GX1 and GX2 wells are located at the Western and South Eastern Miocene Niger Delta Basin respectively. Cored depth for GX1 was 113.5m while that of GX2 was 23.6m. GX1 well is situated within the coastal Swamp of the Niger Delta. It is made up of mostly tidal sands with multiple cycles of fining upwards sequences. These high frequency packages are as a result of local transgressions and regressions. Each parasequence represents regressive set in a prograding sea. High sedimentation and reduced accommodation following lowered sea level resulted in thick layers of sandstone. Reverse is also true. Tidal form structures predominated wave structures. GX2 well located in Shallow Offshore comprised fine sands, silts and shale alternations. Sedimentary wave and tide structures were observed. It is a progradational sequence that commenced with thick shale followed by thin alternations of shale and silt/sand. The fluctuations in sea level in GX2 well are more frequent than in GX1. This is quite significant with thick volumes of sandstones in GX1 cores and thin beds of shale, silts/sands in GX2 cores. Sandstone pinchout was seen in basal shale. The Stratigraphic modelling of both wells places GX2 well above the sequence boundary observed at the top of GX1.The pattern of sea level fluctuations and sedimentation in both tidal environment and shoreface resulted to sharp variations in measured core porosity and permeability. GX1 has average core porosity of 23% and average permeability of 7,000mD, and while GX2 has average core porosity of 29.3% and average permeability 1007mD.

1. Sequence Stratigraphy, Variable Sedimentations, Sea Level Fluctuations and Net Effect on Reservoir Qualities: A Study of Cores from Two Wells GX1 and GX2
in the Niger Delta Basin
Sequence Stratigraphy, Variable Sedimentations, Sea Level
Fluctuations and Net Effect on Reservoir Qualities: A Study of
Cores from Two Wells GX1 and GX2 in the Niger Delta Basin
*Otosigbo Gloria Ogochukwu1, Nzekwe Kenneth Emeka2, Onwe Ikechukwu Moses3
1,3Department of Geology, Federal University, Ndufu-Alike, Ikwo, Ebonyi State, Nigeria
2Delta Terratek Laboratories, Lagos, Nigeria
GX1 and GX2 wells are located at the Western and South Eastern Miocene Niger Delta Basin
respectively. Cored depth for GX1 was 113.5m while that of GX2 was 23.6m. GX1 well is situated
within the coastal Swamp of the Niger Delta. It is made up of mostly tidal sands with multiple
cycles of fining upwards sequences. These high frequency packages are as a result of local
transgressions and regressions. Each parasequence represents regressive set in a prograding
sea. High sedimentation and reduced accommodation following lowered sea level resulted in
thick layers of sandstone. Reverse is also true. Tidal form structures predominated wave
structures. GX2 well located in Shallow Offshore comprised fine sands, silts and shale
alternations. Sedimentary wave and tide structures were observed. It is a progradational
sequence that commenced with thick shale followed by thin alternations of shale and silt/sand.
The fluctuations in sea level in GX2 well are more frequent than in GX1. This is quite significant
with thick volumes of sandstones in GX1 cores and thin beds of shale, silts/sands in GX2 cores.
Sandstone pinchout was seen in basal shale. The Stratigraphic modelling of both wells places
GX2 well above the sequence boundary observed at the top of GX1.The pattern of sea level
fluctuations and sedimentation in both tidal environment and shoreface resulted to sharp
variations in measured core porosity and permeability. GX1 has average core porosity of 23% and
average permeability of 7,000mD, and while GX2 has average core porosity of 29.3% and average
permeability 1007mD.
Key words: Regressive, parasequences, prograding, high stand, sedimentation, fluctuations
INTRODUCTION
Niger Delta basin sedimentation has commenced in early
Tertiary when clastic river input increased (Doust and
Omatsola, 1989). The delta has since prograded over the
subsiding continental-oceanic lithospheric transition zone
extends into the oceanic plate of Gulf of Guinea (Adesida,
1997). Five depositional belts have been described in
Miocene Niger Delta (Figure1). They are Northern Delta
Depobelt, Coastal Swamp Depobelt, Central swamp
depobelt, Ughelli depobelt and Shallow Offshore depobelt
(Ezeh et al. 2016). Depobelt is defined as representative
of successive phase of delta growth. Niger Delta
comprises many sub-environments that have been
identified by many researchers. Oyayan (2013) identified
five sub-environments using of core samples, such as;
Lower shoreface, middle shoreface, distributary channel,
tidal channel and tidal flats. Obaje (2006) interpreted Post
Oligocene Niger Delta as wave dominated with well
developed shoreface sands, beach ridge, tidal channels,
mangrove and swamps. The basic Formations are marine
Akata Shales through sand shale paralic interval of
Agbada Formation to continental sands of Benin
Formation. The delta experiences transgressions and
regressions depending on the sea level condition, local
subsidence, and sediments supply. The overall sequence
of Niger Delta is a regressive sequence as a result of sea
progradation. From the Coastal swamp depobelt to
Offshore depobelts there are variable sedimentations and
sea level fluctuations. This is evidenced in the cores
studied from well in both depobelts
*Corresponding Author: Otosigbo Gloria Ogochukwu;
Department of Geology, Federal University, Ndufu-Alike, Ikwo,
Ebonyi State, Nigeria. Email: gloriaotosigbo@gmail.com, Tel:
+234-1-08037137658
Case Study
Vol. 5(2), pp. 258-268, May, 2019. © www.premierpublishers.org. ISSN: 3019-8261
International Journal of Geology and Mining

2. Sequence Stratigraphy, Variable Sedimentations, Sea Level Fluctuations and Net Effect on Reservoir Qualities: A Study of Cores from Two Wells GX1 and GX2
in the Niger Delta Basin
Otosigbo et al. 259
GX1 well is situated within the coastal Swamp of the Niger
Delta depobelt (Figure 1) and is made up of mostly tidal
sands. The lithology ranges from pebbly to fine
sandstones, occasional shales, thin band of coal, and
carbonaceous shale. It has multiple cycles of fining
upwards sequences. GX2 well is located in Offshore
depobelt and it comprises of shale beds, alternation of
sandy shale/sand, heterolithics units (sand/shale/silts),
and fine sands. It has a coarsening upward cycle.
Sedimentary structures that were observed are flaser
beddings, lenticular beddings, laminations and sometimes
bioturbations (Akagbeobi et al., 2018). This study intends
to compare the style of sedimentation sea level variation
and overall sedimentary deposits in both wells and the
resultant effect on measured core porosity and
permeability.
Fig 1: Location of GX1 and GX2 well within the Niger Delta Depobelts.
Geology and Stratigraphy
The Niger Delta is situated on the continental margins of
the Gulf of Guinea and extends throughout the Niger Delta
Province as defined by Klett et. al. (1997). It located in
equatorial West Africa, between latitude 30 and 50N and
longitude 50 and 80 E. It is bounded to the west by the
Calabar flank, which is a subsurface continuation of Oban
Massif, and bounded in the north by several older
Cretaceous tectonic elements such as the Anambra basin,
Abakaliki uplift and Afikpo syncline (Ejedawe, 1981).
Sedimentation in the Niger Delta started in the
Paleocene/Eocene beyond the trough, at the basement
horst at the northern flank of the present delta area (Weber
and Daukoru, 1975). The Niger delta has prograded into
the Gulf of Guinea at a steadily increasing rate in response
to the evolving drainage area, basement subsidence, and
eustatic sea level changes. The delta has advanced
seaward over 200km and has broadened from a width of
less than 300km to a width of about 500km (Whiteman,
1982) (fig 2). At the initial stage, the delta prograded over
extensionally thinned and collapsed continental crust of
the West Africa margin as far as the triple junction, filling
the basement graben-and-horst topography (Murat, 1972).
According to Weber and Daukoru (1975), the progressive
outbuilding of the Niger Delta as revealed by the landward
position of present coastline relative to the Pliocene
coastline is due to the post glacial rise in sea level. This
resulted in the extension of continental Benin Formation
offshore as far as the shelf break. From Eocene to the
present, the delta has prograded southwest forming
depobelts that represent the most active portion of the
delta at each stage of its development (Doust and
Omatsola, 1990). These depobelts form one of the largest
regressive deltas in the world with an area of some
300,000 km2 (Kulke, 1995), a sediment volume of
500,000km3 (Hospers, 1965), and a sediment thickness of
over 10km in the basin depocenter (Kaplan et. al., 1994)
The early delta –building was river dominated, while the
post Eocene delta environment is typical of a wave-
dominated delta with well-developed beach ridges, bars,
tidal channels, mangrove, and freshwater swamps
(Stacher, 1995).
The Tertiary Niger Delta is divided into three formations;
Akata Formation, Agbada Formation and Benin Formation
representing from top to bottom (Table 1). The Benin

3. Sequence Stratigraphy, Variable Sedimentations, Sea Level Fluctuations and Net Effect on Reservoir Qualities: A Study of Cores from Two Wells GX1 and GX2
in the Niger Delta Basin
Int. J. Geol. Min. 260
Formation is a delta top lithofacies consisting of massive
continental sands and gravel. This graded downwards
into, or overlies unconformably the delta front lithofacies,
the Agbada Formation, which comprises mostly shoreface
and channel sands with minor shales in the upper part, and
an alternation of sands and shale in equal proportion in the
lower section (Reijers et al, 1996). Pro- delta marine
shales belonging to the Akata formation occur deeper in
the section, where associated sandstone units are
generally lowstand turbidite fans deposited in deep marine
setting.
Fig 2: Stratigraphic Cross Section A’A showing Formations and depobelt.(Ezeh et al 2016)
Table 1: Stratigraphy of Niger Delta, Nigeria (Modified
after Akpoyovbike, 1978). The original modified after
(Short and Stauble, 1967)
The aim of this study is to show variable sub-environments
in the Niger Delta through facies and facies association
delineation, differential sedimentations by sea level
fluctuations, sequential layering and the variable effects on
the reservoir properties of the studied wells.
The Niger Delta is situated on the continental margins of
the Gulf of Guinea. (Klett et. al., 1997). It located in
equatorial West Africa, between latitude 50 and 8.50N and
longitude 40 and 60 E. GX1 well is located within the
coastal swamp depobelt along latitude 5o 54'N and
longitude 5o 12‘. GX2 well is located within the shallow
offshore along latitude 7o 08' N and between longitude 4o
02‘.
METHODS OF STUDY
The study carried out on cores from GX1 and GX2 wells
include detailed sedimentological core descriptions carried
out on the cores. GX1 was cored in 3 discontinuities as, A:
3405.6 - 3445.5 , B: 3330 – 3363.3 and C: 3232 – 3264.7
while the cored depth of GX2 ranged from 2416.7- 2440.3.
The similar lithological characteristics were grouped into
facies, facies associations and depositional environment.
The facies log was built by Sedlog 2.1.4 software. The
sequence stratigraphy of the established for the two wells.
The cores were plugged at selected intervals in the both
wells. A total of 81 plugs from GX1 and 20 plugs from GX2.
The diameter of plugs are 1
1
2
inches, while the length is
at least times 1
1
2
of diameter. The plugs were sent to dean
stark for removal of oil with toluene and later with
chloroform for at least 3 days. The cleaned plugs were
dried to stable weights using conventional oven and weight
recorded. Grain density was determined by using the dry
weight previously obtained and measuring grain volume at
constant temperature in a helium expansion porosimeter
using Boyle’s law. Grain density was calculated:
⍴ 𝑮 = 𝑾 𝑮 𝑽 𝑮⁄
Where, ⍴ 𝑮 is grain density in gm/cm3 , 𝑾 𝑮 is corrected
dry weight in gm and 𝑽 𝑮 is corrected grain volume in
cm3. Porosity was determined by utilizing the grain volume
already obtained, then measuring pore volume with
expanding helium in a hydrostatic test cell at net
overburden stress also using Boyle’s law. All the samples
were tested at ambient of 400psi. Porosity, ᶲ values were
calculated as follows:
ᶲ = 𝑉𝑝 (𝑉𝑝 + 𝑉𝐺)⁄ ∗ 100%
The value of porosity is expressed in percentage, 𝑉𝑝 is
corrected pore volume in cm3 , 𝑉𝐺 is corrected grain
volume in cm3. The steady – state gas permeability to
nitrogen was measured in a hydrostatic test cell
immediately after the pore volume measurement. The
manifold was changed to accommodate gas flow. Inlet
pressure was measured directly at the input sample face

4. Sequence Stratigraphy, Variable Sedimentations, Sea Level Fluctuations and Net Effect on Reservoir Qualities: A Study of Cores from Two Wells GX1 and GX2
in the Niger Delta Basin
Otosigbo et al. 261
using three pressure transducers calibrated to measure
high, medium and low permeability. The transducer for low
permeability was in 50psi, while high and medium was in
inches of water. The apparatus used was dependent upon
the pressure drop measured across the sample. Exit
pressure was measured at the pressure transducers. Flow
rate was measured directly by flowing soap film through a
graduated burette. The principal is based on Darcy’s
equation for laminar flow. The flow region has to be laminar
i.e. less than 1cc per second. The time required for a soap
film to flow through a given volume of 30cc was measured
with a stop watch. The average of three measured
consecutive flow rates was used to calculate the final
reported permeability value. Permeability equation is
given as:
𝐾𝑎 =
𝑄 𝑏 𝜇𝑃𝑏(𝐿 𝐴⁄ )
(𝑃𝑖 + 𝑃𝑏)2 − (𝑃𝑒 + 𝑃𝑏)2
𝐾𝑎 is apparent permeability in millidarcys, 𝑄 𝑏 is flow rate
in cm3/s , 𝜇 is viscosity of nitrogen in centipoises (0.176
cp was used), 𝑃𝑏 is the barometric pressure in
atmospheres, 𝐿 is the length in cm3, 𝐴 is sample cross-
sectional area in cm2 , 𝑃𝑖 is inlet pressure converted from
the units read to units of atmospheres, 𝑃𝑒 is the outlet
pressure converted from the units read to units of
atmosphere.
RESULTS AND INTERPRETATION
Facies Association-GX1
The description of GX1 well revealed the lithological
characteristics as; fine to coarse pebbly sands, mixed
sand, silts and shale (heterolithics), coal, etc. The
sedimentary structures observed are planar cross
bedding, herringbone cross bedding, wave ripple
laminations, laminations, lenticular beddings, flaser
beddings, way bedding, convoluted beddings, massive
bedded, reactivation surface and scoured erosional
surfaces. Other features associated with lithologies are
basal lag deposits, load casts and siderite clasts.
Bioturbations are Skolithos, Diplocraterion, and
Thalossionoides trace fossils belonging to Skolithos
ichnofacies assemblages.
Sedimentary Facies is defined as the sum of the
characteristics of a sedimentary rock unit. Related facies
close to each other that depicts a particular sedimentary
environment are grouped into facies associations. Five
facies associations were defined from GX1 from bottom to
top as follows: Lower intertidal flats(Fine Sandstone
Facies- FSF), Middle tidal flat deposits(Bioturbated
Heterolithic Facies-BHF and Shale Facies-SF), subtidal
deposits(Planar Cross-bedded Sandstone-PCS and
Herringbone Cross-bedded Sandstone-HCS), swamp
deposits(Coal Facies-CF), Lower intertidal flat
deposits(Cross Bedded Sandstone-CBS and Shale
Facies-SF), Tidal channel deposits(Pebbly Sandstone-PS,
Planar Cross bedded Sandstone-PCS and Laminated
Heterolithics-LH) and middle tidal flats(Bioturbated
Heterolithics-BH and Shale Facies-SF )(see Figure 6).
Two facies association were repeated along the sequence.
The summary of the lithological characteristics, facies,
facies associations and interpretation are shown in table 2
below.
Figure 3: Lithofacies Log of GX1 showing Facies
Association and System Tracts, MFS and uncored interval.
Figure 4: Reactivation Surface seen in Planar Cross
bedded Sandstone. It is typical of tidal environment

5. Sequence Stratigraphy, Variable Sedimentations, Sea Level Fluctuations and Net Effect on Reservoir Qualities: A Study of Cores from Two Wells GX1 and GX2
in the Niger Delta Basin
Int. J. Geol. Min. 262
Figure 5: Cross Bedded Sandstone Facies from GX 1
showing, planar cross bedding, wave ripplecross bedding
and Diplocraterion trace fossils.
Facies Association GX2
The description of GX2 well revealed the lithological
characteristics as; consists mostly fine sands, mixed sand,
silts, and shale. The sedimentary structures are parallel
laminations, flaser bedding, lenticular bedding, wavy
bedding, cross laminations, convolute bedding and ripple
laminations. Other depositional structures are load casts,
minor faults, synsedimentary micro faults. Bioturbations is
mostly skolithos.
Five sedimentary facies were delineated from GX2 well in
accordance with their lithological characteristics from
bottom to top. They are Shale Facies-SF, Rippled
Heterolithics-RH, Ripple Laminated Sandstone-RLS,
Cross Laminated Heterolithics-CLH, Parallel Laminated
Sandstone-PLS and Symmetrical Rippled Heterolithics-
SRH. These were grouped into four Facies association:
Offshore, Offshore Transition, Shoreface and Foreshore
deposits (see Figure 6). The summary of the facies, facies
association and Interpretation are shown in Table 3.
Figure 6: Lithofacies Log of GX2 showing the Facies
Association, System Tract. Yellow Triangle represent the
position of the Sand wedge-Incised fill.
Table 2: Summary of the Facies, Facies Association, Description and Interpretation of GX1
Facies
Association
Description Interpretation
Lower Intertidal
Deposit
-Fine Sandstone
Facies (FSF)
This consists of laminated very fine sand with
silts and shale streaks. Skolithos and
Diplocraterion ichnofacies at the contact with
Heterolithics facies. Thin band of siderite and
shale laminar, load cast present.
Skolithos Ichnofacies assemblage which
penetrates non cohesive sandy substrates at
high energy shore which is possibly lower tidal
flats
Middle Tidal Flats
Deposits
Bioturbated
Heterolithics
Facies (BHF) and
Shale Facies (SF)
Middle Tidal Flat Deposits consists of two facies
BHF and SF. Basal HF consisting
sandstone/silts =55%, shale = 45%. Mid-
section, sandstone/silts = 75%, shale
decreases to 25%. Sedimentary structures
present are herringbone, planar cross
laminations. SF consists of dark grey shales
interlaminated with heterolithics : Upper HF has
mudstone=35%, siltstone=35%,
sandstone=30%, lenticular bedded.
Alternation of BHF and Shale exhibits two
cycles of fining upwards succession. This is
progradation of regressive tidal flats. There is a
thinning upwards of facies thickness and later
a thickening resembling wave form from the
crest to trough and up to crest. This signifies
short time fluctuations in sea level. The flaser
and lenticular bedding structures reflect the
variations in current or wave activity or
sediment supply due to changing current
strength or wave power normally seen in
environment with mixed sands and mud. The
herringbone cross stratification describes
reversal of current direction typical of tidal
flats(Bloggs, 2007).

6. Sequence Stratigraphy, Variable Sedimentations, Sea Level Fluctuations and Net Effect on Reservoir Qualities: A Study of Cores from Two Wells GX1 and GX2
in the Niger Delta Basin
Otosigbo et al. 263
Table 2 Continue: Summary of the Facies, Facies Association, Description and Interpretation of GX1
Facies Association Description Interpretation
Subtidal Deposits
Planar Cross bedded
Sandstone (PCS) and
Herringbone cross
bedded Sandstone(HCS)
PCS consists of fine to medium grained,
light grey silty sandstone. Presence of
reactivation surfaces from which the cross
beddings build. Base commences with
medium grained texture which graded to
fine grained textured sand. HCS
comprises fine to coarse pebbly brownish
silty sandstone. Skolithos facies present
at HCS. Little bioturbation at PCS.
The course pebbly nature represents a
decreasing current velocity which causes
deposition of the majority of the streams coarse
grained bedload. The PCS forms a fining
upward sequence cycle with basal texture of
medium grained sand. HCS forms 2 cycles of
fining upwards sequences upon PCB with
basal texture of pebbly coarse sand.
Herringbone cross bedding is a strong
influence of bidirectional currents caused by
the ebb and flood of tides. Basal lags seen
represent flooding surfaces. The high
frequency packages are evident of high rate of
sedimentation during the progradation sea.
These are Subtidal channel deposits.
Swamp Deposits Bituminous brown coal The coal facies possibly formed in a swamp or
supratidal environment as a result of aeration
or abandoned channel.
Lower Inter Tidal
Deposits-Cross bedded
Sandstone (CBS) and
Shale Facies (SF)
CBS consists of very fine to fine grained
brownish silty sandstone. The prominent
sedimentary structures are parallel
lamination, symmetrical ripples and
herringbone cross beddings. Skolithos
and Diplocraterion burrows are present
and concentrates more at the lower
region. Load casts are present. SF
comprises dark grey shale interbedded
with shaly sand with high bioturbation and
load casts.
This is indicated of lowered energy and
recession of the tide. Parallel stratification to
symmetric ripple lamination suggests wave and
storm influence. The burrows and bioturbations
are few and restricted to the finer grained
sediments at the top units showing a marine
influence. This is interpreted as Lower Inter
Tidal Deposits
Tidal Channel Deposits-
Pebbly Sandstone (PS),
Planar Cross bedded
Sandstone (PCS) and
Laminated Heterolithics
(LH)
PS consists of medium to coarse grained
pebbly brownish silty sandstone. PCS
consists of a light brownish, fine to
medium grained, planar cross bedded
sandstones with thin laminars of
shale/carbon. It is ripple laminated and
highly bioturbated at the fine sections.
Thallosinoides, Diplocraterion and short
vertical burrows of skolithos are present.
LH is a parallel laminated, dark grey
heterolithic (shale =65% and sand =35%)
with load casts and scoured erosional
surface.
PS and PCS exhibit 2 cycles of fining upwards
trend. These are high frequency progradational
regressive packages of tidal environment. The
thick layers indicate sedimentation rate is
higher than the rate of sea regression. The
scoured erosional surface is associated with
channel. Following long exposure of the
underlying sediment is another built of very
fine-grained silty sand ridge overlain by shaly
sand and thin shale unit, parallel lamination and
symmetrical ripples. The high bioturbation,
symmetrical ripples and sideritic nodules
indicate dominance of marine influence
(Mozley, 1989).
Middle Tidal Flat
deposits- Bioturbated
Heterolithics (BH) and
Shale Facies (SF)
A repeat of bioturbated heterolithics and
3m bed of shale. BH comprises of mottled
facies, sandstone (40%), siltstone (30%)
and mudstone (30%). Sedimentary
structures are flaser bedding, wavy
laminations and convolute beddings. SF
comprises dark grey shales.
Flaser bedding, wavy laminations are formed
as a result of fluctuation in sediment supply or
current. Ripples of sand and silt, mud/shale is
deposited out of suspension during slack
waters. Ripple lamination and convolute
bedding suggests wave and storm influence.
Overlying shale represent quiet environment
and abandonment. Bioturbation indicate low
rate of sedimentation

7. Sequence Stratigraphy, Variable Sedimentations, Sea Level Fluctuations and Net Effect on Reservoir Qualities: A Study of Cores from Two Wells GX1 and GX2
in the Niger Delta Basin
Int. J. Geol. Min. 264
Table 3: Summary of the Lithological characteristics, Facies, Facies Association and Interpretation of GX2
Facies Association Lithological Description Interpretation
Offshore Shale:
Shale Facies
Dark grey shales, very fissile with a
sandstone pinchout
The offshore shale is indicative of Maximum
flooding with increase accommodation and
sedimentation
Off shore Transition
Deposit
Rippled heterolithics
(RH)
RH consists of grey to brownish
heterolithics of clay and silts sizes,
lenticular beddings, and wavy
beddings.Ratio is (sand =50%,
mudstone/siltstone =50%) The
laminars are convoluted.
Synsedimentary micro faulting is
present.
Lenticular and wavy bedding is caused by
short time variation in sedimentations and
sea level. Burrow scarcity implies that
conditions were not conducive for prolific
faunal activity. Synsedimentary faulting is
due to loading
Shoreface Deposit RLS:
Ripple laminated sand
(RLS) Cross laminated
heterolithic(CLH)
RLS facies consists of light grey silty
sand with carbon streaks. It is ripple
laminated, convolute bedded.
Syndepositional faults, soft
deformational structure and scoured
erosional feature. CLH comprises
sand/silt (55%) and shale (45%). It is
cross laminated grayish
heterolithics. The facies association
forms alternations.
Alternations of dark gray and light gray
heterolithic indicating seasonal variations
and differential supply of sediment. Scoured
erosional surface is an evidence of forced
regression occurring as a result of sea level
falling. Soft sediment deformational features
are ubiquitous and a nearly complete lack of
trace fauna indicate inhospitable conditions
caused by substrate instability, turbidity,
salinity, or energy stresses (Tye et. al.,
1999).
Foreshore Deposits:
Parallel laminated
sand(PLS), Symmetrical
Rippled Heterolithics
(SRH)
Foreshore Deposits:
Parallel laminated sand (PLS),
Symmetrical Rippled Heterolithics
(SRH)
The sedimentary structures such as
symmetrical/asymmetrical ripple, low angle
cross beddings and parallel laminations and
well sorted sand are indication of wave and
tidal influence.
Figure 7: Vertical Succession of GX 1 showing, depth,
uncored interval, High frequency cycles and Set
Sequence Stratigraphy –Sedimentation and Sea Level
of GX1
The whole sequence of GX1 consists of about 12 high
frequency cycles of fining upwards cycles. These are
regressive cycles that results from superimposition of
lateral adjacent tidal flats environments. The repeated
cycles are as a result of local transgression and
regressions within the tidal flat environment. Two uncored
intervals that were presumed to be shale with thickness of
25.3 and 42.3m respectively. These thick Shale layer are
Major flooding intervals. Frequency cycles are grouped
into 5 cycle sets (See Figure 7). The GX1 successions is
of Fourth order high frequency (HFC) cycles caused by
Milankovitch orbital cycles
Sedimentation and Sea Level fluctuation in GX1
GX1 succession is mostly tidal deposits at variable sea
level and sedimentation. Tidal environment is confined to
the shallow margins of the sea. It is divisible into 3 zones
in the shallowing order subtidal, intertidal, and supratidal
zone. Subtidal is the deepest zone of the tidal
environment, consists of the coarsest sediments of high
energy, and is submerged most of the time. Intertidal is
the mid zone and consists of the mixed sand and muds
(heterolithics), while the supratidal is mostly aerated and
submerged only twice in a month and consists of only mud

8. Sequence Stratigraphy, Variable Sedimentations, Sea Level Fluctuations and Net Effect on Reservoir Qualities: A Study of Cores from Two Wells GX1 and GX2
in the Niger Delta Basin
Otosigbo et al. 265
or evaporate. So, regression of the sea will bring the
intertidal upon subtidal, and supratidal upon intertidal
forming a fining upwards succession. There are 10 cycles
of fining upwards cycle in a tidal flat setting. These
regression causes deposition of lateral adjacent tidal flat
environment to be superimposed generating
characteristics succession of vertical facies. During tidal
regression, progradation produces a generalised fining
upwards successions. Each Succession represent high
frequency cycle (HFC) occurring at local sea level
reduction being controlled by tide (see Figure 8). A number
of related cycles adjacent or superimposed of which there
is an decreasing grain sizes or thinning of the beds forms
a cycle set (Tucker, 2003). The cycle set is a
representation of major sea regression that gave birth to
the high frequency cycles (see Sinusoidal Curve) About 4
Sets of cycle were delineated in GX1.
GX1 System Tracts
From the base, Lower intertidal sands were followed by
middle tidal flat deposits consisting of the bioturabated
heterolithics and shale (see Table 2). The next is the
shale(mud) of the upper tidal flat. Each is representing a
fining upwards cycle. The trio forms a set. Sedimentation
is low as portrayed by the evidence of bioturbation of
Skolithos Ichnofacies assemblages. The first major
flooding followed immediately after which the next high
frequency cycles, where the sea level variations are within
the subtidal zone. Three cycles of fining upwards
succession were built in 3 series of local regression and
transgression. During this period sedimentation is higher
than the accommodation as the sea level fell and thick
volumes of sands were deposited. Sedimentary facies like
planar cross bedded sandstone and Herringbone cross
bedded sandstone were deposited with features like bed
loads were deposited. The subtidal deposits was by a
major break that brought about exposure and plant growth
resulting in 2 meter- coal of swamp origin. Transgression
possibly brought about deposition of Lower intertidal
deposits which was not well defined. Second phase of
maximum flooding, was followed by another sea level fall
birthing 3 HFC forming a set. Tidal channel deposit
consisting of pebbly coarse sandstone with cross beddings
and scoured erosional surface was followed by middle tidal
flat deposit in a forced regression mode. Sedimentation
exceeded accommodation. Thick deposit of tidal channels
sands was deposited.
The vertical succession of GX1 sequence commenced
with a Falling Stage System Track, FSST to Lowstand
System Tract, LST consisting of Lower Intertidal and lower
part of the Middle Tidal Flats deposits. Starvation of
sediment input during the low tide or local regression
Evidenced by bioturbation skolithos and diplocraterion.
These deposits are made up of Fine Sandstone Facies,
Bioturbated Heterolithics and Shale Facies from bottom to
top. This represents a shift from deeper water to shallower
water of the tidal environment. The Transgressive System
Tract, TST may be represented by Bioturbated Heterolithic
Facies because transgression may have worked deeper
water facies (Boggs,2006). Low sediment input during the
rising tide or local transgression. This is evidenced by
vertical burrows skolithos and diplocraterion. The
Maximum Flooding Surface overlies the TST and consists
of shale facies. Accommodation is increased at this period
and the shelf is starved of the sediment. The Highstand
System Tract is the sediment deposited when the tide is at
highest level. Reactivation surfaces are evident in Subtidal
deposits of HST. High influx of sediment during the
highest tide had inbited faunal activity resulting in scarcity
of bioturbation and burrows.
Within the tidal environment, this consists of the Subtidal
deposits such as Planar Cross Bedded Sandstone and
Herringbone Cross Bedded Sandstone. These are pebbly
fine to coarse grained sandstone that formed 3 regressive
cycles of fining upwards. Sequence Boundary (SB)
bordered the HST and is a surface of subaerial exposure
represented by the coal facies of swamp deposit (Figure
3). The sequence boundary is overlain by Lower inter tidal
deposit of TST which would have been reworked (Boggs,
2006). Low sediment input during the rising tide or local
transgression is proved by vertical burrows of skolithos
and diplocraterion. The next MFS consists of the shale
facies. The next HST comprises of the Tidal channel
deposits se were deposited during regression of the sea
on the tidal environments. Scoured erosional is evidence
of the forced regression. Above the HST, the SB might
have been missing as it is represented by the LST made
up of middle tidal flat deposit. The sediments are
Bioturbated heterolithic and Shale facies.
Figure 8: GX1 Accommodation relative to time and HFC
during local sea regression within a major sea level
regression.
GX2 Sequence Startigraphy Sedimentation and Sea
Level Variation
The whole succession of GX2 progradational regressive
sequence caused by sea regression. The regression is as

9. Sequence Stratigraphy, Variable Sedimentations, Sea Level Fluctuations and Net Effect on Reservoir Qualities: A Study of Cores from Two Wells GX1 and GX2
in the Niger Delta Basin
Int. J. Geol. Min. 266
a result of sea level fall or sea ward movement of shore
line and it is known as forced regression. We have four
facies association each depicting an environment in
relation to its position to the shoreline. From the bottom,
we have thick volume of offshore shale of 15meters thick,
overlain by the rippled heterolithics of offshore transition.
This is followed by ripple laminated sand and cross
laminated heterolithics of Shoreface environment. The cap
deposit is theForeshore deposits consisting of parallel
laminated sand and symmetrical rippled heterolithics. .
Scoured erosional surface is evidence of forced regression
in this zone (see B in Figure). They form a vertical
succession of coarsening upwards sequence. The
presence of skolithos ichnofacies in the foreshore deposit
is indicative of slow sedimentation or scarce sediment as
the sea level fell.
Figure 9: Interplay of Seal level, sedimentation and
Accommodation. VI represent the shoreface and offshore
transition (modified after Nichols, 2009
Figure 10: Seal Level Variation with Accommodation. The
positions of the system tracts along a major rise and fall of
sea level.
Down the bottom offshore shale is sand incised into the
shale or extreme part of channel cutting into the shale. This
is possibly part of a Low stand wedge (see Figure 11).
The bottom tracts started with the top of the shale that
represent maximum flooding surface, where there is
maximum incursion into the land. The shale is incised with
Low Stand wedge of clean sandstone. The high System
Track, HST is represented by the offshore transition and
shoreface deposits. This is the period of highest sea level
rise. This is followed by the Sequence Boundary (SB)
represented by the unconformity (angular) showing a
period of non deposition and probably erosion of the
surface and tectonics. Falling Stage System Tract, FSST
is represented by the fore shore deposits. FSST is
represented by parallel laminated sand and symmetrical
ripple laminated heterolithics
Figure 11: GX2, A- Incised fill within the thick shale layer,
A and the white line is the position of minor faults, B-
Sequence boundary marked by an unconformity, short
burrows of Skolithos perpendicular to bedding plane.
Figure 12: Lenticular bedding in the Rippled Heterolithics
(RH), Offshore Transition Deposits of GX2

10. Sequence Stratigraphy, Variable Sedimentations, Sea Level Fluctuations and Net Effect on Reservoir Qualities: A Study of Cores from Two Wells GX1 and GX2
in the Niger Delta Basin
Otosigbo et al. 267
Effects of Sedimentation Rate and sea level changes
on Reservoir Property calculated.
The core porosity and permeability calculated for both well
revealed that reservoir properties are variable for both
wells even though both were mostly deposited during
regression of the sea. The porosity values for the entire
tidal environments close ranging from 22 to 26.2% but
variable permeability. High Stand System Tract
represented by the tidal channel deposits and subtidal
deposits recorded the highest permeability value, 4805
and 4010mD respectively (see Table 4). Their porosities
for both are equally high.
Table 4: Mean value result of 5 samples for each
Facies Association measures at reservoir condition,
3600Psi: Porosity and permeability for GX1 well.
In GX2, there is a general high porosity values throughout
the environment, but much lower permeability. The
porosity value for GX2 ranges from 26.3 to 32% (Table 5).
Table5: Mean value result of 5 samples for each Facies
Association measures at 400Psi: Porosity and
permeability for GX2 well.
Facies Association
Mean
Porosity (%)
Mean
Permeability
Foreshore 26.3 603.8
Shore face 32.4 1693
Offshore Transition 27.2 194
Offshore Not measured Not measured
The Shoreface environment representing the upper
Highstand System Tract (HST) has the highest porosity
and permeability, 32.4% and 1693mD respectively. Falling
Stage System Tract (FSST) sediment is the second best
in porosity in both wells. In GX1, the FSST represented by
the fine sandstone facies has a permeability of 1214mD.
In GX2, the FSST has a permeability of 603.8mD.
SUMMARY AND CONCLUSION
In the Coastal Swamp Deposit and Shallow offshore of
Niger Delta had experienced regression as a result of
progradation of the shoreline. Delta environment globally
is a complex environment being controlled by many
processes. Both tides and wave processes played vital
role in sedimentation of GX1 and GX2. These are
explainable by numerous waves and tidal structures such
as cross beddings, wave ripple laminations, herringbone
cross bedding, reactivation surfaces, flaser and lenticular
bedding. Coastal swamp deposit which is interpreted as
mostly tidal environment experienced progradation being
represented by high frequency cycles of fining upwards
vertical successions. GX2 vertical succession was
deposited as the as sea level fell but it is represented by 2
cycles of coarsening upward vertical succession. HST in
GX1 formed thick beds while in GX2, the beds are thinner.
It can be deduced from both wells that in a progradational
sequences, HST are the best reservoir. Bioturbations in
both wells have not favoured the reservoir property. This
is partly in accordance with Lemoine et al (1987) study
that Clay-lined burrow walls would create numerous
permeability barriers which significantly alter fluid flow
patterns within rock facies. Although not all the burrows
were lined. Other factor in the burrow could be the
connectivity.
ACKNOWLEDGEMENT
My sincere gratitude to Delta Terratek Limited for
permission to use their core laboratory and their support
and assistance throughout the study.
REFERENCES
Adesida, A. A, Reijers T. J. A, and Nwajide C. S. (1997).
Sequence stratigraphic framework of the Niger Delta.
Paper presented at the AAPG international conference
and exhibition, Vienna, Austria
Akaegbobi I.M,, Ola-Buraimo A. O., Otosigbo G.O., Eluwa
N. N., 2018. Textural Characteristics and Post
Depositional Effects on the Reservoir Rock: A Case
Study of Core Samples from Wells GX1 and GX2
Located on the Western Offshore of the Niger Delta,
Nigeria. International Journal Geology and Mining Vol.
4(2), pp. 223-236,
Akpoyovbike, A. A. (1978). Tertiary lithostratigraphy of
Niger Delta. The American Association of Petroleum
Geologists Bulletin, 62(2): 295-306.
American Petroleum Institute, (1990). Core Analysis, 2nd
Edition, Recommended Operating Practices RP40,
Washington, DC. USA.
Boggs, S. Jr. (2006). Principles of stratigraphy and
sedimentology. Pearson Education Inc. 4th edition.
Pp.483
Evamy, D. D., J. Haremboure, P. Kemerling, Knaap, W. A.,
Morlly, F. A. and Rowlands, P. H. (1978). Hydrocarbon
habitat of Tertiary Niger Delta. The American
Association of Petroleum Geologists Bulletin, 62: 1-39.
Eze, S. C., Mode, A.W., Adejinmi, A. and Ozumba, B. M.
(2016). Ichnological Characteristics and variability of
Miocene deposits in the Cenozoic Niger Delta:
Examples from cores in the Coastal Swamp and
Offshore Depobelts.
Facies Association Mean Porosity(&) Mean Permeability(mD)
Middle Tidal Flat 24 721
Tidal channel 24.8 4805
Lower Inter tidal 23.1 1376
Swamp Not measured Not measured
Subtidal 24 4010
Upper Tidal Flat Not measured Not measured
Middle Tidal Flat 26.2 551.5
Lower Inter tidal 22 1214

11. Sequence Stratigraphy, Variable Sedimentations, Sea Level Fluctuations and Net Effect on Reservoir Qualities: A Study of Cores from Two Wells GX1 and GX2
in the Niger Delta Basin
Int. J. Geol. Min. 268
Hospers, J. (1965). Gravity field and structure of the Niger
Delta, Nigeria, West Africa: Geological Society of
American Bulletin, 76: 407-422
Kaplan, A., Lusser, C. U. and Norton, I. O. (1994). Tectonic
map of the world, in Tuttle, M.L.W., Charpentier, R. R.,
Brownfield, M. E., 1999. The Niger Delta Province,
Nigeria, Cameroon, and Equatorial Guinea, Africa.
USGS, open-file Report, 99-150
Knox, G. J. and Omatsola, E. M. (1989). Development of
the Cenozoic Niger Delta interms of the Escalator
Regression model and impact on hydrocarbon
distribution.Proceedings KNGMG symposium Coastal
Lowlands, Geology and Geotechnology,181-202.
Kulke, H. (1995). Nigeria, in Kulke, H., Ed., Regional
Petroleum Geology of the World. Part II: Africa,
America, Australia and Antarctica: Berlin, Gebruder
Borntraeger, 143-172.
Lemoine, R. C., Moslow, T. F. and Lowry, P., 1987, Effects
of Primary Sedimentary Processes on Reservoir
Quality of "Deep Wilcox" (Eocene) Sandstones in
Fordoche Field, Louisiana. AAPG Search and
Discovery Article #91038©1987 AAPG Annual
Convention, Los Angeles, California.
Nichols G., 2009: Sedimentology and Stratigraphy. Willey
Blackwell 410pp
Mozley, P. S. (1989). Relation between depositional
environment and the elemental composition of early
diagenetic siderite. Geology, 17: 704-706.
Murat, R. C. (1972). Stratigraphy and Paleogeography of
the Cretaceous and Lower Tertiary in Southern Nigeria
in T. F. J Dessauvagie and A. J. Whiteman (Eds);
African Geology, Geology Department, Ibadan, Nigeria.
Obaje(2006). Obaje Obaje, N. G., 2009, Geology and
Mineral Resources of Nigeria. Springer Verlag-Berlin
Heidelberg 221pp.
Ola-Buraimo, A. O. (2013c). Biostratigraphy and
paleoenvironment of the Coniacian Awgu Formation in
Nzam-1 well, Anambra Basin, southeastern Nigeria.
International Journal of Scientific and Technology
Research, 2(3): 112-122.
Ola-Buraimo, A. O. (2013d). Biostratigraphy and
paleoenvironment of deposition of Bima and Gongila
Formations in M-1 well, Bornu Basin, Northeastern
Nigeria. International Journal of Scientific and
Technology Research, 2(8): 320-328.
Ola-Buraimo, A. O. and Akaegbobi, I.M. (2013C).
Palynological evidence of the oldest (Albian) sediment
in the Anambra Basin, southeastern Nigeria. Journal of
Biological and Chemical Research, 30(2): 387-408
Oyanyan, R.O., Soronnadi-Ononiwu, C.G., Omoboriowo,
A.O., 2013. Depositional Environments of sam-bis oil
field reservoir sands, Niger Delta, Nigeria. Adv. Appl.
Sci. Res 3, 1624-1638
Readings, H. G. (1996). Sedimentary Environments,
Processes, Facies and Stratigraphy. Blackwell, Oxford,
688p.
Reijers, T. J. A., Petters, S.W. & Nwajide, C. S. (1997).
The Niger Delta basin in Reijers, T. J. A ed; Selected
Chapters in Geology: Warri-Nigeria, SPDC Corp.
Reprographic Services, 103-117.
Short, K. C. and Stauble, A. J. (1967). Outline of Geology
of Niger Delta. The American Association of Petroleum
Geologists Bulletin, 51: 761-779.
Stacher, P. (1995). Present understanding of the Niger
Delta hydrocarbon habitat, in, Oti, M. N., Postma, G.,
eds., Geology of Deltas: Rotterdam, A. A. Balkema,
257-267.
Weber, K. J., and Dakouru, E. (1975). Petroleum Geology
of Niger Delta; Proceedings of the Ninth World
Petroleum Congress, v. 2. Geology: London, Applied
Science Publishers, Ltd., 210-221.
Whiteman, A. J. (1982). Nigeria its Petroleum Geology,
Resources and Potential: London, Graham and
Trotman, 394 p
William H. (1996). Atlas of reservoir geology, procedures
and guidelines. The Shell Petroleum Development
Company of Nigeria Limited.
Accepted 22 May 2019
Citation: Otosigbo GO, Nzekwe KE, Onwe IM (2019).
Sequence Stratigraphy, Variable Sedimentations, Sea
Level Fluctuations and Net Effect on Reservoir Qualities:
A Study of Cores from Two Wells GX1 and GX2 in the
Niger Delta Basin. International Journal of Geology and
Mining 5(2): 258-268.
Copyright: © 2019: Otosigbo et al. This is an open-access
article distributed under the terms of the Creative
Commons Attribution License, which permits unrestricted
use, distribution, and reproduction in any medium,
provided the original author and source are cited.