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Petroleum of the Deep: Palynological proxies for palaeoenvironment of deep offshore upper Miocene-Pliocene sediments from Niger Delta, Nigeria

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Better understanding of the palaeoenvironments under which the lithologies of the deepwater petroleum systems were deposited is necessary to unravel the problem surrounding the deep offshore petroleum exploration and production. Therefore, the integration of palynological, lithological and gamma ray log data of ditch-cutting samples from wells A and B from the Niger Delta region are utilized to delineate the upper Miocene-Pliocene depositional environments. The detailed palynological analysis revealed diverse and abundant palynomorph assemblages, which consisted of angiosperm pollen 85.7%, monolete spores 5%, fungal elements 4%, trilete fern spores 4%, freshwater algae 1% and marine elements 0.3%. Eight informal palynological assemblage zones (PAZ I–PAZ VIII) with corresponding eustatic sea level changes are delineated in Wells A and B. Four lithofacies, namely sandstones, siltstones, claystones and mudstones, are recognized in association with three depositional environments in the studied wells. Distributary channels are characterized by the erosive base and filled with moderate to fine, uniform and blocky sand-grain size sediments that are of good reservoir quality. Mud-rich sediments, which are of excellent sealing rock potential, capped this sand formation. Moreover, tidal channels are typified by the erosive base and filled with fining-upwards sand sequences with tops covered by muddy sediments. Finally, the regressive barrier sands are filled by coarsening-upwards sediments with basal organic-rich deposits that are likely to be good quality source rock. The oil potential of these sites is of interest to the oil company and the reconstructed palaeoenvironments will be useful for deepwater explorationandexploitation,andprobably remove or minimize the risks that arecommonlyinvolved in this task.
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Petroleum of the Deep: Palynological proxies for
palaeoenvironment of deep offshore upper
Miocene-Pliocene sediments from Niger Delta, Nigeria
Moshood A. Olayiwola1,2*& Marion K. Bamford1
1Evolutionary Studies Institute (ESI), and School of Geosciences, University of the Witwatersrand, Private Bag 3,
WITS 2050, Johannesburg, South Africa
2Natural History Museum, Obafemi Awolowo University, Ile-Ife, Osun State, Nigeria
Received 21 September 2015. Accepted 1 March 2016
INTRODUCTION
The pursuit of petroleum resources has driven explora-
tion and production from onshore terrains into offshore
environments, which are geographically and geologically
complex.
Deep-water exploration and production is a high-risk
venture (Fadiya 2014). These include harsh offshore envi-
ronments such as severe weather, ice and storms that pose
engineering challenges to offshore drilling equipment,
and risk due to incidents that may represent challenges
for emergency preparedness. In addition, there are risks
due to major hazards, occupational injury and illness
(Vinnem 2010). The extremely high cost of maintenance of
drillship/semisubmersibles, installation, and the decom-
missioning of the offshore structures also pose another
problem to the deep-water exploration and production
(Saifullah & Pudjihanto 1996; Byrd et al. 2014). Target
missing and subsea blowout due to overpressured shale
during drilling further complicates the risk and expenses
for the oil and gas industry in the offshore terrain. The
2010 deepwater horizon drilling rig explosion and oil
spills in the Gulf of Mexico are a typical example of this
scenario, where many lives were lost and several billions
of dollars were spent to stop and clean oil spillage.
Therefore adequate knowledge of the palaeoenvironment
ofthepetroleumsystem,suchassource,reservoir, and cap
or seal rocks of the deep offshore area, is essential to guar-
antee the safety of the rig personnel and marine life in par-
ticular, and to reduce the overall cost of deep offshore
exploration and production (Magoon & Dow 1994;
Magoon & Schmoker 2000). In order to achieve this goal, it
is imperative for us to have a better understanding of the
environment under which the lithologies of this petro-
leum system were deposited.
The common deep offshore petroleum system was the
shale-sandstone interaction where shale formed the
source and seal rocks and sandstone formed the reservoir
rock. The Gulf of Mexico (McDade et al. 1973; Hood et al.
2002), Niger Delta (Ekweozor & Daukoru 1994; Kulke
1995;Tuttle et al. 1999) and Bengal (Shamsuddin et al. 2001)
basins are typical examples of this system. However, lime-
stone occasionally can serve as both cap rock, when its
matrix is tightly connected, and reservoir rock when it is
fractured (Shamsuddin et al. 2001). Evaporite is, in addi-
Better understanding of the palaeoenvironments under which the lithologies of the deepwater petroleum systems were deposited is
necessary to unravel the problem surrounding the deep offshore petroleum exploration and production. Therefore, the integration of
palynological, lithological and gamma ray log data of ditch-cutting samples from wells A and B from the Niger Delta region are utilized
to delineate the upper Miocene-Pliocene depositional environments. The detailed palynological analysis revealed diverse and
abundant palynomorph assemblages, which consisted of angiosperm pollen 85.7%, monolete spores 5%, fungal elements 4%, trilete
fern spores 4%, freshwater algae 1% and marine elements 0.3%. Eight informal palynological assemblage zones (PAZ I–PAZ VIII) with
corresponding eustatic sea level changes are delineated in Wells A and B. Four lithofacies, namely sandstones, siltstones, claystones
and mudstones, are recognized in association with three depositional environments in the studied wells. Distributary channels are
characterized by the erosive base and filled with moderate to fine, uniform and blocky sand-grain size sediments that are of good
reservoir quality. Mud-rich sediments, which are of excellent sealing rock potential, capped this sand formation. Moreover, tidal
channels are typified by the erosive base and filled with fining-upwards sand sequences with tops covered by muddy sediments.
Finally, the regressive barrier sands are filled by coarsening-upwards sediments with basal organic-rich deposits that are likely to be
good quality source rock. The oil potential of these sites is of interest to the oil company and the reconstructed palaeoenvironments will
beusefulfordeepwaterexploration and exploitation,andprobably remove or minimizetherisksthatare commonly involvedinthistask.
Key words: palynological proxies, palaeoenvironments, lithofacies, deep offshore, ditch-cuttings, late Miocene-Pliocene, petroleum
systems, Niger Delta
Palaeontologiaafricana 2016. ©2016 Moshood A. Olayiwola & Marion K. Bamford. This is an open-access article published under the Creative Commons Attribution 4.0
Unported License (CC BY4.0). To view a copy of the license, please visit http://creativecommons.org/licenses/by/4.0/. This license permits unrestricted use, distribution,
and reproduction in any medium, provided the original author and source are credited. This item is permanently archived at: http://wiredspace.wits.ac.za/han-
dle/10539/19865
Palaeontologia africana 50: 31–47 — ISSN 2410-4418 [Palaeontol. afr.] Online only
Permanently archived on the 18th of March 2016 at the University of the Witwatersrand, Johannesburg,
South Africa (http://wiredspace.wits.ac.za/handle/10539/19865)
tion, a potential deep-water reservoir rock (Hood et al.
2002). The shale-sandstone facies which generally make
up the petroleum system elements occur either as
transgressive-regressive sequences during the gradual
rise and fall of the sea level relative to the land, or as
turbidite deposits in deepwater offshore (Tuttle et al. 1999;
Hood et al. 2002).
Duringthesealevelrisethefining-ordeepening-upward
sediment sequences will be formed in the subaerial delta
plain as channels, inter-distributary bay, crevasse splay
and levee deposits (Horne et al. 1978; Coleman & Prior
1981). In contrast, coarsening- or shallowing-upward
sediment sequences will be deposited during sea-level fall
in the subaqueous delta plain that comprises distributary-
mouth-bar, river-mouth tidal-ridge and subaqueous
slump deposits (Coleman & Prior 1981). High representa-
tion of coastal miospores such as those from mangrove,
beach, brackish and rainforest environments compared to
minimal hinterland miospores, i.e. savanna and montane
types , characterize subaerial sediments that were depos-
ited during sea level rise; the opposite represents sea level
fall (Morley 1995; Rugmaia et al. 2008).
Variations in the abundance of some spore and pollen
taxa relate well to changes in eustatic sea-level and hence
palynological analyses offer a proxy for the understand-
ing of sea-level rise and fall (Poumot 1989; Morley et al.
2004). For instance, low eustatic sea level that is correlated
with dry climatic conditions is characterized by an abun-
dance of Poaceae pollen whereas mangrove pollen is pre-
dominant during transgressive-highstand that suggests
humid climatic intervals (Morley & Richard 1993). For
instance, low eustatic sea level that is correlated with dry
climatic conditions is characterized by an abundance of
Poaceaepollen,whereasmangrovepollenispredominant
during transgressive-highstand that suggests humid
climatic intervals (Morley & Richard 1993), or retreat of
mangroves with rising sealevel and humid conditions
(Ellison 1993, 2008).
Many other authors have also successfully employed
the relative diversity and abundance of palynomorphs
that make up the lithologies of different petroleum
systems to reconstruct the palaeoenvironment of their
deposition in different basins worldwide. Prominent
among these are the Niger Delta Basin (Oboh 1992, 1995;
Oboh et al. 1992; Durugbo et al. 2010), Sergipe Basin
(Carvalho 2001, 2004), Barmer Basin (Tabaei & Singh
2002), McMurray Formation (Demchuk et al. 2008), Bornu
Basin (Ola-Buraimo & Boboye 2011; Ayinla et al. 2013),
Anambra Basin (Ola-Buraimo & Akaegbobi 2012), Dahomey
Basin (Durugbo & Aroyewun 2012; Adeigbe et al. 2013),
Larsen Basin (Carvalho et al. 2013) and the Gulf of Mexico
(Barron & Oboh-Ikuenobe 2014). In particular, Oboh
(1992) used palynomorphs from the middle Miocene
onshore Kolo Creek Field to show that the depositional
conditions were similar to today’s but the climate was
somewhatdrier. Pollenanddinoflagellatecystsfromcores
from the western Niger Basin (Durugbo et al. 2010)
showed that the Plio-Pleistocene (5.0–1.3 Ma) environ-
ment was mostly dry and there were two cooling events,
possibly related to global glacial events, at –2.7 Ma and
2.0 Ma. In depth correlations by Evamy and colleagues
(1978) and Morley & Richard (1993) are used as the frame-
work for correlations in this project.
Data other than palynology have also been helpful for
palaeoenvironmental reconstructions (Morley 1986). For
example, gamma ray log and lithological data have been
employed to delineate sub-environments of deposition
such as distributary channel fill, lagoon and tidal delta,
coastal barrier and delta fringe in the Niger Delta Basin,
Nigeria (Oboh 1990, 1991, 1992, 1993, 1995). Seismic data is
another useful tool for delineating the depositional envi-
ronment (Owoyemi & Willis 2006).
The value of palaeoenvironmental interpretation in oil
exploration is immense and is a major contributor to the
characterization of facies, which may be of reservoir,
source and seal rock potentials. Therefore palaeoecological
studies will significantly increase the precision and reso-
lution of the palaeoenvironmental interpretation that can
provide templates for the distribution of lithologies and
reservoir qualities to aid in the vertical and horizontal
planning of wells’ development (Bann et al. 2004; Powell &
Riding 2005; Gregory et al. 2007).
Most previous research has been concentrated on the
onshore deposits. The aim of this study, therefore, is to use
the integration of palynological, lithological and gamma
ray log data of ditch-cutting samples from Wells A and B
from the deep offshore Niger Delta to delineate the past
depositional environments or shoreline positions. This
will provide a better understanding of the complex nature
of the subsurface geology that is usually associated with
the deep offshore settings. The findings of this study will
enhance the exploration and exploitation of hydrocarbon
in particular, and basin analysis in general, in the deep-
water environments.
Geological setting of the study area
The study area is situated within the deep offshore
(>200m)part of the Niger Delta Basin that is located in the
Gulf of Guinea, on the edge of the West African continental
margin (Doust & Omatsola 1990). This basin lies between
latitudes 2° and 7° N and longitudes 3° and 9° E and it is in
contact with the older Cretaceous sedimentary rocks in
the north, the 4000 m (13 100 ft) bathymetric contour line
in the south, and the Benin and Calabar Flanks in the west
and east, respectively (Tuttle et al. 1999; Fig. 1). This basin
originated because of the tectonic activities that led to the
opening of the South Atlantic in the Gulf of Guinea in the
Late Jurassic-Cretaceous period. This is evidenced by the
fractured zones along the oceanic crust, e.g. the Charcot
fracture zone, expressed as trenches and ridges that are
associated with the basin (Wolak 2011; Fig. 1).
Stratigraphically, the Niger Delta is made up of three
diachronous formations, namely, in descending order:
Benin Formation (continental sediments), Agbada Forma-
tion (transitional sand/marine sediments) and Akata
Formation (marine sediments) (Short & Stauble 1967;
Knox & Omatsola 1989; Doust & Omatsola 1990; Fig. 2).
These formations are filled with time-transgressive regres-
sive sequences of siliciclastic sediments that prograded
basinward, forming depo belts, namely, Northern Delta,
32 ISSN 2410-4418 Palaeont. afr. (2016) 50: 31–47
ISSN 2410-4418 Palaeont. afr. (2016) 50: 31–47 33
Figure 1. Location of the study area within the MPNU acreage in the southeastern offshore Niger Delta (modified after Wolak 2011).
Figure 2. Stratigraphic equivalence of facies in the subsurface Niger Delta with the inferred locations of Wells A and B (modified after Wright et al.
1985).
Greater Ugheli, Central Swamp, Coastal Swamp, Shallow
Offshore and Deepwater.
METHODS AND MATERIALS
Fifty and forty ditch-cuttings samples with Gamma ray
logs of sedimentary rocks from Wells A and B, respec-
tively, were acquired from the deep offshore Niger Delta,
Nigeria. The names and locations of these wells are confi-
dential for proprietary reasons. Therefore, these wells
have been code-named Wells A and B and are located
within the Mobil Producing Nigeria Unlimited (MPNU)
acreage in the southeastern offshore Niger Delta (Fig. 1).
MPNU acreage has been previously described (see
Olayiwola & Bamford 2016). The Department of
Petroleum Resources (DPR) and National Petroleum
Investment and Management Services (NAPIMS)
granted permission to MPNU to provide the material for
the study. These acquired ditch-cuttings samples are
mostly paralic Agbada Formation sediments while a few
are likely to be continental Benin Formation sediments
(Fig. 2).
Sample preparation for palynological data
Ninety samples from Wells A and B were used for this
study. Ditch-cuttings samples are usually loose sediments
and hence crushing was not needed. These samples were
processed for palynomorphs following standard proce-
dures(Faegri&Iversen1989),whichincludestheaddition
of HCl, HF, KOH and heavy liquid separation within a
saturated ZnCl2solution. Identification, counting and de-
scription were performed with a light microscope under
×400 and ×1000 magnification with the aid of reference
photomicrographs published in the literature, namely:
Clarke (1966); Hopping (1967); Germeraad et al. (1968);
Legoux (1978); Salard Cheboldaeff (1979, 1990); Oboh &
Salami (1989); Moore et al. (1991); Wood et al. (1996);
Traverse (2007) and Durugbo et al. (2010). Approximately
250 pollen grains spores are counted per sample, exclud-
ing marine palynomorphs.
Species identification and pollen diagram
Theidentified palynomorphs are shown in Figs 3–5 with
ranges and zones in Figs6&7which are based on previous
workbyEvamyetal.(1978)andMorley&Richards(1993).
The percentage abundance and the microfloral distribu-
tion trends of Wells A and B are illustrated in the Tilia
(version 1.7.16) pollen diagrams (Grimm 1987; Locatelli
2011; Figs 8 & 9).
Pollen diagrams are divided into palynological assem-
blage zones (PAZ) for ease of interpretation. PAZ is a
reflection of palynomorph compositions that can be inter-
preted by following an individualistic approach or CONISS
(Olayiwola & Bamford 2016). Recovered pollen and
spores were classified into ecological groups, namely,
mangroves, brackish water swamp, freshwater swamp,
lowland rainforest, savanna, freshwater algae, fungal
elementsand marine origin species (Table1;Figs6&7).The
classification of palynomorphs into different ecological
groups was based on the general assumption that nearly
all the botanical families that existed during Paleogene
times have an affinity with their present-day living rela-
tives (Muller 1981).
The palaeoenvironment is herein reconstructed by
reference to the models of Posamentier et al. (1988) as
modified by Poumot (1989) and Morley (1995), which
assumed landward shifting of coastlines during sea level
rises and resultant deposition of marine sediments in the
subaerial delta plain. There was also shifting of the
mangrove and other coastal swamp plant belts during
these periods due to their preference for saline water.
Therefore, high representation of mangrove, other coastal
swamp plants (from beach, brackish, freshwater swamp,
rainforest and palm) miospores, fungal elements, fresh-
water algae and marine origin species characterized this
depositional environment (Poumot 1989; Morley 1995;
Morley et al. 2004; Rugmaia et al. 2008). In contrast, as sea
level fell the coastline shifted basinward and the shelf area
which was initially covered by marine water, was exposed
and probably incised due to erosion caused by fluvial
activities. Deposition of terrestrial sediments occurred in
the subaqueous delta plain and was characterized by
widespread savanna vegetation belts. Maxima spectra of
savanna and montane pollen characterized this deposi-
tional environment (Poumot 1989; Morley 1995; Morley
et al. 2004; Rugmaia et al. 2008).
Palynostratigraphy
The palynostratigraphical biozonation of Wells A and B
are based on the recovered floral events and diagnostic
bio-event species as defined by Evamy et al. (1978) and
Morley & Richards (1993) (Figs 6 & 7). Evamy et al. (1978)
used pollen abundance and diversity to subdivide the
Palaeogene-Neogene periods of the Niger Delta Basin
into eight super zones comprising P200–P900 Zones.
Moreover, sixteen palynological zones were further
erected for the Middle Eocene to Plio-Pleistocene segment
of the Niger Delta Basin (G1–J3) by Legoux (1978); Morley
& Richards’ (1993) floral zonation scheme further subdi-
vided the Miocene-Pleistocene epochs into 21 floral
zones.
Sample preparation for lithological data
This analysis was carried out to provide the detailed
lithological description of the strata penetrated by Wells A
and B. An average of 80 g of each of the ditch-cutting
samples were soaked in 250 ml aluminum containers with
hot water and Sunlight™ liquid detergent for about 24
hours to disaggregate the material. The soaked samples
were efficiently washed under a distilled water nozzle tap
using a 63 µm mesh sieve. The retained samples on 63 µm
sieve were dried over hot plates and enclosed in well-
labelled drug dispensing cellophane bags.
The lithologic description followed a standard laboratory
procedure whereby the washed samples, which were
spread on a black anodized aluminum foraminiferal pick-
ing tray, were viewed using the binocular reflected light
microscope. Gamma-ray and resistivity logs enhanced
descriptions, since high and low values of GR-log and
RES-log signified shale and sand lithologies, respectively
(Adegoke 2002). The essential parameters studied were
34 ISSN 2410-4418 Palaeont. afr. (2016) 50: 31–47
grain size, textures and facies changes. These lithological
data were plotted, with the aid of SEDLOG™ software, to
generate the vertical lithofacies profiles that were embed-
ded in Figs 8 & 9.
Gamma ray logs (GR-logs) data
Acquired GR-logs from Wells A and B are utilized for the
understanding of depositional environment changes.
Natural radioactivity of rocks, which can be determined
by GR-logs, shows generally a close relationship to grain
size. The naturally radioactive elements are normally con-
centrated in shales (clay-size grains; Rider 1990). Clay is
herein used in a textural sense, i.e. grain size less than
0.002 mm, which can also be used to describe lithologies,
following the format of Rider (1990). Grain size and bed-
ding of sedimentary rocks have an influence on the shape
of the gamma ray log curve (https://www.spec2000.net/2-
1-strat1.htm-01/12/2015). Therefore, GR-log allows the
qualitative distinction of zones of shale lithology (high
gamma values) from sand lithology (low gamma values).
The values of these GR-logs ranged from 0–150 API units
(American Petroleum Institute).
There is usually no fundamental relationship between
gamma ray log data and the palynology. However, the
palynological assemblages and gamma ray log exhibit
cyclic changes, which have been used previously to corre-
late the sedimentary evolution (Sarah et al. 1995; Ziegler
et al. 1997; Wagstaff et al. 2006; Adebayo 2014).
Hence, gamma ray curve shapes in association with
vertical lithological profiles are used to subdivide the
main subaerial and subaqueous delta plains, previously
defined by palynomorph assemblages, into sub-environ-
ments (Selley 1985; Oboh 1995). The trend of changes of
GR-log curve is herein used for the interpretation of the
sedimentary depositional environments (Figs 8b & 9b).
RESULTS
The top of Well A is 716 m below sea level and the base is
3024 m below so the core represents 2308 m. Well B is
presumably farther from shore and its top is 1472 m below
sea level and extends down to 2542 m, so the core is 1070 m
long. Nonetheless, the cores represent more or less the
same time frame (based on the palynology) although
core A is more than twice the length of core B. The core
material from the two wells will be presented and dis-
cussed together.
Palynology
Thirty-one and 29 palynomorph species belonging to 33
generaare recorded in Wells A and B,respectively, and the
palynomorph assemblages are dominated by angiosperm
pollen that comprises 85.7% (Figs 8a & 9a). Monolete
spores (5%), fungal spores (4%), trilete fern spores (4%),
freshwater algae (1%) and marine palynomorphs (0.3%)
followed this in abundance.
Palynostratigraphy
Since the palynomorphs recovered from Wells A and B
were the same as those described by Evamy et al. (1978)
and Morley & Richard (1993), their zonation schemes are
used here to determine the age of the strata as well as
the palaeoenvironment. The palynostratigraphic bio-
zonation of the wells was defined using the recovered
diagnostic markers and ecologically significant species.
The first two informal subzones, which are established in
these wells, are correlated with P860–P870 Subzones of
Evamy et al. (1978) (Figs 6 & 7).
P800 Zone and subzone P860 (late Miocene)
The P860 Subzone is the older subzone of zone P800 in
Wells A and B with the tops marked at 2548 m and 2185 m,
respectively, by the quantitative base occurrence of
Retistephanocolpites gracilis. The bases were tentatively
marked at 3024 m and 2542 m which are the deepest
samples analysed in Wells A and B, correspondingly. This
subzone may continue below. The continuous co-occur-
rences of Gemmamonoporites sp., Nymphaeapollis clarus,
Peregrinipollis nigericus, Stereisporites sp. and Cyperaceaepollis
sp. characterized this subzone and, therefore, their pres-
ence indicates a late Miocene age according to the
schemes of Evamy et al. (1978) and Morley & Richard
(1993).
P800 Zone and subzone P870 (early to late Pliocene)
P870 was the youngest subzone documented in the
analysed intervals of Wells A and B (Figs 6 & 7). The tops of
the subzone were tentatively marked at 716 m and 1472 m
which are the depths of the first samples analysed in these
wells, respectively. The bases are marked by the quantita-
tive base occurrence of Retistephanocolpites gracilis at
2548 m and 2185 m in Wells A and B, respectively.
Co-occurrence of Gemmamonoporites sp. and Retistephano-
colpites gracilis together with regular occurrences of
Nymphaeapollis clarus and the absence of Podocarpus
milanjianus confirmed early–late Pliocene age in this
subzone.
Floral Zonation for Wells A and B
Secondly, the P860 and P870 Sub-zones of Evamy et al.
(1978) in Wells A and B are further subdivided into seven
floral zones of Morley & Richards (1993). P1 (914–716 m),
Upper P3–Upper P2 (1487–914 m), Lower P3
(1679–1487 m), P4 (2045–1679 m), P6–P5 (2393–2045 m), P7
(2548–2393) and M1 (3024–2548 m) comprised floral zones
in Well A (Fig. 6) while Well B comprised Upper P3
(1609–1472 m), Lower P3 (1692–1609 m), P4 (1774–1692 m),
P6–P5 (1993–1774 m), P7 (2145–1993 m), M1 (2350–2145 m)
and M2 (2542–2350 m) floral zones (Fig. 7).
Palynological, lithological and gamma ray data and the
palaeoenvironmental reconstruction
Thirdly, eight informal palynological assemblage zones
and eustatic sea level changes are recorded in Wells A and
Bwhich are designated as PAZ-1toPAZ-VIIIfromthebase
to the top (Figs 8 & 9). Four lithofacies comprised of sand-
stones, siltstones, claystones and mudstones are well
represented in Well A, whereas mudstones are predomi-
nant in Well B. Eight and five depositional sequences
accompanied by nine and seven inferred depositional
environments are recognized in Wells A and B, respec-
ISSN 2410-4418 Palaeont. afr. (2016) 50: 31–47 35
36 ISSN 2410-4418 Palaeont. afr. (2016) 50: 31–47
Figure3.Commonpalynomorphs from Wells A and BfromNiger Delta. (a)Nympheaepollis clarus;(b)Pachydermitesdiederixi;(c)Peregrinipollis nigericus;
(d)Proteacidites cooksonni;(e)Proxapertites cursus;(f)Psilatricolporites crassus;(g) Psilamonocolpites sp.; (h)Psilastephanocolpites sp.; (i)Psilatricolpites
operculatus; (j)Retitricolporites irregularis; (k)Retimonocolpites sp.; (l)Retibrevitricolporites obodoensis.
ISSN 2410-4418 Palaeont. afr. (2016) 50: 31–47 37
Figure 4. Common palynomorphs from Wells A and B from Niger Delta. (a)Acrostichum aureum;(b)Crassoretitriletes vanraadshooveni;(c)Laevigato-
sporites sp.; (d)Polypodiaceoisporites sp.; (e)Stereisporites sp.; (f)Verrucatosporites sp.; (g)Arecipites exilimuratus;(h)Chenopodipollis sp.; (i)Crototricolpites
densus; (j)Cyperaceaepollis sp.; (k)Gemmamonoporites sp.; (l)Monoporites annulatus.
38 ISSN 2410-4418 Palaeont. afr. (2016) 50: 31–47
Figure 5. Common palynomorphs from Wells A and B from Niger Delta. (a)Retistephanocolpites gracilis (b)Striatricolpites catatumbus;(c)Zonocostites
duquei;(d)Zonocostites ramonae;(e)Botryococcus braunii;(f)Concentricytes circulus;(g)Pediastrum sp.; ( h) Diatom frustule 1; (i) Diatom frustule 2;
(j) Scolecodont sp.; (k)Distephanus boliviensis (Silicoflagellate sp. 1); (l)Naviculopsis robusta (Silicoflagellate sp. 2).
tively (Figs 8 & 9). These PAZs with their allied litho-
logies and likely inferred depositional environments are
described below.
It should be noted that the ecological interpretations of
the palynomorphs by Germeraad et al. (1968), Evamy et al.
(1978) and Morley & Richards (1993), who have worked
on the Niger Basin, have been followed here. There are
other interpretations, however, for example, palms pro-
ducing Arecipites pollen can grow along the beach
(Povilaskas 2013) and inland along water courses.
Verrucatosporites spp. represent several families of ferns
(Stuchlik 2001). Cyperaceae grow in a range of poorly
drained soils from permanent wetlands to dambos to
patches in savanna and brackish wetlands. The
Chenopodiaceae also grow in a variety of habitats but
overall they indicate dry environments
PAZ-IA (2728–2289 m) and PAZ-IB (2542–2516 m); P860
Subzone (late Miocene)
PAZ-IA (from Well A) and -IB (from Well B; Figs 8 & 9) are
the basal parts of the studied sections of Wells A and B.
These palynofloral assemblages are correlated with P860
Subzone (of Evamy et al. 1978). Hence, the age of these
palynozones are likely to be late Miocene. PAZs-IA and IB
bases are defined by the first quantitative base increase of
Zonocostites ramonae. The tops of this zone are marked by
the first quantitative decrease of Zonocostites ramonae and
increase of Psilatricolporites crassus,Monoporites annulatus
and Verrucatosporites sp. PAZ-IA and 1B are dominated by
mangrove, freshwater swamp, freshwater algae and
marine palynomorphs (Figs 8a & 9a; Table 1). The rare
occurrence of savanna pollen, fungal elements and low-
land rainforest characterized this zone. Mudstone
lithologies and bell-shaped gamma-ray curves are recorded
for this zone.
PAZ-IIA (2899–2469 m) and PAZ-IIB (2515–2457 m);
P860 Subzone (late Miocene)
These palynofloral zones are located within the upper-
most parts of P860 Subzone of the analysed interval of
ISSN 2410-4418 Palaeont. afr. (2016) 50: 31–47 39
Figure 6. Chart showing palynomorph zones and subzones recognized in well A based on the frameworks of Evamy et al. (1978) and Morley &
Richard (1993).
Wells A and B and occurred within the uppermost late
Miocene. PAZ-IIA and IIB (Figs 8 & 9) bases are marked by
the first quantitative decrease of Zonocostites ramonae and
increase of Psilatricolporites crassus,Monoporites annulatus
and Verrucatosporites sp., while the quantitative increase
of Zonocostites ramonae and the quantitative decrease of
Psilatricolporites crassus,Verrucatosporites sp. and
Monoporites annulatus defined the tops. These zones are
characterized by the predominance of Monoporites
annulatus,Cyperaceaepollis sp. Verrucatosporites sp. and
Acrostichum aureum with reduced proportions of
Zonocostites ramonae, beach and brackish miospores, algae
and marine palynomorphs (Figs 3–5). Sandstone
lithologies and boxcar gamma ray signatures are recog-
nized in these zones.
PAZ-IIIA (2469–2335 m) and PAZ-IIIB (2463–2350 m);
P870 Subzone (early Pliocene)
These zones that are located within P800 Zone and
occurred within the early Pliocene (Zanclean) period of
the analysed interval of Wells A and B. Base boundaries of
these palynofloral zones are marked by the quantitative
increase of Zonocostites ramonae and the quantitative
decrease of Psilatricolporites crassus,Verrucatosporites sp.
and Monoporites annulatus, while the quantitative decrease
of Verrucatosporites sp. and quantitative decrease of fungal
elements defined their tops. The overall prevalence of
mangroves, Laevigatosporites sp., Verrucatosporites sp. and
fungal elements, with reduced percentages of savanna
palynomorphs, characterized these palynological zones.
In these zones mudstone facies were recorded in Well A,
whilesandstones were upgraded to siltstone lithologies in
Well B, with bell and funnel-shaped gamma ray curves,
respectively, in these wells.
PAZ-IVA (2335–2155 m) and PAZ-IVB (2350–2179 m);
P870 Subzone (early Pliocene)
These zones occurred in zone P800 within the early Plio-
cene (Zanclean). The base boundaries of these palyno-
floral zones are defined by the quantitative decrease of
40 ISSN 2410-4418 Palaeont. afr. (2016) 50: 31–47
Figure 7. Chart showing palynomorph zones and subzones recognized in Well B based on the frameworks of Evamy et al. (1978) and Morley &
Richard (1993).
Verrucatosporites sp. and fungal elements, with the upper
boundaries marked by the quantitative decrease of
Psilatricolporites crassus,Monoporites. annulatus, peak of
Verrucatosporites sp. and the peak of fungal elements. This
palynofloral zone was characterized by the dominance of
savanna, spores (Verrucatosporites sp. and Acrostichum
aureum) and freshwater algae with corresponding low
occurrences of mangrove, brackish water swamp, fresh-
water swamp, and fungal elements. Mudstones with
intermittent claystones were deposited and accompanied
with bell-shaped gamma ray signatures (Figs 8 & 9).
PAZ-VA (2155–1798 m) and PAZ-VB (2179–1996 m);
P870 Subzone (early Pliocene)
Thesepalynofloralassemblages are correlated with P870
Subzone within the early Pliocene (Zanclean stage).
PAZ-VAandVB(Figs8&9)bases are defined by the quan-
titative decrease of Psilatricolporites crassus,Monoporites
ISSN 2410-4418 Palaeont. afr. (2016) 50: 31–47 41
Table 1. Distribution of recovered palynomorphs in Wells A and B.
annulatus, peaks of Verrucatosporites sp. and fungal spores,
while the quantitative peak of Zonocostites ramonae, quan-
titative increase of Monoporites annulatus and fungal
spores marked their tops. PAZ-VA and VB are dominated
by mangrove, brackish water swamp, freshwater swamp,
freshwater algae, fungal elements and marine paly-
nomorphs with the rare occurrence of savanna. Mudstones
upgraded to sandstones are recognized in Well A, while
mudstones facies are recorded in Well B. The accompa-
nied gamma ray curves recorded in these zones are boxcar
and bell-shaped in Wells A and B, respectively.
PAZ-VIA (1798–1469 m) and PAZ-VIB (1996–1710 m);
P870 Subzone (mid-Pliocene)
These palynofloral zones are located in the middle parts
of P870 Subzone of the analysed interval of Wells A and B
42 ISSN 2410-4418 Palaeont. afr. (2016) 50: 31–47
Figure 8. Chart showing integration of palynological, lithological and gamma ray log data with inferred depositional environments in Well A.
(Figs 8 & 9), and occurred in the mid-Pliocene. The bases
of these micro-floral zones are marked by the quantitative
peak of Zonocostites ramonae, the quantitative increase of
Monoporites annulatus and fungal elements, while the
quantitative decrease of Verrucatosporites sp., the quantita-
tive increase of Monoporites annulatus and the decrease of
fungal elements defined their tops. PAZ-VIA and VIB are
characterized by the predominance of Monoporites
annulatus,Cyperaceaepollis sp. Chenopodipollis sp., fungal
elements, Verrucatosporites sp., Acrostichum aureum, fresh-
ISSN 2410-4418 Palaeont. afr. (2016) 50: 31–47 43
Figure 9. Chart showing integration of palynological, lithological and gamma ray log with inferred depositional environments in Well B.
water algae and marine palynomorphs with a reduced
proportion of Zonocostites ramonae, beach and brackish
miospores. The basal parts of these zones are dominated
by deposition of mudstones while sandstones/siltstones
lithologies were deposited in the upper parts. Bell-shaped
gamma-ray signatures are recorded in these micro-floral
zones (Figs 8 & 9).
PAZ-VIIA (1469–1097 m) and PAZ-VIIB (1710–1585 m);
P870 Subzone (mid-Pliocene)
These palynological zones that are located within P870
Subzone and occurred within the mid-Pliocene period of
the analysed intervals of Wells A and B. Base boundaries
ofthesepalynologicalzonesaremarkedbythequantitative
decrease of Verrucatosporites sp., the quantitative increase
of Monoporites annulatus and the decrease of fungal
spores. The quantitative decrease of Zonocostites ramonae
and quantitative increase of fungal spores defined the
tops of these floral zones. The overall prevalence of
Zonocostites ramonae,Psilastephanocolporites sp., fungal spores
and marine palynomorphs (silicoflagellates and diatoms),
with rare to absent savanna, freshwater swamp, freshwa-
teralgaepalynomorphscharacterizedthesepalynological
zones. In these zones, sandstone facies were deposited in
Well A, while mudstone lithologies were deposited in Well
B. Boxcar and bell-shaped gamma-ray curves are re-
corded in Wells A and B, respectively, (Figs 8 & 9).
PAZ-VIIIA (1097–701m) and PAZ-VIIIB (1585–1463 m);
P870 Subzone (late Pliocene)
These palynofloral zones are the uppermost part of
Wells A and B and are the youngest stratigraphical zones
recorded in these wells. The palynofloral assemblages
occurred within P870 Subzone and dated to late Pliocene
(Piacenzian statge). PAZ-VIIIA and VIIIB (Figs 8 & 9)
bases are defined by the quantitative decrease of
Zonocostites ramonae and quantitative increase of fungal el-
ements, while the quantitative decrease of Zonocostites
ramonae, fungal elements and quantitative increase of
Monoporites annulatus tentatively marked their tops.
PAZ-VIIIA and VIIIB are dominated by mangrove, fresh-
water swamp, lowland rainforest, fungal spores and ma-
rine palynomorphs (silicoflagellates and diatoms) with
the rare occurrence of savanna. These zones are typified
by the deposition of sandstones in Well A, while
mudstone facies were deposited in Well B. The accompa-
nied gamma ray curves in these zones are funnel and
bell-shaped in Wells A and B, respectively.
DISCUSSION
The detailed palynological analysis revealed that
palynomorphs form diverse and abundant assemblages
in Wells A and B from the deep offshore Cenozoic Niger
Delta and is supported by the previous reports from the
Niger Delta (Oboh 1992; Ige 2011; Ige et al. 2011; Adebayo
et al. 2012; Ojo & Gbadamosi 2013; Adebayo & Ojo 2014).
The age of Wells A and B ranges from late Miocene (P860
Subzone) to Pliocene (P870 Subzone) (Evamy et al. 1978).
The quantitative base occurrence of Retistephanocolpites
gracilis at 2548 m and 2185 m in Wells A and B, respectively,
defined the boundary between late Miocene and Pliocene
that suggests the age is around 5.2 Ma (Discoaster
quinqueramus, Morley & Richard 1993; Olayiwola 2014).
Palynofloral assemblages of Wells A and B are similar to
those of West Africa and farther afield, for example,
subzones P860 and P870 are correlated with the pan-
tropical Echitricolporites spinosus Zone of Germeraad et al.
(1968) and J2–J3 Zone of Legoux (1978). The base of
micro-floral zone P3 of subzone P870, defined by in-
creased occurrences of Psilastephanocolporites sp. at 1679 m
and 1692 m in Wells A and B, respectively, suggest an age
of about 4.1 Ma (Gephyrocapsa demitre, Morley & Richard
1993; Olayiwola 2014).
The palynological assemblages revealed that there is a
landward shifting of the mangrove vegetation due to the
sea level rise as shown in PAZ-IA, PAZ-IB, PAZ-IIIA,
PAZ-VB, PAZ-VIIB and PAZ-VIIIB. Thick bodies of
mudstone/claystone lithologies recorded at depths
2768 m, in Well A, 2469 m, and 1807 m, in Well B, on top of
sandstones bodies, are likely to be overpressure zones in
the studied wells. Conversely, the palynological assem-
blage zones PAZ-IIA, PAZ-IIB, PAZ-IIIB, PAZ-VA, and
PAZ-VIIA revealed that there is a shifting of the savanna
vegetation seaward during sea level fall. PAZ-VIIIA does
not show any significant change in savanna pollen. It
should be noted that Poaceae, e.g. Monoporites annulatus,
can grow in diverse habitats, e.g. close to the coast (Rebata
et al. 2006), and not only in savannas, although for
Germeraad (1968) M. annulatus is a savanna element.
There is no new evidence to contradict this.
Palynological assemblages in PAZs-1A, 1B, IIIA, IIIB, VA,
VB, VIIA, VIIB, VIIIA and VIIB suggested deposition in a
subaerial delta plain during sea level rise, while those of
PAZs-IIA, IIB, IVA, IV, VIA and VIB suggested deposition
in a subaqueous delta plain during sea level fall. The
sedimentological analysis revealed two to three lithologies.
The sandstones facies are recognized to have been depos-
ited as the regressive sequences in the palynofloral
zones PAZs-IIA, IIB, IIIB, IVA, IVB, VA, VIA and VIB
(upper part), VIIA and VIIIA. In addition, mudstones or
claystone are delineated to have been deposited as the
transgressive sequences in palynozones PAZs-IA, IB, IIIA,
VB, VIA and VIB (basal parts),VIIB, and VIIIB. In addition
analysis of the gamma-ray signatures showed three
patterns. These are bell, funnel, and boxcar-shaped,which
indicated tidal channel (in PAZs-IA, IB, IIA, IVA, IVB, VB,
VIA, VIB, VIIB and VIIIB), deltaic distributary channel (in
PAZs-IIA, IIB, VA and VIIB), and regressive barrier (in
PAZs-IIIB and VIIIB) depositional environments. The
deltaic distributary channel that belongs to the deltaic
system complex is recorded in Wells A and B and was
previously interpreted to have been formed during the
first stage of prograding sea level fall (Emery & Myers
1996). It usually has an erosive base that is interpreted as a
sequence boundary and is commonly filled with moder-
ate to well-sorted, uniform and blocky sand-grain size
sediments with a good reservoir quality (Unukogbon et al.
2008). In addition, tidal channels, which are usually domi-
nated by moderate to well-sorted sand deposits (Oboh
et al. 1992; Emery & Myers 1996), are recognized in this
44 ISSN 2410-4418 Palaeont. afr. (2016) 50: 31–47
study. They are characterized by an erosive base that is
commonly filled with fining-upward sand sequences in
association with the muddy sediments in the topmost
part. The tidal channel sand usually forms a good quality
reservoir rock with a sealing rock capacity on top
(Unukogbon et al. 2008). Furthermore, the regressive
barrier sand that formed when there is a drop in the base
level of the sea, are recorded in Wells A and B. This sub-
environment is characterized by deposition of coarsen-
ing-upwards sediments ranging from basal marine shale
to continental coarse-grained sand at the top (Selley 1985).
The basal organic mud-rich deposits underlying the
regressive barrier sand are likely to be of a high-quality
kerogen supply in the study area (Chow et al. 2005).
Many earlier authors have used a combination of
palynologic, lithologic and well-log data to outline differ-
ent sub-environments in the onshore portion of the Niger
Delta. Amongst these are Oboh (1991, 1992a,b, 1993, 1995);
Kulke (1995), Unukogbon et al. (2008) and Oyedele et al.
(2012). They reported distributary and tidal channel fills
that contained few palynomorphs (chiefly Poaceae) and
were associated with erosive based, fining-upward and
bell-shaped to blocky gamma-ray patterns. The regressive
barrier has equally been described to be rich in palyno-
morphs (mainly grass, mangrove and ferns spores) in
association with coarsening-upward and funnel-shaped
patterns. However, three categories of interbedded shale,
namely clay smears along faults, interbedded sealing
units, and vertical seals, were previously recorded in the
Agbada Formation that formed the chief seal rock in the
Niger Delta (Doust & Omatsola 1990).
Overpressure zones have earlier been reported to be
associated with young Tertiary basins such as the Niger
Delta, Gulf of Mexico, Nile Delta and Baram basins
(Nwozor et al. 2013). This is as a result of high rates of sedi-
ment accumulation, which rapidly generated sufficiently
thick intervals of shales that commonly characterize these
basins. However, the tops of the overpressure zones vary
greatly from depo-belt to depo-belt in the Niger Delta
(Krusi 1990). Consequently, some authors tried to detect
and predict overpressure zones in this region by applying
different methods. Amongst these are well logs (Owolabi
et al. 1990; Osinowo et al. 2007) and seismic data (Alao et al.
2014; Omolaiye et al. 2011; Ayuku & Olorunniwo 2004).
CONCLUSION
The Late Miocene-Pliocene Niger Delta is characterized,
in order of abundance and diversity, by angiosperm
pollen, monolete spores, fungal elements, trilete fern
spores, freshwater algae and marine palynomorphs. The
alternation of sandstones and mud/claystone lithologies
typified Wells A and B in this study. The sandstone
lithologies likely served as reservoir rocks while mud/
claystones are suggested to be the good source and seal
rocks in the study area. Three different depositional
sub-environments, namely, distributary channel, tidal
channel, and regressive barrier sands, are delineated in
this study. The findings of the multi-pronged approach
used here will encourage further studies on offshore wells
and help the exploration geologists in recognizing
oil-prone deposits from sub-environments such as the
distributary channel, tidal channel, and regressive barrier
sands and in detecting/predicting the overpressure zones.
Therefore, this will help to reduce the costs and risks that
are usually associated with deepwater exploration and
exploitation of petroleum sources.
The support of the DST-NRF CoE in Palaeosciences, University of the Witwaters-
rand, South Africa; Nigerian Tertiary Education Trust Fund (TETFund) through
Obafemi Awolowo University (OAU), Ile-Ife, Nigeria, Palaeontological Scientific
Trust (PAST) and JC Carstens Trust, South Africa (to the corresponding author)
towards this research are hereby acknowledged. Opinions expressed and conclu-
sions arrived at, are those of the authors and are not necessarily attributed to the
supporters. Thanks are also due to the management of Mobil Producing Nigeria
Unlimited (MPNU) for the provision of ditch cutting samples used for this study
and for permission to publish the results of this study.My very sincere appreciation
goes to Professors M.O. Odébòdé and M.A. Rahaman of the Department of
Geology, OAU, Ile-Ife, for their suggestions and support during the acquisition of
the ditch-cutting samples. The help of O.J. Olaleye-Otunla, of Natural History
Museum, OAU, Ile-Ife, on cartographic work is acknowledged.
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ISSN 2410-4418 Palaeont. afr. (2016) 50: 31–47 47
... The bagged samples were spread on a black anodized aluminium foraminiferal picking tray and viewed using the standard binocular reflected light microscope (Fisher Scientific, No. 62416). The lithologic description was enhanced by the gamma-ray and resistivity logs since high and low values of gamma log and deep induction resistivity log signified shale and sand lithologies, respectively (Olayiwola and Bamford, 2016). The essential parameters studied were: (i) the rock types; (ii) colour and texture such as grain size, sorting and grain shape (roundness); and (iii) accessory mineral and fossil contents. ...
... Montane and savanna taxa have the lowest representation ( Figure 6). Some authors (Adojoh et al., 2015;Olayiwola and Bamford, 2016) agree that landward shifting of coastlines during sea level rise result in deposition of marine sediments in the subaerial delta plain. This period is also associated with shifting of the mangrove and other coastal swamp plant belts due to their preference for saline water. ...
... This period is also associated with shifting of the mangrove and other coastal swamp plant belts due to their preference for saline water. Therefore, the subaerial delta plain depositional environment is characterised by high representation of mangrove, other coastal swamp plants (from beach, brackish, freshwater swamp, rainforest, and palm) miospores, fungal elements, freshwater algae, and marine species (Adojoh et al., 2015;Olayiwola and Bamford, 2016) (Figure 7). Similarly, during sea level fall, the coastline is shifted basinward and the shelf area ini- tially covered by marine water become exposed and probably incised due to erosion by fluvial activities. ...
... Fissile, platy and flagy samples indicate shale while samples with fine to coarse grain sizes indicate sandstone units. The lithologic description was enhanced by the Gamma Ray and Deep Induction Resistivity Logs since high and low values of Gamma Ray Log and Deep Induction Resistivity Log signify shale and sandstone lithologies, respectively (Adegoke, 2002;Olayiwola and Bamford, 2016). The Gamma Ray Log patterns (fining and coarsening upward signatures) description by Sneider et al. (1978) and Beka and Oti (1995) were adopted in this research. ...
... This period is also associated with shifting of the mangrove and other coastal swamp plant belts due to their preference for saline water. Therefore, the subaerial delta plain depositional environment is characterized by high representation of mangrove, other coastal swamp plants (from beach, brackish, freshwater swamp, rainforest and palm) miospores, fungal elements, freshwater algae and marine origin species (Adojoh et al., 2015;Olayiwola and Bamford, 2016). ...
... This results in deposition of terrestrial sediments in the subaqueous delta plain which is characterized by widespread savanna and montane vegetation belts. This depositional environment is characterized by maxima spectra of savanna and montane pollen (Adojoh et al., 2015;Olayiwola and Bamford, 2016). ...
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... Fissile samples were denoted to be shale while fine to coarse grained samples are the sandstones. Gamma ray logs were also employed in describing the lithologies because high and low values of gamma ray indicate shale and sandstone lithologies respectively (Olayiwola and Bamford, 2016). The fining and coarsening upward signatures of the gamma ray patterns description by Sneider et al. (1978) and Beka and Oti (1995) and Onyekuru et al., (2012) were employed in this study (Figure 2 and 3). ...
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Lithological analysis was carried out on 96 ditch cuttings samples from HD-001 well located within the shallow offshore Niger delta basin, Nigeria. Three lithofacies sequences were delineated by the integration of wireline logs textural/lithologic attritudes and the distribution of index accessory minerals. They are transitional paralic, paralic and marine paralic sequences. The lithologic, textural and wireline log data indicate that the entire interval studied in the HD-001 well belongs to the Agbada Formation. The Formation is made up of alternating sand and shale units which suggests rapid shoreline progradation. The grain size increases from essentially fine to medium-grained at the basal part of the well to dominantly coarser grain at the upper part. The index accessories recognize shallow marine to coastal deltaic settings environment of deposition. Sand bodies which represent sub-environments within those settings are deposited in sequences. Each sequence begins with a transgressive phase followed by significant regressions.
... They conducted the first palynostratigraphic study in the Onshore Kwanza Basin, and assigned an early Miocene (Aquitanian/Burdigalian) age to the lowermost Quifangondo Formation. Further north along the West African shore, some essential regional palynological studies were accomplished in Nigeria (Legoux, 1978;Williams, 1978;Biffi and Grignani, 1983;Demchuk and Morley, 2004;Adeigbe et al., 2013;Olayiwola and Bamford, 2016;Adeonipekun et al., 2017) as well as in the Congo deep sea fan (Dale et al., 2002). These studies are essential for this study in their potential for age correlation. ...
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... The palynostratigraphic zones proposed in this study were based on the international stratigraphic guide -an abridged version of Murphy & Salvador (1999). The works of Germeraad et al. (2015) and Olayiwola & Bamford (2016) were also consulted. Age diagnostic marker species were used to determine the age of the studied interval in the well. ...
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... Each residue was oxidized using concentrated nitric acid (HNO 3 ) and prepared for study as strewn mounts using Loctite. (2014) and Olayiwola & Bamford, (2016). The distribution of each palynomorph present on each slide was plotted against depth using Tilia™ and the distribution chart (Fig. 2) produced. ...
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The need to increase our knowledge of palaeo-flora is important in palaeoclimatic and palaeoenvironmental reconstruction of the Tertiary Niger Delta as to highlight possible changes in the depositional environments over time. Hence, palynological data from the M1well from the western Niger Delta region were employed in an attempt to reconstruct the Middle Miocene palaeoenvironment and palaeoclimate. The detailed palynological analysis revealed diverse and abundant palynomorph assemblages. This consisted of pollen species 60.14%, spores 25.86%, algae (Botyococcus braunii, Pediastrum sp., and Concentricytes circulus) 10.53%, iscellaneous palynomorphs (fungal elements, diatom frustules and charred Gramineae cuticle) 2.62%, dinoflagellate cysts 0.79% and acritarchs 0.06%. The well is dated Middle Miocene based on the common occurrences of diagnostic middle Miocene Niger Delta palynomorphs.Four informal palynofloral assemblage zones (MPAZ) I–IV were defined and correlated with major cycles of alternating dry and wet climatic conditions. Sediments within MPAZ I and MPAZ II were assumed to have been deposited during dominantly wet periods while MPAZ IV and III showed brief dry pulses coupled with periods of marine transgressions. The palaeoenvironment fluctuated between nearshore and marginal marine inferred from abundant records of land-derived palynomorphs and the spotty records of the dinoflagellate cysts Nematosphaeropsis labyrinthus, Nematosphaeropsis lemniscata and Impagidinium sp.
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The Niger Delta of Nigeria is one of the world’s most prolific Cenozoic hydrocarbon provinces. The Sample_B well from Niger Delta Basin encountered the upper Miocene-lower Pliocene Agbada Formation. This present study analyzed 125 ditch cutting samples for palynomorphs and foraminifera in conjunction with well log data to illustrate the sequence stratigraphic framework for the understanding of the distribution of system tracts, key surfaces, and depositional systems. Three lithologies (sandstones, siltstones, and mudstones) were recorded. The recorded palynomorph assemblages that enabled the delineation of two palynological subzones, ten palynozones, and five palynocycles of transgressive and regressive depositional sequence were recognized. A detailed biostratigraphic based on foraminifera allowed the subdivision of Sample_B well into two zones. Integration of lithologic, palynologic, biostratigraphic, and well log data were used to constrained 9.2Ma, 7.0Ma, 5.8Ma, 5.49Ma, 5.0Ma, 4.15Ma, 4.0Ma, and 3.2Ma maximum flooding surfaces, and 8.2Ma, 6.3Ma, 5.5Ma, 4.2Ma, and 3.8Ma sequence boundaries that were correlated with dating of Haq et al. (1988). The alternation of the HST sands and TST shales provided a seal for reservoir facies, which are essential for the hydrocarbon accumulation and its stratigraphic trapping. The LST sands served as the reservoir rocks. Near-shore to marginal marine (coastal deltaic) and inner to middle neritic paleoenvironments were recorded. Two distributary channels, two tidal deltas, and four tidal channels were revealed to fluctuating from littoral to shallow marine environments. The results of this study will serve as a predictive tool for determining the likely presence and distribution of source, seal, and reservoir rocks and optimizing production prospective in the Niger Delta region of Nigeria.
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The graphic correlation method provides better and higher resolution to the stratigraphic problems than zonation biostratigraphy. The measured total stratigraphic ranges of 44 palynomorphs present in five wells from the Niger Delta Basin have been assembled by graphic correlation method into a chronostratigraphic framework. The analysis by graphic correlation reveals that for a meter of sediment accumulation in Well E, correspondingly amounted to only 0.8794 m, 0.7586 m, 0.8677 m and 0.8686 m of sediments that were deposited in wells A, B, C and D, respectively. These relative rates of sediment accumulation indicate that either there was less erosion taking place and/or more accommodation space in Well E than Wells A, B, C and D. A graphic correlation horizontal terrace was recorded in each of the Wells A, C and D, which are interpreted as condensation of sediments that had truncated the sediments accumulation profile in these wells. These condensed sections are interpreted as sediment starvation and very slow rates of sedimentation during the Early Pliocene, Late Pliocene and Late Pleistocene due to marine transgression(s) at these stages in the Niger Delta Basin. The generated correlation equations, slopes and intercept values from graphic correlation plots allow the correlation of six biostratigraphic and sedimentological events in Wells A, B, C, D and E. The resulting chronostratigraphic framework in this study is essential for sequence stratigraphic interpretations and basin analysis, and in particular, to correlate widely separated wells.
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Biostratigraphic studies on Neogene deltaic sequences very often fail to provide sufficient resolution to answer stratigraphic questions posed by petroleum geologists. This is particularly the case for the offshore Niger Delta area, where a biostratigraphic framework has proved particularly elusive. This paper presents an approach involving a combination of quantitative palynological studies, micropalaeontology and lithostratigraphy to provide a sequence of thirteen palynological zones through the Plio-Pleistocene of the offshore area. The palynological zonation scheme is then augmented by the identification of over twenty stratigraphically significant environmental events relating to (a) changes in delta plain style, and (b) marine transgressions, which can be correlated by reference to the scheme of palynological zones. The resulting composite stratigraphic framework provides a valuable new insight into the stratigraphy of the offshore Niger Delta region.
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Abstract An investigation of the overpressure condition in Afam field was carried out using a suite of borehole logs and 3D seismic data obtained from Shell petroleum Development Company of Nigeria (SPDC) with a view to detecting and predicting abnormal pressure zones in the area.The methodology employed knowledge of well lithology and more detailed information extracted from inverted seismic traces. The interval velocity data of some shot lines within the study field and its immediate environment were computed to ascertain the pressure variations and the geological model of the less known areas from seismic data. Using well data near the seismic trace for calibration, precise stratigraphic interpretation of the constructed interval velocity section was carried out. The overpressure zones were identified and probable hydrocarbon distribution pattern within the field was established and correlated with the geology of the study area..The results obtained revealed five tops of overpressure (TOV) namely TOV 1, TOV 2, TOV 3, TOV 4, and TOV 5 at 1608.8m, 1884.1m, 2387.3m, 2708.9m and 3001.4m respectively derived from seismic and lithologic logs including their lateral variations. .The plot of the Normal Compaction Trend versus velocities obtained from sonic logs also confirmed the identified overpressure zones. The five horizons representing the identified overpressure zones were picked on the 3D seismic sections at 1.42 s, 1.75 s, 2.01 s, 2.11 s and 2.30 s respectively, with thickness varying from 4.08 m to 50.58 m. In-addition, the water saturation (Sw) and porosity (f ) values calculated showed that the overpressure zones were generally characterized by high water saturation (52 % to 80.36%) and low porosity (16.55 % to 30.80 %). Furthermore, four hydrocarbon-bearing zones, which were overlain by thin over pressured shale beds, were delineated at 2657 m, 2804 m, 2916.9 m and 3048 m respectively.This case study shows how knowledge of well site lithology and detailed information from seismic data could enable prediction match well conditions with high fidelity thereby reducing drilling cost, prospect risk and improving safety.. Keywords: overpressure, compaction, interval velocity, Shale, amplitude inversion