

latu

s

Stri

atric

olpo

rites

cat

atum

bus(

Ber

linia

)S

triam

onoc

olpi

tes

rect

ostri

atus

Psi

lam

onoc

olpi

tes

spp.

Psi

latri

colp

orite

s cr

assu

s

Can

thiu

mid

ites

spp.

Cro

totri

colp

orite

s cr

oton

oisc

ulpt

us

Pac

hyde

rmite

s di

eder

ixi

Psi

latri

colp

orite

s sp

p.

Ret

itric

olpo

rites

irre

gula

ris

Are

cipi

tes

spp.

Ech

itric

olpo

rites

spi

nosu

sP

odoc

arpu

s sp

p.R

etib

revi

trico

lpor

ites

obod

oens

is

Ret

ibre

vitri

colp

orite

s pr

otru

dens

Pol

yado

polle

nite

s va

ncam

pol

Ret

itric

olpi

tes

gage

onne

tiS

ynco

lpor

ites

spp.

Rac

emon

ocol

pite

s hi

ans

Psi

latri

colp

orite

s on

itsha

ensi

sP

roxa

perti

tes

curs

usR

etis

teph

anoc

olpi

tes

will

iam

sB

revi

colp

orite

s gu

inet

tiS

pini

zono

colp

ites

echi

natu

sN

umul

ipol

lis n

eoge

nicu

sR

etid

ipor

ites

mag

dale

rieris

isR

etitr

icol

porit

es c

rass

icos

tatu

s

1 1 3 1

1 5 1

1 4 1?

1 1

1 2 2 6 1

1 1

1 2 1 2 1

2

1

1 1 1

2 2 8 2 1 2 1

1 1

1 1 2 1 1 1 1

1 3 1

6 1 2 1 1 1 1 2 1 6

1 7 1 1 1 1 1 1 1 1 1

1 6 2 1 1 1 3

6 3 2?

1 1 2 2 2

1

2 1

5 1 1 2 1 2 3 5 1 1 1

1

1 1 1 1?

1

2

1 1

1

1 2 2 1 1 1 2 1 1

1 2 1 1

1 1 1

5 2 1 1 2

1

2 1 1

6 2 2 1 7 1 2 10 1

4

1 1 1 1 1 2

1 1

3 1 1 1 1

1 1

1 6 3 2 1 2 1 3 1

1

1 1 1 3 1 1 2 1 1

11 1 1 2 1 3 3 1 1

1 4 6 1 1 1 1 1

1

PollenAbsolute abundance (10mm=10 counts)

Fun

gal s

pore

Laev

igat

ospo

rites

spp

.

Leio

trile

tes

adrie

nsis

Mag

nast

riatit

es h

owar

dii

Sap

otac

eoid

aepo

lleni

tes

spp

Ste

reio

spor

ites

spp.

Ver

ruca

tosp

orite

s sp

p.

Cya

thid

ites

min

orS

pore

inde

term

inat

eC

orru

gatis

porit

es iv

oire

nsis

Ale

tesp

orite

s sp

p.C

rass

oret

itrile

tes

vanr

aads

hoov

eni

Sel

agin

ella

myo

soru

sV

erru

cato

spor

ites

usm

ensi

sA

cros

tichu

m a

ureu

m

10 5 4 2 3 1?

2

16 11 3 5 9

8 4 2 2 5

17 3

4

19 1

1

4

4 1 1 4 1

2

17 2

22 1 4 1?

3 1

13 3 1 5 1 1?

2 1

4 1

23 11 7 9 1

64 7 1 7 1 5

57 1 2 4 1 1 4

85 4 4 8 1 7

7

35 3 2 2

113 14 6 4 1 3

64 1?

1

10 1 2

35 1 1

9

12 1 1

6 1

45 1 1 1

7 1

10 1 1 1

4 1 2

21 7 3 3 13

4 1 1 1

3

2 1

3 2

2

1

1

9 5

15 9 1 4 7 1 6

1 1

25 4 3 6 1 1

9 2 2 4

1 1

3 1

5 3 2 1

1

3

4 2 1

16 8 10 9 2 1 4

7 1 1

29 9 2 13 3

29 25 1 13 2

5 1

1

4 1 1

5

4

Spore*2

For

amin

ifera

wal

l lin

ing

1

1

FT

Absolute abundance (10mm=10 counts)

Leio

spha

erid

ia s

pp.

Leje

unec

ysta

falla

xLi

ngul

odin

ium

mac

haer

opho

rum

Ope

rcul

odin

ium

cen

troca

rpum

Spi

nife

rites

ram

osus

Tub

ercu

lodi

nium

van

cam

poae

Din

ocys

ts in

dete

rmin

ate

Hom

otry

bliu

m fl

orip

esP

olys

phae

ridiu

m z

ohar

yl

Sel

enop

emph

ix s

pp.

Hom

otry

bliu

m s

pp.

Pro

tope

ridin

ium

spp

.S

pini

ferit

es s

pp.

Hys

trich

okol

pom

a rig

audi

aeIn

verto

cyst

a sp

p.Le

jeun

ecys

ta s

pp.

Spi

nife

rites

pse

udof

urca

tus

Brig

ante

dini

um s

pp.

Impa

gidi

nium

spp

.T

asm

anite

s sp

p.N

emat

osph

aero

psis

laby

rinth

us

1 1 1 1 3 2

2 1 4 3 2 3

2 1 1 1

1

2 1

1 8 2 1 6 1

1 1 1

2 1 1 1

1

1 1 1

1 2 1

1 1

2 1 1 2 1 1 1

1 1

1

1

2 1 1

1

1

2 1

2

1 1

1 1 1

1 1

Dinoflagellate Cysts

Tot

al c

ount

: Pol

len

150

6

7

8

12

2

7

2

1

3

18

2

8

5

22

17

15

19

1

3

23

1

5

2

2

1

12

5

3

11

1

4

32

4

7

2

7

2

20

1

12

24

16

1

Pollen

Div

ersi

ty: P

olle

n

15

4

3

5

5

2

5

1

1

3

7

2

7

3

10

11

7

8

1

2

11

1

5

1

2

1

9

4

3

5

1

3

9

1

6

2

5

2

9

1

9

9

8

1

Pollen

Tot

al c

ount

: Spo

re

10

27

44

21

20

4

20

1

4

11

2

19

28

4

27

5

51

85

70

109

7

42

141

66

13

37

9

14

7

48

8

13

7

47

7

3

3

5

2

1

1

14

43

2

40

17

2

4

11

1

3

7

50

9

56

70

6

1

6

5

4

Spore

Div

ersi

ty: S

pore

5

7

5

5

2

1

2

1

1

5

1

2

4

2

8

2

5

6

7

6

1

4

6

3

3

3

1

3

2

4

2

4

3

5

4

1

2

2

1

1

1

2

7

2

6

4

2

2

4

1

1

3

7

3

5

5

2

1

3

1

1

Spore

Tot

al c

ount

: For

am T

est L

inin

gs

10

1

1

FT

Div

ersi

ty: F

oram

Tes

t Lin

ings

1

1

1

FT

Tot

al c

ount

: Din

ofla

gella

te C

ysts

10

9

15

5

1

3

19

3

5

1

3

4

2

9

2

1

1

4

1

1

3

2

2

3

2

DC

Div

ersi

ty: D

inof

lage

llate

Cys

ts

1

6

6

4

1

2

6

3

4

1

3

3

2

7

2

1

1

3

1

1

2

1

2

3

2

DC

?Base occurrence Multiareolitesformosus

Base occurrenceCrassoretitriletesvanraadshooveni/ BaseTuberculodinium vancampoae

Comments

Dep

th (m

)

300m

350m

400m

450m

500m

550m

600m

650m

700m

750m

800m

850m

900m

950m

1000m

1050m

1100m

1150m

1200m

1250m

1300m

1350m

1400m

1450m

1500m

1550m

1600m

1650m

1700m

1750m

1800m

1850m

1900m

1950m

2000m

2050m

2100m

2150m

2200m

2250m

2300m

2350m

2400m

Well Name : EMI-1Interval Various PALYNOMORPH DISTRIBUTION CHART OF WELL EMI-1

Scale : 1:5000

EMI-1Chart date: 15 January 2009 MOROHUNKOLA ADEYEMI OLADAPO, DEPT. OF GEOLOGY, UNIVERSITY OF IBADAN, NIGERIA

CRYSTAL AGE LIMITED

LAGOS

Project

Chart

: DEMO

: Yemi Paly Chart

Base Lithology

shale/mudstone

si lty mudstone

sandy mudstone

sandy si ltstone

Lithology Qualifiers

Lithology Accessories

Lithology Stringers

IGD Boundary KeyPossible

Probable

Confident

Unconformable

? ?Unconformable

f Fault

?f ?Fault

Correlation lines

Text Keys*1 Absolute abundance (15mm=25 counts)

*2 Absolute abundance (10mm=10 counts)


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Paleoenvironment of Deposition The reconstruction of the depositional environment of the studied well is dependent on some parameters such as relative percentage of Zonocostites ramonae to grass pollen (Monoporites annulatus), Palynomorph frequency distribution, organic wall microplankton, palynomorph abundance and diversity, palynomorph assemblage, freshwater algae and lithologic characters. The relative percentage of Zonocostites ramonae (Rhizophora type) to grass pollen (Monoporites annulatus) to the total palynomorph sum is an important factor. Muller [32], and [29], indicated that high percentage of Rhizophora occur in mangrove environment and very close to marine environment. Therefore, the frequency of Rhizophora is expected to decrease towards the offshore. Hence, its quantitative distribution makes it significantly useful for environmental interpretation [33]. Zonocostites ramonae is

moderate to abundant in frequency throughout most of the sequences described but decreases down the well; suggesting mangrove environment. Percentage varies from about 2.2% to about 64.8%. However, absence or rarity of the species to increase in grass pollen (Monoporites annulatus) in some horizons indicates sediment deposition in areas away from the mangrove edge, probably an upland that is associated with unfavourable ecological conditions. Palynomorph frequency percentage distribution shows that there are more pollen and spores than dinoflagellates in the palynomorph sum in each palynozones; that is, there are more terrestrially derived miospores than marine dinoflagellates which indicates that the sources of organomaceral are plant and the environment of deposition is likely to be fluviatile to brackish systems of marginal marine system (Figure 6).

Figure 6: Histogram of frequency percentage (%) of palynomorphs in Emi- 1

Well, offshore Dohomey Basin, Nigeria.

Palynomorph marine index (PMI) is another important factor that suggests paleoenvironment of deposition. The PMI shows environmental variation from freshwater through brackish to typical marine setting at the top. The interval 1720-1780ft contains more freshwater forms in association with PMI forms such as Tuberculodinium vancampoae, Operculodinium centrocarpum, Selenopemphix sp,

Leiosphaeridia sp, Histrichokolpoma rigaudiae, and Lejunecysta sp which are indicative of inner neritic to outer neritic setting {[34], [35], [31]}. However, the co-occurrence of the freshwater algae and PMI is suggestive of admixture of freshwater and saline water resulting to a fluviomarine depositional environment formed by prograding sedimentational system (Figures 7 and 8).

0102030405060708090

100

Frequency (%)

900-2600 2600-6500 6500-8000

Depth

Frequency (%) Distribution of Palynomorphs

Groups

%spores

%pollen

%dinoflagelates


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Figure 7: Depth versus PMI values, Diversity and Abundance of EMI-1 well


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Figure 8: Depth versus Diversity of Pollen, Spores and Dinoflagellates

The underlying older sequence (6500-8000ft) is mainly fluviatile system; containing more of terrestrially derived and freshwater forms with few dinoflagellate cysts compared with overlying intervals (Figures 7 and 8). However, the palynomorph abundance and diversity plot complements other paleoenvironmental indices; this tends to show the effect of the sea level changes and its resultant effect on plant growth and distribution. The periods of high palynomorph abundance and diversity (Figures 7, 8 and 9) may suggest rise in sea level, leading to landward marine incursion, while a low palynomorph abundance and diversity suggests drop in sea level and adverse ecological conditions. The presence and association of some environmentally diagnostic miospores such as

Crassoretitricolporites vanraadshooveni, Zonocostites ramonae, Magnastriatites howardii, Peregrinipollis nigericus, Verrutricolporites rotundiporus and Pachydermites diederixi are indicative of coastal plain environment, while the presence of Botryococcus brunii- colonial alga which produces a biomarker- botrococcane could suggests two things; either the environment indicates freshwater input (fluviomarine), or as a biomarker which suggests that oil production could be generated from the interval if subjected to adequate temperature [36].


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Figure 9: Depth versus Total Pollen, Total Spores and Total Dinoflagellates

CONCLUSION Palynological investigations of Emi-1 well located in the deep offshore Eastern Dahomey Basin resulted in the establishment of lithofacies units, palynological zones, ages and paleoenvironment of deposition of the studied stratigraphic interval (900-8000ft). The litho-section was informally sub-divided into four lithfacies units varying from sandy shale, silty shale, through shale to clayey shale. They are generally dark gray in colour, blocky to fissile, calcareous, fossiliferous, micaceous and ferruginized at various stratigraphic horizons. The zones established include Magnastriatites howardii/P680 Zone 1, characterized by quantitative base occurrence of Pachydermites diederixi, Magnasriatites howardii and Racemonocolpites hians, dated Early Miocene; Crassoretitriletes vanraadsooveni/P700 Zone 2 marked by co-occurrence of Crassoretitrilets vanraadsooveni, Verruticolporites rotundiporus and Belskipollis elegans, dated Middle Miocene and Echitricolporites spinosus/P820 Zone 3, characterized by co-occurrence of Multiareolites formosus and Anthocerus sp dated Late Miocene. The paleoenvironment of deposition varies from fluviatile at the base through marginal marine to typical marine setting

strongly influenced by the relative abundance of terrestrially derived miospores and palynomorph marine index (PMI). This work could help in optimizing hydrocarbon exploration and exploitation through the use of the chronostragraphic framework generated and adequate correlation of the study with other well drilled nearby.


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PLATE 1

1 2 3 4

5 6 7 8

9 10 11 12

13 14 15 16

All magnification at ×800 Plate 1 1 Zonocostites ramonae 2,7 Laevigatodporites sp 3 Pachydermites diederixi

4 Fungal spore

5 Retibrevitricolporites protrudens

6 Verrutricolporites rotundiporus

8 Achrostichum aureum

9 Spiniferites ramoxux

10 Magnastriatites howardii

11 Operculodinium vancampoae

12 Histrichokolpoma rigaudae

13,14 Lingulodiniummachaerorophorum

15 Tuberculodinium vancampoae

16 Retitricolporites irregularis


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PLATE 2 All magnification at ×800

1 2 3 4 5 6

7 8 9 10 11

12 13 14 15 16

17 18 19 20 21

Plate 2

All magnification at ×800

1 Monoporites annulatus

2 Crassotriletes vanraadshooveni

3 Botryococcus brunii

4 Polysphaeridium zoharyi

5 Nymphae lotus

6 Anthocerus sp

7 Impagidinium sp

8,12 Psilatricolporites crassus 9 Canthimidites sp 10 Tricolporites sp

11 Striamonocolpites sp

13 Multiareolites formosus 14 Belkipollis elegans

15 Verrucatosporites sp

16 Pachydermites diederixi 17 Anthocerus sp


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18 Microforaminiferal wall lining

19 Nymphae lotus

20 Elaies guineensis

21 Taxodinum sp

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Documents

INTERNATIONAL JOURNAL OF SCIENTIFIC & TECHNOLOGY … · 2013. 1. 29. · by Cretaceous and Tertiary sediments. Seven lithostratigraphic sequences were recognized in the basin which