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

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
IJGM
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
1Akaegbobi Izuchukwu Micheal, 2Ola-Buraimo Abdulrazaq Olatunji, *3Otosigbo Gloria
Ogochukwu, 4Eluwa Ndidiamaka Nchedo
1Department of Geology, University of Ibadan, Ibadan, Nigeria
2Department of Geology, Federal University, Birnin Kebbi, Kebbi State, Nigeria
3,4Department of Geology, Federal University, Ndufu-Alike, Ikiwo, Ebonyi State, Nigeria
Textural parameters in terms of grain size and sorting play a vital role in determining the
sandstone reservoir characters such as porosity and permeability. Core samples of two wells
were described lithologically in terms of grain size, sorting, colour, structures and bioturbation.
A total of 101 samples were plugged for porosity and permeability measurement under steady
state flow. Textural properties exhibited by the lithofacies are resultants of energy of deposition
which yielded various lithofacies associations which are dominantly tidal channel, tidal flat,
deltaic shale, lower shoreface and marine shale. Reservoir genetic unit is an upgrade of
lithofacies association whereby the reservoir genetic units are map able over distance and across
wells based on peculiar characteristics. Therefore, the core GX1 is mainly tidal channel and tidal
flats while core GX2 is mainly lower shoreface to foreshore. Reservoir quality shows that the tidal
reservoir genetic units possess moderately high porosity and excellent permeability values
compared to lower shoreface reservoir genetic units of higher porosity values and relatively lower
permeability values. These differences are as a result of textural properties. Structures and
bioturbation which tend to increase or decrease the pore throat size. The effect is visible in the
high permeability of the channel deposits. Fine grain size and good sorting are responsible for
high porosity in lower shoreface deposits.
Keywords: Texture, Post depositional, Lithofacies association, Genetic unit, Porosity and Permeability
INTRODUCTION
Extensive studies have been carried out in the Niger Delta
over the last decades using seismic, wire line logs, coring
and core analysis to estimate petroleum reserves. In
Nigeria, over 1300 exploratory wells have been explored,
out of which about ¾ have been drilled for oil production.
Only about 30% of the oil is produced while 70% remains
bound to the reservoir rock. Core analysis result can help
prepare for effective secondary recovery.
This study is based on the conventional core data from two
wells located in the Tertiary Niger Delta and made
available by Delta Terratek Laboratory. The well names
chosen for this research are GX1 and GX2 for the offshore
and the deep offshore well respectively. The original
names and precise locations on map were omitted for
propriety reasons. The geographical location of the studied
wells in relation to the coastline of the Niger Delta is seen
in Figure 1.
*Corresponding Author: Otosigbo Gloria Ogochukwu,
Department of Geology, Federal University, Ndufu-Alike,
Ikiwo, Ebonyi State, Nigeria. E-mail:
gloriaotosigbo@gmail.com; Tel: +2348037137658
Case Study
International Journal Geology and Mining
Vol. 4(2), pp. 223-236, December, 2018. © www.premierpublishers.org. ISSN: 3019-8261

Akaegbobi et al. 224
Figure 1: Map of Niger Delta showing the location of wells GX1 and GX2 in relation to the coast
Previous core studies in the Miocene Coastal Swamp by
Egbu et al (2009) described four lithofacies associations
such as: foreshore, upper shoreface, middle shoreface
and lower shoreface.
Geologic Setting and Stratigraphy
Niger Delta is located in equatorial West Africa, between
latitudes 30 and 60N and longitudes 50 and 80 E. The
Cenozoic Niger Delta is situated at the intersection of the
Benue Trough and the South Atlantic Ocean where a triple
junction developed during the separation of the continents
of South America and Africa in the Cretaceous (Whiteman,
1982). The Benue Trough represents the failed arm of the
triple junction during the opening of the South Atlantic.
Niger Delta Province consists of Onshore and Offshore
section. The onshore section is outlined by geology of
southern Nigeria and western Cameroon. The offshore is
marked in the east by Cameroon volcanic line and in the
west by Dahomey basin. The province covers
300,000km2and includes the Cenozoic Niger Delta
petroleum system (Michele et al., 1999).
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 Dakouru, 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 (Whiteman, 1982; Figure 2).
Generally, the delta prograded over the subsidizing
continental-oceanic lithospheric transition zone and during
the Oligocene spread onto oceanic crust of the Gulf of
Guinea (Adesida et al., 1997).
Figure 2: Progradation of the coastline of the Niger Delta
(After Whiteman, 1982)
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:
Benin Formation, Agbada Formation and Akata Formation
representing from top to bottom (Table 1).

Int. J. Geol. Min. 225
Table 1: Stratigraphy of formations in Niger Delta Area, Nigeria (Modified after Akpoyovbike, 1978). The original modified
after (Short and Stauble, 1967)
Figure 3: Stratigraphic cross-section A-A` showing the three formations of the Niger Delta. Adapted after (Eze et al., 2016)
The Benin 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 shale in
the upper part, and an alternation of sands and shale in
equal proportion in the lower section (Reijer et al., 1997).
Pro-delta marine shale belonging to the Akata Formation
occur deeper in the section, where associated sandstone
units are generally lowstand turbidite fans deposited in
deep marine setting. The studied lithofacies sections fell
within the Agbada Formation, which is revealed in the
stratigraphic section A-A (Figure 3). GX 1 falls within the
Coastal Swamp while GX2 falls within the Shallow
Offshore.
METHODOLOGY
This study was carried out on conventional cores from two
wells (GX1 and GX2) in Delta Terratek Laboratory. The
lithological core description on the 1/3 slab sections of
whole cores were carried out under white light in the core
description room. These led to the establishment of
various facies. Related facies are grouped into facies
associations in order to interpret the depositional
environment.
The 2/3 sections of the cores were plugged following the
procedure of America Petroleum Institute, 1990 for core
analysis using liquid nitrogen. The diameter of plugs are 1
1/2 inches, while the length is at least times 1 ½ times that

of diameter. 20 plugs from GX2 were encapsulated prior
to analysis because of the unconsolidated nature of the
formation, while 81 samples from GX1 were plugged and
ran directly because of the consolidated nature, 71 were
plugged horizontally so as to be parallel to the bedding
planes, while 10 were plugged vertically so as to be
perpendicular to the bedding plane. About 101 plugged
samples were taken to Dean stark where the oil was
removed using methanol and chloroform for about 52
hours after which they were later dried to stable weight
using conventional drying oven. Absolute porosity and
permeability of 101 dried samples were determined using
porosimeter and permeameter respectively. Firstly, the
grain density was determined using Helium expansion
porosimeter at constant temperature using Boyles Law.
The porosity was also determined using the grain volume
already obtained, then measuring pore volume with
expanding helium in a hydrostatic test cell using Boyles
Law at overburden pressure.
The permeameter was run with nitrogen gas and the
principle was based on Darcy’s Law of laminar flow.
Following completion of the pore volume measurement
described above, the manifold was changed to
accommodate gas flow. Inlet pressure was measured
directly at the input sample face 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 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. The
flow was carried out under steady state (constant
temperature and pressure). They were run at both ambient
and over-burden pressure so as to mimic the reservoir
condition.
RESULTS AND DISCUSSION
Lithological Core Descriptions
The cores were described and grouped into sedimentary
facies. Sedimentary facies is defined as a distinctive rock
unit formed under certain condition of sedimentation
reflecting a particular process or environment. Facies is
distinct in terms of the lithology, colour, grain size,
sedimentary structure, biogenic structure and degree of
bioturbation. Eleven facies were described from GX1 and
six were described from GX2. Closely related facies that
reflect a particular environment were grouped into
lithofacies association.
GX1A, GX1B and GX1C were cored in a single well with
two discontinuities. The uncored intervals were presumed
to be shale thicknesses. However, there are textural
characteristics along the sequence of GX1 which tend to
vary based on their energy of deposition/environment;
their post depositional effects based on structures and
various degrees of bioturbation imparted on the lithofacies.
The characteristics are quite contrasting to that of GX2
which is situated deeper offshore. The litho facies are
described below and used to characterize the facies into
lithofacies associations.
From GX 1A, three sedimentary facies 1 to 3 were
delineated from bottom to top (see Figure 4) : Fine
sandstone facies(FSF) , Bioturbated heterolithics(BH) ,
and Shale Facies. Bioturbated facies occurred thrice
along the succession as well as Shale facies, a sort of
alternating beds.
FSF
This consists of laminated very fine sand with silts. It is
Skolithos burrows and Diplocraterion ichnofacies present
especially at the upper section. The vertical burrows
typical of skolithos and sometimes clogged with clay.
Occasional thin band of siderite and shale interlamination
are present. Tiny streaks of shale are present too. Load
cast present.
BH
Bioturbated Heterolithics facies occurs thrice along the
succession in alternation with Shale. The facies consists
of gray to dark sand, silts and shale rocks. The ratio of
sand: silt: shale = 45%: 40%, :15%. It has flaser beddings
and also convolute beddings. Few lenses of Siderite
occur. It is intensely bioturbated especially at the upper
section. The second layer of BH facies consists of about
55% sandstone/silts while the shale approximates 45%.
From the mid-section, the sandstone/silts ratio increases
to 75% while shale decreases to 25%. This is as a result
of minor fluctuation in sea level rise. The bioturbation is
intense at the mid-section of this facies. Sedimentary
structures present are herringbone and planar cross
laminations. Occasional sideritic lenses are present.
Boring feature of Diplocraterion trace fossil present
especially at upper sections.
The third upper layer of BH is a lenticular bedded
heterolithics (mudstone=30%, siltstone=35%,
sandstone=35%). Vertical burrows of skolithos are present
and bioturbations. Load casts are present with occasional
sideritic lenses.
Shale Facies
Shale facies is consistently layered in alternation with the
BH. The duo forms fining upwards successions in 3
phases.
GX1B

PCS: Planar Cross bedded Sandstone 3354-3363.3m
(9.3m)
Planar Cross bedded Sandstone(PCS) consists of fine to
medium grained, light grey silty sandstone. There is
presence of reactivation surfaces from which the cross
beddings build out (Figure 13 C). The base commenced
with medium grained texture which graded upwards to fine
grained textured sand. The facies exhibits a fining upward
sequence. There are few bioturbations at the fine grained
section. It is generally massive bedded.
HCS: Herringbone Cross-bedded Sandstone(3341.3-
3354m) 12.7m
HCS comprises fine to coarse pebbly brownish silty
sandstone. It commenced from the base with coarse
grained textured sandstone with pebbles, which graded
into medium grained sandstone at the mid-section. The
latter graded to fine grained sandstone at the upper
section. The succession is repeated to make up 2 cycles
of fining upwards sequence (Figure 5). The bedding
thickness thins upwards from the base. Coarse particles
are aligned along the bedding planes to form basal lag.
The base represents a flooding surface. It is poorly
consolidated. This represents prograding parasequences
in Tidal flat. The upper cycle has carbon streaks. It has
vertical burrows of Skolithos ichnofacies present. Total
thickness is 12.7meters
Coal (3340.5-3341.3) 0.8m
This is about a meter band of shaly bituminous brown coal.
This Floodplain environment
FSF: Fine Sandstone Facies(3332.7-3340.5m) 7.8m
FSF comprises very fine to fine grained brownish silty
sandstone. The sedimentary structures present are
parallel lamination, symmetrical ripples and herringbone
cross beddings. Herringbone cross beddings are limited.
Skolithos and Diplocraterion burrows are present and
concentrates more at the lower region. Load casts are
present.
S:Shale 3330-3332.7m ( 2.7m)
The Shale Facies consists of dark grey shale interbedded
with a thin bed of shaly sand. It is highly bioturbated within
the shaly sand with load casts are present. This is the zone
of mfs (maximum flooding surface)
GX1C
PS: Pebbly sandstone 3258.6-3264.7m (2.1m)
It is a parallel laminated and also massive bedded,
medium to coarse grained pebbly brownish silty
sandstone. It has Diplocraterion and skolithos ichnofacies.
Minor cross beddings occur.
LH: Laminated heterolithics 3255.6-3258.6m(3.0m)
LH Facies is a parallel laminated (Figure 13B) very fine
grained dark grey heterolithic (shale =65% and sand
=35%). Soft deformational structure occurs at the contact
with the overlying sandstone facies and the contact is
sharp.
PCS: Planar Cross-bedded sandstone 3247-3255.6m
(8.6)
PCS consists of a light brownish, fine to medium grained,
and planar cross bedded sandstones. It is also ripple
laminated. It has very thin laminars of shale/carbon. It is
highly bioturbated mostly at the fine grained sections
(figure 13A). Branching structures of Thallosinoides and
short vertical burrows of skolithos are present. It has
multiple synsedimentary faults. It exhibits a fining upwards
trend. It has sharp contact with the underlying formation.
PS: Pebbly sandstone 3237-3247m (10m)
PS is comprised of light grey to brownish sandstone. It is
medium to coarse grained pebbly sandstone. It is parallel
laminated Skolithos ichnofossils are present. There is
synsedimentary fault with throw of about 0.36m. It has a
scoured erosional base. It exhibits 2 cycles of fining
upward trends.
BH: Bioturbated heterolihtic 3235-3237m (2m)
Bioturbated heterolithics comprises of mottled sandstone
(40%), siltstone (30%) and mudstone (30%). BH is
burrowed. Sedimentary structures are flaser bedding and
wavy laminations which are formed as a result of
fluctuation in sediment supply or current (Figure 13B).
Ripples of sand and silt move while mud/shale is deposited
out of suspension times of slack waters. Convolute
beddings are also present.
S: Shale 3232-3235 (3m)
Dark grey shales. The thickness is about 3 m
GX-2Well
Shales 2424- 2440.3m(16.3m)
Dark grey shales, very fissile with a sandstone pinchout.
See figure 14AA(A) showing sand pinchout within the
shale unit.
RH: Rippled heterolithics 2421.6 – 2424m (2.4m)
RH consists of grey to brownish heterolithics of clay and
silts sizes. It has lenticular beddings, and wavy beddings
(Figure 14BB). It is (sand =50%, mudstone/siltstone
=50%) The laminars are convoluted. Synsedimentary
micro faulting is present. Slump and load structure
occurred at the contact with Shale facies and RH is seen
in figure 14BB(B).

RLS: Ripple laminated sand 2419.5-2421.6m (2.1m)
RLS facies consists of light grey silty sand. It has ripple
laminated beddings and also convoluted bedding.
Syndepositional faults and soft deformational structure
owing to loading. It has carbon streaks.
CLH: Cross laminated heterolithic 2419.1 –
2419.5m(0.4m)
CLH comprises sand/silt (55%) and shale (45%). is a cross
laminated very fine grained grayish heterolithics It has
angular unconformity with RLS facies below (Figure
13E(B). This is an evidence of forced regression occurring
as a result of sea level falling. There are alternations of
dark gray and light gray heterolithic indicating seasonal
variations and differential supply of sediment. Normal
micro-faulting was observed in this facies (Figure 13E (A)
PLS: Parallel laminated sand 2418.4 – 2419.1m (0.7m)
PLS is parallel laminated, very fine grained, grayish silty
sand. The laminations are sometimes convoluted. Shale
laminars are present. Vertical burrows of Skolithos
ichnofacies present.
SRH: Symmetrical Rippled Heterolithics 2416.7-2418.4
(1.7m)
SRH comprises grayish sand (55%) and shale (45%). The
ripple structures are symmetrical and wavy. It has
syndepositional faultings and loading structures (Figure
13E). It has stunted trace fossils burrows especially
Skolithos (Figure 13E(c). Strike slip fault was observed in
this facies (Figure 13E(D).
Figure 4: Core description and facies units in core GX1A
Five lithofacies units were delineated in core GX1B and
indicated as Facies 7 to 11 as seen and presented in
Figure 5 below.
Figure 5: Core description and facies units in core GX1B
Figure 6: Core description and facies units in core GX1C

Figure 7: Core description and facies units in core GX2
Lists of symbols
Table 2: Summary of GX1 Facies with Mean Porosity and Permeability values
Facies Depth (m) No of samples Porosity Range Mean Porosity
(%)
Permeability Range Mean Permeability
(mD)
1 3146.5 - 3150.6 5 15.8-219 22.0 72-3553 1214
2 3136.6 – 3145.7 8 23.3-27.3 25.4 432-1597 818
3 3129.8 – 3134.3 2 23.9-28.5 26.2 155-948 551.5
4 3124.2 – 3129.7 Not plugged Not plugged
5 3120.2 – 3123.7 6 21.3-24.5 22.4 237.2-266 104
7 3062.2 – 3074.9 12 24.1-27.8 25.3 915-10592 3496
8 3055.3 – 3060.3 7 22.3-27.4 24.0 977-12914 4010
10 3047.6 – 3054.5 8 20.9-26.6 23.1 441-5176 1376
11 3046.2 – 3047.2 2 14.4-15.8 15.1 11-18 13.0
12 2980.7 – 2985.6 6 24.7-27.1 24.8 639-8902 4805
13 2977.3 1 15.9 181
14 2970.7 – 2975.5 5 24.5-29.6 25.9 857-3628 2049
15 2962.2 – 2969.1 7 19.2-27.1 21.8 1167-7819 3247
16 2959.4 – 2960.0 2 23.7-24.3 24.0 637-787 712
 Note that unplugged sections are complete shale and mudstone sections
Routine Core Analysis Result - GX 1A: Porosity and
Permeability Result
All the 81 samples (70 horizontal and 11 vertical samples)
from well GX 1 were run at ambient pressure of 400 psi.
Subsequently GX 1A, GX 1B, GX 1C were subjected to
overburden pressure net effective of 3200, 3100, 3000 psi
respectively. The 17 samples from well GX 2 were
subjected to ambient pressure of 400 psi and overburden
pressure of 1500 psi. The mean values of the porosity and
permeability for each facies are shown in the Tables 2 and
3 below. The cross plots of porosity-permeability for the all
the values run at 400 psi and overburden pressure were
shown in subsequent Figures 8-11.

Figure 8: Permeability versus Porosity plot of samples of
well GX1A at 400 psi
Figure 9: Permeability versus porosity plot of samples of
well GX1B at 400 psi
Figure 10: Permeability versus porosity plot of samples of
well GX1C at 400psi
Figure 11: Permeability versus Porosity of samples of well
GX2 at 400 psi pressure
Table 3: Summary of GX2 Facies with Mean Porosity and Permeability values
Facies Depth (m) No of Sample Porosity Range Mean
Porosity (%)
Permeability
range
Mean
Permeability (mD)
1 2216-5 – 2217 Not Plugged Not Plugged
2 2214.5 – 2216.4 6 25.7-29.5 27.2 259-715 194
3 2212.9- 2213.9 7 32.0-35.5 32.4 1465-2870 1693
5 2211.4 – 2212.0 5 22.8-34.5 29.1 121-1896 688
6 2210.6 – 2210.9 2 20.3-26.5 23.4 174-865 519.5
DISCUSSION
Facies Associations, depositional environments and
reservoir characterization
Facies associations constitute several facies that occur in
combination and typically represent one depositional
environment (Readings, 1996). In most cases the
lithofacies which occur in groups or associations were
related in terms of energy levels within an environment of
deposition. Therefore, here lithofacies associations are
used for particular sediment body. Furthermore, an up-
scaling of lithofacies association is the resultant genetic
reservoir units whereby it is a practical subdivision of
reservoir into components which have consistent range of
reservoir properties, a consistent external geometry and
where available a set of log responses which can be
consistently recognized. Three lithofacies associations
were established in GX 1 well in a vertical succession.
GX 1A
Facies Association Tidal Flat
From the bottom the deposits grade from FSF (Fine
Sandstone Facies to Bioturbated Heterolithics
Facies(BHF) to Shale (Fig 12). The BHF composed of
interlaminated silts, silty sands and shale. This is a typical
fining upwards succession which is typical of a
progradation of sea. The multiple herringbone cross
beddings, flaser beddings, lenticular beddings are
significance in tidal environment where bi-current direction
indicate reversal of current (Boggs, 2009). Subsequently

there is repeated alternations of Bioturbated Heterolithics
overlain by Shale which potrays 2 successive cycles
progradation of shale(muds) on mixed sand and
muds(heterolithics). That is upper intertidal overlying
middle intertidal deposits The intense bioturbation at mid-
section of the facies and siderite nodules indicate low
sedimentation rate that allowed fauna to browse within the
sparse sediments. The presence of asymmetrical and
symmetrical ripple lamination suggests wave action. The
tidal flat deposits exhibit high permeability value averaging
1214 mD and porosity value averaging 22 % (Table 2, Fig.
12). BHF characterized by Ophiomorpha ichnofossil
presence; show both wavy lamination, flaser bedding but
much lower permeability averaging 818 mD and 515.5 mD
and higher porosity value averaging 25 % and 26.2 %
respectively (Table 2; Fig. 12). The relative increase in
average porosity might be due to intense bioturbation
exhibited by the facies, while the decrease in permeability
could be as a result of increase in clay content and lenses
of siderite mineral crystallization present.
Figure 12: Section of GX1 litho-log showing structures, effect of bioturbation and lithofacies environment of deposition

Wavy laminations, flaser
beddings in BH facies
Intense bioturbation
at upper PCS unit
Reactivation surfaces in
PCS facies
Herringbone
Cross bedding
in HCS facies
Parallel lamination in LH
Facies
(A)
(B)
(C)
(D)
(E)
Fig 13
Fig 13: Cores of GX 1 and GX2 wells showing core samples from different facies showing sedimentary structures and
bioturbation

A : S a n d p i n c h o u t
In Shale facies of GX2
Shale Facies
RH
B: Load and slump
structure at the contact
of RH and Shale facies
Of Gx2
Shale Facies
AA
BB
Figure 14
GX 1B
Facies Association: Subtidal Environment
The Subtidal deposits consists of three cycles of fining
upwards succession. The lithofacies are Planar cross-
bedded Sandstone(PCS) and Herringbone Cross-bedded
Sandstone(HCS). PCS comprises massive bedded
sandstone which grades from medium to fine to very fine
sandstone exhibiting fining upwards succession.
Reactivation surfaces occur as a result of current reversals
in tidal deposits (Tucker, 2003). HCS commenced from the
base with coarse grained textured sandstone with pebbles,
which graded upwards to medium grained sandstone, then
to fine grained sandstone at the upper section. It exhibits
2 successive fining upwards cycle. Herring bone cross
bedding indicates tidal deposits. Very few burrows and
bioturbation shows high sedimentation rate and energy.
The base represents a flooding surface. (Figure 13B).
Core porosity value averages 25.3 % and the permeability
has an average value of 3496 mD; thus, Subtidal deposits
exhibiting an excellent reservoir quality (Table 2, Figure.
12). The core porosity average value is 24.0 % while the
permeability average value is 4010 mD, implying a very
good to excellent reservoir quality (Table 2). Thereafter,
Coal facies of swamp origin represent a period of non-
deposition and aeration that supported vegetation. Lower
intertidal deposits which consist of Fine Sandstone facies
reoccurred immediately overlies the coal facies along the
sequence exhibiting herringbone cross beddings, parallel
laminations and wavy laminations. The textures are mostly
very fine to fine sands. Thick shale facies of upper tidal flat
overlies sequence. Skolithos Ichnofacies, Diplocraterion,
and bioturbation with load and cast structure which
suggests deposition in tidal channel environment. Subtidal
deposits have average porosity value of 23.1 % and
permeability averaging1376 mD. Shale Facies defined by
interbedding of thin bed of shaly sand, highly bioturbated
and associated with load casts deposited in a tidal flat
environment. The porosity and permeability values
average 15.1% and 13 mD respectively (see Table 2). The
reduction in reservoir quality of Subtidal deposits could be
as a result of sufficient increase in clay particles which clog
the pore spaces or pore-throat of the interparticullar
spaces.
GX 1C
Facies Association: Tidal channel Deposits
Tidal channel deposits consists of Pebbly Sandstone(PS),
Laminated Heterolithics(LH) and Planar Crossbedded
Sanstone facies. long to various environments of
deposition suggesting different energy of deposition and
textural characteristics. The pebbly sandstone texture
grades from fine to coarse, pebbly and becomes silty
sandstone upward. Bioturbation is evident. The sorting
ranges from moderate to well sorted with average porosity
and permeability of 24.8 % and 4805 mD respectively
(Figure 6, Table 2). PS was deposited in a tidal channel
environment characterized by excellent reservoir quality
as a result of coarser grain sizes, poor sorting, intense
bioturbation and greater pore throat size which permit
larger pore spaces and better connectivity of the pore
spaces. LH defined by mixture of sands, silts and shale
forms part of the middle intertidal deposits. It has average
porosity and permeability values of 15.1% and 159mD
respectively (Table 2; Figure 6). The poor reservoir quality
is as a result of high clay particle content and in-filling of
the burrow parts by finer particles which plugged the pore
spaces created by the effect of bioturbation.
The PCS and overlying PS consists of fining upwards
successions of fine to pebbly coarse grained. The
sedimentary structures such as planar cross bedding and
ripple lamination and frequency of erosional sharp contact

are indications of tidal channel deposits. The intense
bioturbations at the fine grained section represents tidal
flat deposits. Multiple synsedimentary faults would have
increased porosity. The average porosity and permeability
of PCS is 25.9 % and 2049 mD respectively suggestive of
good to excellent reservoir quality (Table 2; Figure 6). The
reservoir quality is attributed to integration of relative
medium grain size, synsedimentary faults and intense
bioturbation and planar cross bedding. which increased
the porosity and interconnectivity of the pore spaces? PS
exhibits higher average permeability value of 3247 mD due
to coarser grain size associated with larger pore throat,
reduced bioturbations, synsedimentary faults presence,
scarce cross bedding running contrary to flow direction for
fluid flow secondary erosional base serve as excellent
conduits. (Figure 6).
BH and overlying Shale facies comprised mottled sand,
silts and shale. It is majorly characterized by flaser
beddings, wavy lamination, convolute bedding and
bioturbations typical of tidal flat (Readings, 1996; .13D).
BH gives porosity and permeability value of 23.1 % and
660 mD respectively (Table 2). It is relatively low in
reservoir quality due to increase in clay particles and
bioturbation.
GX2: Shoreface to Foreshore
The Shale facies at the bottom of the GX2 core is an
offshore marine deposit with sandstone pinchout or wedge
at the upper section at the upper section on. The pinch out
structure is possibly part of extant part of incised valley fill.
This is cut during the relative sea level fall. The rippled
heteroliths consists of sand, slits and clay. The presence
of lenticular bedding, wavy bedding and convolute
beddings are all indications of fluctuation in currents and
sediments supply(Tucker, 2003). The slump and load
structures indicate sediments instability, which possibly
lead to micro faults (Figure 13E). Average porosity and
permeability values are 27.8 % and 194 mD respectively
which is fair in quality (Table 3). Rippled Laminated
Sand(RLS) is shoreface deposits consisting of silty
sandstone with ripple laminations convoluted beds. Soft
deformational structures are present (Figure 13E). Its
average porosity and permeability is 32.4 % and 1693 mD
respectively; characterized as good reservoir quality
(Table 3).
Cross Laminated Heterolithic (CLH) and Parallel
Laminated Sand(PLS) both form the Foreshore deposits.
CLH has equal proportion of sand and shale ratio (50:50).
It shows patches of yellow fluorescence under the UV light
implying presence of distributed hydrocarbon within. PLS
is planar laminated sand with evidence of shale laminars
and bioturbation (Figure13E). It has porosity and
permeability average values are 29.1 % and 688 mD
respectively with a relatively good reservoir quality (Table
3). Symmetrical Rippled Heterolithics is also laminated
with greater percentage of sand. SRH is bioturbated and
shows evidence of syndepositional fault and load structure
(Table 3; Figure 13E). Average porosity and permeability
value is 23.4 % and 519.5 mD respectively,thus indicating
a good reservoir.
Reservoir Genetic Units
Genetic unit association is a practical subdivision of
reservoir unit components which have consistent range of
reservoir properties, a consistent external geometry and a
set of log responses as the different measuring tools might
be available for assessment of the core samples.
Therefore, three main reservoir genetic units are
recognized. The main identified reservoir genetic units
present in the studied core samples are subtidal deposits,
tidal channel, mid intertidal flat, tidal flat deposits from
GX1, while there are offshore, offshore transition,
Shoreface and foreshore deposits in GX2. GX1B core
section, the reservoir genetic unit present is mainly
channel fill deposits consisting of lithofacies association-
tidal channel deposits and tidal flat deposit The GX1C is
characterized by mainly by tidal channel deposits reservoir
genetic unit comprising of lithofacies association of tidal
flat and deltaic shale. However, the GX2 core section is
mainly characterized by lower shoreface reservoir units
and the associated underlying marine shale. The
associated underlying marine shale with the lower
shoreface facies is the basis of distinction between lower
shoreface sediments and upper shoreface facies in log
and core sections.
In terms of reservoir quality the channel reservoir genetic
units possess moderately high porosity and excellent
permeability values (Table 2; Figure 8-10) compared to
lower shoreface reservoir genetic units characterized by
relatively higher porosity values and relatively lower
permeability values (Table 3; Figure 11). The relative
improvement of porosity values in lower shoreface
reservoir sands over channel fill sediments is not
unconnected to their relation with finer grain sizes, better
sorting due to winnowing by bidirectional currents
compared to channel fill deposits which are characterized
by coarse grains poor to relatively good sorting due to one
directional water current. However, the channel fill
reservoir genetic units exhibit higher permeability values
compared to lower shoreface due to poor sorting and
coarser grain sizes which provided larger pore throat and
intergranular connectivity, other factors include formation
of both syndepositional structures and post depositional
ichnofossil activities of various intense bioturbations
contributed to both improved or reduced porosity and
permeability of the reservoir rocks (Droser et al, 1986).
It is here suggested that the reservoir genetic units of core
sections in both GX1 and GX2 constituting the mid
intertidal and lower shoreface reservoir units,
characterized by high porosity and low permeability should
be subjected to artificial simulation for improved
permeability for effective secondary hydrocarbon
recovery.

CONCLUSIONS
Detailed study was carried out on the reservoir properties
of two wells: GX1 and GX2 of several kilometers apart at
the western part of the offshore Niger Delta. The
depositional environments were delineated based on
textural parameters including the grain size and sorting;
lithology, textural trend, and post depositional activities
including sedimentary/biogenic structures. Energy of the
depositional environment is the resultant of various
lithofacies associations which are dominantly tidal
channel, tidal flat, deltaic shale, lower shoreface and
typical marine shale deposit at deeper water depth.
Reservoir genetic units upgrade the lithofacies
associations into a more definite and characterizing
reservoir units that are mappable over long distance using
different measuring tools. Such mechanism categorizes
the GX1 into channel fill units and lower shoreface units
while GX2 is mainly of lower shoreface reservoir unit and
an impervious adjoining and underlying marine shale. The
Tidal channel reservoir units have lower porosity values
than the lower shoreface reservoir sands but possess
higher permeability values than lower shoreface reservoir
genetic units. The main controlling factors of better
reservoir properties of channel fill deposits over lower
shoreface reservoir genetic units include coarser grain
sizes with diminished clay/silts/fines and intergranular
connectivity. Presence of clays and silts reduces the pore
throat and it was evidenced in GX2 (Lemoine et al, 1987).
Other factors include formation of both syndepositional
structures and post depositional ichnofossil activities of
various intense bioturbation which the effect is not clear on
both well since a further study on ichnofabric study has to
be carried out in relation to the pore throat in order to
determine the positive or negative effects on both wells.
There is distinct variation from tidal flat to Foreshore
environment in relation to reservoir genetic unit of GX1 and
GX2well.
ACKNOWLEDGEMENT
The authors would like to appreciate Delta Terratek
Laboratories Services for QC core samples and
permission to use their facilities. Our appreciation also
goes to Palystrat Limited for her kind gesture in providing
materials used for interpretation.
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Accepted 27 November 2018
Citation: Akaegbobi IM, Ola-Buraimo AO, Otosigbo GO,
Eluwa NN (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 4(2): 223-236.
Copyright: © 2018 Akaegbobi 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.

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

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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