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Scientific Research Journal (SCIRJ), Volume V, Issue II, February 2017 1 ISSN 2201-2796
Facies Architecture and Reservoir Properties of
Campanian-Maastrichtian Nkporo Formation in the
Anambra Basin, Nigeria
Cyril E. Ukaonu1, Samuel O. Onyekuru
2and Diugo O. Ikoro
2
1First Exploration and Petroleum Development Company Limited, Ikoyi Lagos, Nigeria
2Department of Geology, Federal University of Technology, Owerri, Nigeria
Abstract: Exploration for hydrocarbons in Nigeria’s inland basins
has not been commercially successful to-date, principally because of
lack of good knowledge of the geology, facies architecture and
reservoir properties of sediments in the inland basins. The facies
architecture and reservoir properties of the Nkporo Formation
sediments in parts of the Anambra Basin is evaluated in this study.
Five different facies were identified on outcrop sections, namely:
Interbedded shale-oolitic ironstone facies, Heterolithic Facies,
Fluvial Facies, Shallow Marine Facies and Basal Offshore Mud
Facies. These facies were also fingerprinted in the sub crop data
using bio stratigraphic and wireline log data of Well_3 and Well_4
drilled by SPDC Nigeria in Igbariam and Alor, respectively. The
relatively well-sorted sandstone units of the shallow marine deposits
and marginal marine facies have been observed as better
characterized reservoir rocks compared to the fluvial and open
marine facies with obvious clogging of pore throats by clays and
clay-filled minerals that limit reservoir quality. The stratified
nature of the shales and sandstones provides likely favorable
pathways for migration of fluids into potential reservoir rocks.
Introduction
The Anambra Basin(Fig. 1) is the second most prospective basin
in Nigeria with a very high gas potential (Ekweozor, 2006). The
basin is relatively unexplored (frontier basin) with only about 40
exploratory wells drilled since 1952, compared to the nearby
Niger Delta Basin. Five of the wells including Akukwa -1, Alo -
1, Amansiodo-1, Igbariam -1 and Ihandiagu -1 encountered gas
while one well- Anambra River -1 encountered oil (resulting to
about five discoveries (Avbovbo and Ayoola, 1981; Njumbe,
2002).. Besides a few other wells, have encountered oil shows,
while oil seepshave been observed at a few other locations in the
basin (Ekweozor, 2006; Nwajide, 2006; Onyekuru and Iwuagwu,
2010). Following several years of exploration for hydrocarbons in the
Anambra basin, oil was first found in this basin in 1967 by a
company then called Safrap, the fore-runner of what later
became Elf Petroleum Nigeria Limited (EPNL) and now Total
E&P Limited (Ndefo et al.,1987). Since that early discovery, no
other significant oil discovery has been madein the basin despite
significant investments in exploration and drilling activities
(Onuoha, 2005).Between 1953 and 1986, Anambra Basin has
experienced improved exploration activities but the attendant
results of these exploration activities have not been very
rewarding compared tothat in theNiger Delta basin.For instance,
OPL 917 in the Anambra Basin contains the Igbariam gas and oil
discoveries with estimated in place gas volumes of about
300billion cubic feet (bcf) and an oil in place of about 80
mmbbls (Ndefo et al., 1987).
A number of prospects and leads have also been identified south
of the discovery well.OPL 907 licence covers 1,462 km2 and
contains the Akukwa gas and condensate discovery, with an
estimated in place volumes of about 400 billion cubic feet (bcf).
Nigeria’s current oil reserve estimates stand at about 35 billion
barrels while the average annual reserves addition in the last ten
years is about 800 million barrels (Avuru, 2006). These reserves
are mainly from the onshore, offshore and recently the deep
offshore parts of the Niger Delta. Presently Nigeria is striving to
attain a daily oil production rate of about 4 million barrels. If this
feat is achieved, the reserve/production ratio for oil will be a
cause for concern as the country would be drawing closerto the
zero flat line in net reserves. While efforts are geared towards
getting the best exploration and production results from the
current assets and plays and from the new deep water offshore
prospects, there is great need to aggressively explore the
Nigerian inland frontier basin like the Anambra Basin to increase
hydrocarbon reserve.
The total exploration depth penetrated in the basin as recorded is
less than 3700m (Nwajide, 2006). This implies that significant
thickness of sediments in the basin is yet to be penetrated based
on the results of magnetic and gravity studies that have shown
clearly sediment thickness in excess of 5km (Avbovbo,
1978).Innovative technology is required to provide important
information on the facies architecture and internal
heterogeneities in the properties of reservoirs of formations in
the Anambra Basin. This study has investigated the facies architecture, reservoir
properties and depositional environments of the Nkporo
Formation sediments in the Anambra Basin using sequence
stratigraphic technique.
The study area covered prominent exposures at Leru area along
the Enugu-Port Harcourt Express Way(Fig.1). The data from the
sections are complimented with subcrop data including: ditch
cuttings; sidewall samples wire line logs, bio stratigraphic data
and paleobathymetric data.
Methods of Study
The selected outcrops of the formation were carefully logged and
the physical, biogenic and chemical sedimentary structures were
Scientific Research Journal (SCIRJ), Volume V, Issue II, February 2017 2 ISSN 2201-2796
described. Sandstone samples were also collected and analysed.
From these, lithological logs were made and used to delimit
lithofacies, assemblages, architecture, stacking patterns and
depositional environments.
Subsurface wire line logs from the exploratory wells that
penetrated the Nkporo Formation in the Anambra Basin were
acquired from the Department of Petroleum Resources, Nigeria.
The data include gamma ray, neutron, density, compressional
sonic and resistivity (deep and shallow resistivity).The
information extracted from these data were used for formation
evaluation to establish the petrophysical properties of the Nkporo
Formation reservoirs. Additionally,results of biofacies analysis
carried out on sidewall and ditch cutting samplesof the two
exploratory wells were also obtainedthrough the Department of
Petroleum resources (DPR) Nigeria. The data were studied and
interpreted using the strata bug software.
Systems tracts as well as the key stratigraphic surfaces together
with the defined faunal zones, ages and faunal events were
delineated usingbiostratigraphic and paleobathymetric datain
combination with the wireline logs. The key surfaces were
datedby calibration to the global cycles chart of Haq et al. (1988)
and Hardenbol et al. (1998).
Geological Setting and Previous Work on the Anambra Basin
The Anambra Basin is one of the inland basins in Nigeria that
has recorded significant level of hydrocarbon exploration
activities over the past three decades (Obaje et al., 2004). The
basin is bounded on the west by the Precambrian Basement
Complex rocks of western Nigeria and on the east by the
Abakaliki Anticlinorium.It issituated at the southwestern
extremity of the Benue Trough (Fig. 1). Several authors such
asBurke et al., 1972; Murat, 1972; Kogbe,1978; Whiteman,
1982;Agagu et al., 1985;Onuoha, 2005, have written on the
geological setting of Anambra Basin. Sediment deposition in the
AnambraBasin started in the Campanian with a short marine
transgression followed by a regression. The Nkporo Shale and its
lateral equivalents, the Enugu Shale and Owelli Sandstone
(Nkporo Group), constitute the basal beds of the Campanian
period (Table.1).
Fig.1:Geological map of Anambra Basin showing the study area(Adapted Ojo et al., 2009)
Table 1: Lithostratigraphic Units of Anambra Basin
Architectural Facies of Nkporo Formation
The late Campanian Nkporo Shale interpreted by Zaborski
(1983) as deltaic in origin marks the beginning of active
sedimentation in the Anambra Basin after the Santonian folding
episode. This formation together with its lateral equivalent, the
EnuguShale represent the marine and fossilferousprodelta facies
of the late Campanian–early Maastritchian depositional Nkporo
Cycle (Nwajide and Reijers, 1997). The Nkporo Shale in the
study area is predominantly mudrock.Five different facies were
defined on outcrop sections(Fig.2): (a) Interbedded shale-oolitic
ironstone facies (b) Heterolithic Facies (c)Fluvial Facies (d)
Shallow Marine Facies and (d) Basal Offshore Mud Facies
The best exposure of NkporoShale Formation which was logged
and sampled in this study occurs at Leru (Lokpaukwu) about
Km-56.5 –Km-72 south of Enugu on the Enugu–Port Harcourt
express road. The outcrops are exposed in a cascading
topography visible on both sides of the Enugu – Port Harcourt
expressway
Imo Formation
Nsukka Formation
Ajali Formation
Mamu Formation
NkporoFm
NkporoShale
EnuguFm
OwelliSs
AfikpoSs
OtobiSs
LafiaSs
Agwu Formation
Niger Delta
Anambra Basin
SouthernBenueTrough
Thanetian
Danian
Maastrichtian
Campanian
Santonian
Akata Formation
Eocene Ameki/Nanka Fm/Nsugbe Sandstone (Ameki Group)
Agbada Formation
Oligocene-Recent
Benin FormationOgwashi-Asaba Fm
Age Basin Stratigraphic Units
Imo Formation
Nsukka Formation
Ajali Formation
Mamu Formation
NkporoFm
NkporoShale
EnuguFm
OwelliSs
AfikpoSs
OtobiSs
LafiaSs
Agwu Formation
Niger Delta
Anambra Basin
SouthernBenueTrough
Thanetian
Danian
Maastrichtian
Campanian
Santonian
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(a)Interbedded Shale –Ooliticlronstone Facies. This facies unit
is made up of thick shale beds with alternating thin
ooliticlronstone beds (Fig.2). Thin section study of the ironstones
carried by Nwajide and Reijers (1996) show that they consist of
chamositeooids in a groundmass of granular siderite and pyrite.
The oolitic ironstone bands are dark grey in colour with pockets
of oolites and non- oolitic cobbles and mud. Some tiny burrows
exist which were identified as burrows of Planolites by Nwajide
and Reijers (1996).The Interbedded concretionary Shale-
ironstone faciesis interpreted as marginal marine deposits. This
facies unit is about 20m thick exhibiting sharp contact between
the black shales and the oolitic bands. There is an observed
thickness variation of the shale beds from 75cm to 390cm while
the oolitic bands range from 15cm – 60cm averaging 30cm. The
shales are parallel laminated while the oolitic ironstones are
devoid of any physical sedimentary structure.
(b)Heterolithic Facies
The heterolithic facies is made up of alternation of laminated
shale and fine to medium grained sand. Sand thickness increases
upwards (Fig.2).Dominant sedimentary structures in this facies
are ripple lamination and bioturbation.Contact between the
shales and sandstones is sharp with the shales exhibiting parallel
lamination. Burrows of Ophiomorpha and Thalassinoidesexist in
this facies (Bown, 1982, Frey et al., 1978). Prominent load casts
were also identified in the heterolithic facies (Plates. 1,2,3). This
faciesis interpreted as marginal marine deposits.
(c)Fluvial Facies
The stacking pattern of this facies isretrogradational exhibiting a
fining upward sequence(Fig.2). It is medium to coarse
grained.Dominant sedimentary structures are ripple lamination,
planar cross bedding and bioturbation.Faciesis interpreted as
fluvial deposits.The shales are parallel laminated. The basal
sandstones exhibit erosional contact with the underlying shale
bed. Burrows of Ophiomorpha and Thalassinoidesexist in this
facies (MacEachern and Pemberton 1992) (see Plate.4)
(d) Shallow Marine Facies
The facies is medium grained.Dominant sedimentary structures
are ripple lamination, trough cross bedding and bioturbation (see
Plate. 5).Sorting vary from moderate to good.The shales are
parallel laminated with wavy and lenticular bedding. Contact
between the shales and sandstone is sharp interpreted as shallow
marine deposits(Fig. 2).
(e) Black Shale Facies
The black shale facies which form the base of Nkporo Formation
consist of highly fissile, micaceous, pyritic and fossilferous
shales with thin lenses of clean fine grained quartz sands. Okoro
(1985) established the faunal assemblage to consist of low
diversity, low abundance and stunted ammonites, bivalves,
gastropods, crabs, foraminifera and ostracods.The dominant
sedimentary structure of this facies unit is the thin parallel
lamination with platy partings(Fig. 2). This facies is interpreted
as full marine (Unomah and Ekweozor 1993) (see Plate. 5).
In the subsurface, these facies have also been inferred on the logs
from Well_3 and Well_4 especially the facies associated with
open marine deposits, fluvial deposits, the shallow marine
deposits and marginal marine deposits based on the log signature
in combination with biostratigraphic data. (Figs. 3a&b).
Fig. 2: Sedimentary Log of Nkporo Formation at Leru (Lokpaukwu) about
Km-56.5-Km-72 Enugu Port Harcourt Expressway
Plates: 1, 2 & 3: Load Casts Structures Observed on the Heterolithic
Facies of NkporoFormation;Plate: 4: Ophiomorpha observed in the Fluvial facies
Plate: 5: Bioturbation observed on the Shallow Marine Facies
Depositional Environments and Model
Thepalaeoenvironmentof the CretaceousNkporoFormationin the
Southern Anambra basin was deduced from stratigraphic,
sedimentologic and faunal characteristics. Lithofacies units and
theiridentification and interpretation was a useful tool
foridentifying the depositional conditions under which
thesediments were deposited and preserved.The prominent
depositional environments identifiedin the study area, include
shallow marine, marginal marine,open marine and fluvial/tidally
influenced channel depositional environments.
Shallow Marine and Marginal Marine Environments. These are high energy environments (Fig.4). The sandstones are
well sorted and cross-bedded.From the subsurface using the
biostratigraphic interpretation and logs (Figs. 3a&b), backshore,
1 2 3
4 5
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foreshore and shoreface facies are present in the Nkporo
Formation. Globally, these facies occur in high energy shallow
marine environments. On the outcrop section, they are evidenced
by the presence ofSkolithosand Ophiomorpha burrows(Ekdale et
al., 1984, Frey et al., 1978).On theoutcrop section ofNkporo
Formationat Leru,as shown on the section in figure-2,the shallow
marine facies (Cross Stratified Sandstone Facies),the interbedded
concretionary lateritic ironstone facies and the heterolithic facies
belong to thisdepositional environments. They exhibit cross
bedding which is attributed to wave induced unidirectional
currents as well as shallow tidal currents developed in the open
nearshore environment. Hence they are interpreted as storm and
wave dominated environmentsbyWalker and Plint (1992). The
section also show that they exhibit thin bedded wave rippled
sandstones with a variety of trace fossils.Reineck and Singh
1973, Reading 2001) interpreted such deposits as shoreface
deposits of shallow marine origin.Swift and Niedoroda, 1985;
MacEachern andPemberton,1992) reported that when they are
strongly bioturbated sandstones with abundant Thalassinoides,
Skolithos andOphiomorpha traces, they probably represent the
upper shoreface and foreshore (see Plate. 5).
Figs. 3a&b: Architectural Facies of Nkporo Formation in Well_3 and Well_4
Fig. 4: Depositional Environments and Bathymetric Ranges(after, Allen, 1965;
1970).
It is believed that the provenance of some of the sandstones that
constitute these facies may have originated from the eroded
region of the AbakalikiAnticlinorium and the basement area of
the Cameroon basement (Hoque, 1976;Okoro, 1985).Presence
ofThalassinoides burrows were alsoreported in the shale fraction
of the heterolithic facies suggesting slow rate of sedimentation
(Ekdale et al., 1984;Okoro, 1985). Hence the alternation of rapid
and slow rates of deposition, the occurrence of Thalassinoides,
Skolithosand Ophiomorphaburrows, the dominance of sandstone
facies over shale facies suggest gradual shallowing of the sea and
a marginal shallow marine environment.
Reconstruction of the palaeoenvironment of the interbedded
concretionary lateritic ironstone facies suggests also a shallow to
marginal marine environment. Ironstones form in shallow marine
seas at no great distance from extensive low lying, well
vegetated warm humid climate (Halam and Bradshaw, 1979).
Sellwood (1971)proposed a model for precipitation of sideritic
ironstone beds of Yorkshire Lias (England) in shallow marine.
These ironstones are however known to occur in shallow agitated
waters but the mode of transport and mechanism responsible for
the formation and separation of the oolites still remains
controversial (Brookfield, 1971; Halam and Bradshaw 1979,
Sellwood, 1971).
In the middle part of theinterbedded concretionary lateritic
ironstone facies, there are plant remains and burrows. Okoro (
1985) observed bivalve and gastropod moulds. All these support
a shallow to marginal marine environment for this facies. The
lithologic and textural characteristicssuggest that the rate of
sedimentation involves rapid and slow rates of sedimentation
that gave rise to the rhythmic alternation of oolitic ironstone
bands and shale beds of this facies. Pettijohn (1975) reports that
oolitic deposits are poorly sorted and this is an indication of
accumulation in a turbulent medium.Oolites are generally known
to occur in shallow agitated waters suggesting that the oolitic
ironstone bands are probably products of rapid sedimentation
while the shale units were deposited during the slow and low
energy phase of the depositional medium. The shale portions
exhibit parallel and continuous laminations suggesting a period
of episodic suspensions in relatively quite waters. Potter et al.
(1980) attributed such preserved laminations to ineffectiveness
of wave and current action and product of episodic suspension.
This study therefore proposes a model of rapid and relatively
slow rate of deposition for this facies unit and a shallow marginal
marine environment with storm deposited oolitic ironstone
alternating with shallow marine muds.
Open Marine Environments
The black shale facies which form the base of Nkporo Formation
belong to this environment (Fig.2,Plate. 5). They are also evident
on the subsurface logs. it consist of highly fissile, micaceous,
pyritic and fossilferous shale with thin lenses of clean fine quartz
sand.The dominant sedimentary structure of this facies unit is the
thin parallel lamination with platy partings. The faunal
assemblage consist of low diversity, low abundance and stunted
ammonites, bivalves, gastropods, crabs, foraminifera and
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ostracods(Okoro, 1985).This facies is interpreted as full marine
(Unomah and Ekweozor, 1993) and is similarly interpreted here.
Fluvial Channel and Tidally Influenced Channel
Environments.
These arenon-marine to fluvio- marine environments. The fluvial
channel deposits are characterised by a fining upward or
retrogradational sequence(Allen, 1964, ArcherandKvale, 1989).
The thickly bedded fluvial facies deposits observed on the
outcrops of the Nkporo Formation are made of fining upward
gravel, sand and silt sequences and sometimes sand and gravel to
the exclusion of fine grained overbank silt and clays (Fig.2). The
fining upward gravel, sand and silt sequences are attributed to
waning current velocities as a channel is gradually filled
(Williams and Rust, 1969). From the subsurface logs of Well_3
and Well_4, it can be observed that they are identified with more
or less uniform sandy sequences with a single shale unit on the
gamma ray log (Figs. 3a&b). These intervals are interpreted as
fluvial deposits based on the log signaturesandbiostratigraphic
data. There is also presence of cross stratification and mud in the
outcrop section. Theseprobably relate to ebb-flood tidal
cycles(Yang and Nio, 1985; Leckie and Singh,1991; Shanley et
al. 1992).Lin, et al 1995. In the NkporoFormation, the fluvial
deposits are suggestive of meandering channels due to the
presence of burrows. In the subsurface well logs of Well_3 and
Well_4, these channel deposits show irregular fining upward
sequence but high rate of shale to sand (low Net-to-gross ratio)
as reflected by the bell shape of gamma ray log(Heckel 1972).
The sandstones are rarer and discontinuous due to abundance of
fine sediments..
Multivariate Discriminant Analysis Results The linear discriminant function which combines all the grain
size parameters into one linear equation was used in
differentiating depositional environments by substituting the
grain size parameters of the unknown into the equation to give a
value that can be compared to values obtained from modern
depositional environments. Thislinear discriminant function
serves to calibrate the environmental interpretation done. In this
study, the linear discriminant function of Sahu (1964) was used
to deduce the depositional environment of the sandstone units
(see Equations1-3). This Discriminant analysis was tested out for
shallow marine, fluvial and beach environments.The function
distinguishes Aeolian from beach, shallow marine and beach,
shallow marine and fluvial-deltaic and also fluvio-deltaic and
turbidite environments.
YU: Aeolian: Beach: =
-3.5688Mz+3.7016ð2-2.0766Ski+3.1135KG…Eq (1)
YU less than (-2.7411) =Aeolian deposit
YU greater than (-2.7411) =Beach environment
YU: Beach: Shallow Marine:
=15.6534Mz+65.70916ð2+18.1071Ski+18.5043KG…Eq (2)
YU less than (65.3650) =Beach environment
YU greater than (65.3650) =Shallow Marine
YU: Shallow Marine: Fluvial:
=0.2852Mz-8.7604ð2-4.8932Ski+0.0482KG………Eq (3)
YU less than (-7.4190) =Fluvial Deltaic Deposit
YU greater than (-7.4190) =Shallow Marine
From the result of the discriminant analysis, the environments
are predominantly fluvial to shallow marine. This agrees with the
result of outcrop,biostratigraphic and
paleobathymetricinterpretation data(Table-2).
Table. 2: Nkporo Formation (Outcrop) Multivariate Analysis Result (Sahu,
1964)
N – 1 -7.7004 Fluvial
N – 2 -1.641588 Shallow marine
N – 3 -6.93262 Shallow marine
N – 4 1.90503 Shallow marine
N – 5 -9.45023 Fluvial
N – 6 -6.14846 Shallow marine
N – 7 -3.13144 Fluvial
N – 8 -5.3065 Fluvial
N – 9 -7.36967 Fluvial
Depositional Model.
The stacking patterns, lithofacies association, sedimentary
structures, paleocurrent pattern and biostratigraphy shows that
marginal marine to shallow marine depositional model fits the
Nkporo Formation in the southern Anambra Basin.
Biostratigraphic information revealed the presence of bivalves,
gastropods, forams, dinoflagillates, cysts and pollens and spores
all indicate shallow marine(Zarboski, 1983).Presence of dwarfed
and juvenile bivalves and gastropods indicate fluvial /tidal sands
suggesting deposition in marginal marine (estuarine)
environment..Hence this study proposes a marginal marine to
shallow marine depositional model for the Nkporo Formation in
the southern Anambra formation.Figure- 5 shows the 3-
dimentional paleo-depositional model of the Nkporo formation.
Fig. 5:3-Dimensional paleodepositional model for the Nkporo Formation
Reservoir Properties and Quality.
Shallow Marine
Deposits
Offshore
Deposits
Deep Marine
Deltaic
Meandering Rivers
Delta Plain KeyMeandering Rivers
Delta Plain
Deltaic
Shallow Marine Deposits
Offshore Deposits
Marine Fan
Deep Marine Deposits
Shallow Marine
Deposits
Offshore
Deposits
Deep Marine
Deltaic
Meandering Rivers
Delta Plain KeyMeandering Rivers
Delta Plain
Deltaic
Shallow Marine Deposits
Offshore Deposits
Marine Fan
Deep Marine Deposits
KeyMeandering Rivers
Delta Plain
Deltaic
Shallow Marine Deposits
Offshore Deposits
Marine Fan
Deep Marine Deposits
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Nkporo Formation sandstonesreservoirs qualities are clearly
related to depositional facies, environment and
diagenesis.Depositional environment had its most profound
control on reservoir quality by way of dictating initial sediment
composition, texture and pore
fluid composition and compaction.
Among the factors that affect reservoir properties is compaction.
Compaction in the Nkporo Formation was investigated using
cross-plot of density and depth and confirmed by the cross-plot
of compressional sonic and depth as this plot is unaffected by carvings like the density. The
plots show that effect of compaction was evident in the
Nkporoformation(figs.6a&b).. However, the formation
atIgbariam (Well-3) area seem to have experienced more
compaction than at Alo (Well-4) area based on the compaction
trends of the sonic and density logs with depth
(Figs.6a&b)..Hence it is expected that petrophysical properties
will be reduced accordingly in the Nkporo reservoirs
encountered by Igbariam Well-3 compared to those seen by Alo
Well-4 (see Table-3).
Figs. 6a&b: Cross-plots of depth against compressional Sonic and Density in
Nkporo Formation.
The porosity values range from good to very good for the
shallow marine reservoirs of Nkporo Formation in Alo area.
However, this property is degraded towards the Igbariam area.
Same trend applies to the marginal marine and fluvial deposits
with the values ranging from poor to good (see Table-2).
Permeability was computed usingTimur-Coates modelbased on
the Free Fluid Volume (BFV),
Bound Fluid Volume (BFV), Total Porosity (PHIT) and Volume
of Clay (VCL). The permeability K was deduced from the
relation: K=aPHIb(FFV/BFV)
C……………………….Eq(4)
Where, a=empirical constant, typically equal to 10000
PHI=Total Porosity, FFV=Free Fluid Volume
BFV=Bound Fluid Volume, b is typically equal to 4 and c to 2,
but can vary according to local conditions
where BFV=VCL*PHIT_CL, FFV=PHIT-BFV
VCL=Volume of Clay, PHIT_CL=Total Porosity in Clay
The permeability result also follow the trend of the facies and
their depositional environment (see Figs 13-16). The shallow
marine reservoirs have a better permeability range compared to
the fluvial facies and the open marine facies.
Facies Definition from logs and Reservoir Quality
Net sand was evaluated using the Thomas Stieber Method by
computing continuous Net to Gross using the “fan chart” on the
Phie vs. Vclxplot.
From the Thomas Stieber Method four Facies were defined
based on their sand quality (NTG)(Figs.8-10):
1 – Shale Facies: NTG_Stieber<0.3
2 - Shaly lamination Facies: 0.3 ≤ NTG_Stieber< 0.55
3 - Sandy lamination Facies: 0.55 ≤ NTG_Stieber< 0.8
4 – Massive sandFacies: 0.8 ≤ NTG_Stieber
All the identified facies fall into one of these facies definitions
based on the sand quality ( NTG). The results showed that the
facies respected the trend ofthe reservoir properties. Facies 4
interpreted to be massive sands with NTG_Stieber>08 occur in
the blocky sand intervals of the shallow marine facies and the
channel bases of the fluvial facies(see Figs 13-16).
Sequence Stratigraphy. In the outcrop section, two maximum flooding surfaces are
inferred based on the stacking patterns at 4m and 61m from the
top of the section at Leru (Fig.7). Also a sequence boundary is
placed at 40m from the top of the section due to the erosional
surface (hard ground) observed (Plate.6). From the well logs,
delineation of the systems tracts as well as the key stratigraphic
surfaces also relied on the defined faunal zones, ages and faunal
events. Dating of these key surfaces was by calibrationto the
global cycle’s chart of Haq, et al (1988) and Hardenbol, et al
(1998).Three sequence boundaries (SB) are interpreted in
Well_3 andtwoin Well_4. In Well_3, the boundaries are placed
at 2225, 2410m and 3100m while in Well-4; they are placed at
1740 and 2170m based on minimum Foraminiferal faunal
abundances. Two Maximum Flooding Surfaces (MFS) were also
interpreted in Well_3 at depths 2720m and 3200m and Well-4 at
depths 1635m and 2110m based on stacking pattern and the high
kick in the gamma ray signatures. These key surfaces are
correlatable across the wells (see Figs.11&12 and table-
4).Overall retrogradational log motif characterizes the
Transgressive System Tracts (TST)while theHigh Stand System
Tracts (HST) are characterized by a progradational log motif. Of
interest isinterval: 3100m – 2830m interpreted as (Lowstand
Systems Tract (LST) – Prograding Wedge Complex) in Well-3.
This predominantly sand interval is characterized by blocky log
motif suggesting a further regression. The TS that terminated
this phase of deposition and initiated the next transgression is
characterized by abrupt shift of the GR log to the right and
resistivity log to the left (see Fig.11).
VCL < 0.40 V/V
PHIE > 0.08 V/V
SWE < 1 V/V
GROSS NET NTG PHIE_AV
M M M/M V/V
SHALLOW_MARINE_R1 57.00 31.07 0.55 0.22
SHALLOW_MARINE_R2 15.00 7.77 0.52 0.14
SHALLOW_MARINE_R3 17.00 14.77 0.87 0.22
SHALLOW_MARINE_R4 23.30 9.14 0.39 0.17
FLUVIAL_R1 39.00 23.30 0.60 0.19
GROSS NET NTG PHIE_AV
M M M/M V/V
FLUVIAL_R1 156.00 77.32 0.50 0.13
MARGINAL_MARINE_R1 56.00 13.11 0.23 0.12
FLUVIAL_R2 39.00 1.22 0.03 0.13
SHALLOW_MARINE_R1 61.00 19.81 0.32 0.20
MARGINAL_MARINE_R2 95.00 3.66 0.04 0.14
SHALLOW_MARINE_R2 87.00 53.51 0.62 0.14
FLUVIAL_R3 159.00 28.48 0.18 0.11
SHALLOW_MARINE_R3 33.00 3.05 0.09 0.11
FLUVIAL_R4 43.00 1.07 0.02 0.11
CUT-OFFS
ALO-1 RESERVOIRS
IGBARIAM-1 RESERVOIRS
TABLE-3: NKPORO FORMATION RESERVOIR PROPERTIES
Scientific Research Journal (SCIRJ), Volume V, Issue II, February 2017 7 ISSN 2201-2796
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Fig.7: Sequence Stratigraphic interpretation of Nkporo Formation at Leru
(Lokpaukwu) about Km-56.5 –Km-7 Enugu - Port Harcourt Expressway
Fig.8: NTG_Facies Definition forNkporo Formation in Alo
Fig.9: NTG_Facies Definition forNkporo Formation in Igbariam
Fig.10: NTG_Facies Definition forNkporo Formation in Alo and Igbariam.
Plate: 6: Erosional surface (hard ground) observed on the Nkporo formation.
Discussion and Conclusion This research on the Nkporo formation was carried out with the
objectives to investigate the facies architecture and stacking
patterns, document the stratigraphic sequences, the depositional
environments andprovide insights into the reservoir properties
especially in relation to the facies architecture,stratigraphic
sequences and depositional environments.The major
architectural facies elements of the deltasystems identified in this
study include: foreshore, backshore and shoreface deposits and
channel fills (tidally influenced estuarine and fluvial channels).
The stacking patterns range from progradational, retrogradational
and aggradational patterns giving rise to HighstandSystem Tracts
(HST), Transgressive System Tracts (TST) and Low Stand
System(LST) -Prograding Wedge Complex(PWC).
Sand Quality Variations The sand quality of Nkporo Formation appears to vary across the
area. The reservoir properties were observed to be lower in
Igbariam area compared to Alo area.This has been attributed to
the effect of compaction. Nkporo Formation at Igbariam (Well-
3) area seem to have experienced more compaction than at Alo
(Well-4) area as revealed by the compaction trends of the sonic
and density logs with depth (Figs. 6a&b). The area occupied by
the sandstones of the Nkporo Formation in Igbariamsuggest an
Scientific Research Journal (SCIRJ), Volume V, Issue II, February 2017 8 ISSN 2201-2796
areasubjected to deeper burial and more pronounced tectonic
activity during the end of Cretaceous (Odigi and Amajor,
2009c).Previous petrographic and diagenetic studies of thepost-
Santonian sandstones have shown that kaoliniteand illite are the
most frequently occurring authigenicclay minerals in the
Campanain-Maastrichtainsandstones (Ukaonu, 2009, Odigi,
2007).With such burial, an early diagenetic-clay coating around
detrital grains, kaolinite formation at intermediate burial depth
and growth of pore-filling illite at the deepestburial stage impact
on the sand quality. This explains the reduced sand
quality(reduced porosity and permeability) in the Nkporo
reservoirs encountered by Well-3 drilled at Igbariam compared
to reservoirs seen by Well-4 drilled at Alo.
Reservoir qualities in the sandstones of Nkporo Formation are
clearly related to depositional facies, environment and
diagenesis. However depositional environment had its most
profound control on reservoir quality by way of dictating initial
sediment composition, grain size sorting, facies nature and
stacking pattern.
The shallow marine depositional environments were observed to
have a better reservoir sand quality (Table-3).These are high
energy environments (Fig. 4). The sandstones are well sorted
with a better grain to grain contact.
NTG Faciesand Sand Quality
NTG facies in this study was modelled using the Thomas Stieber
method. Four NTG facies were realised; namely :shale
facies(NTG_Stieber<0.3), the shaly lamination facies(0.3 ≤
NTG_Stieber< 0.55), sandy lamination facies(0.55 ≤
NTG_Stieber< 0.8) and the massive sand facies (0.8 ≤
NTG_Stieber). The facies were found to reflect the sand quality,
the depositional environments and stacking patterns of the
system tracts.InAlo area, the sand quality as reflected by the
NTG seem to display a better recognition of the individual facies
with minimal overlap of facies in contrast to the observation in
the Igbariam area (Figs.8-10.).
Stratigraphic Sequences and Sand Quality
The system tracts were observed to exhibit different sand
qualities which are essential for hydrocarbon accumulation and
trapping.
The sediments of the Transgressive Systems Tract (TST) that
terminate atMaximum Flooding Surfaces were identified by
upward increasing microfossil abundance and diversity
(Figs.11,12) andretrogradational stacking patterns that suggest
upward increase in clay-shale contents. These system tracts
exhibit poor sand quality at the base and non reservoirs towards
theMFS. Their NTG using Stieber estimation ranges
fromNTG_Stieber<.3(for the shale facies)to 0.3 ≤ NTG_Stieber<
0.55 (for the shaly lamination facies, and sometimes from 0.55 ≤
NTG_Stieber< 0.8 (for the sandy lamination facies).On the shelf,
relatively regular changes in facies assemblages, forming
heteroliths of sand-shale packageswith maximum shale content
at the Maximum Flooding Surfaces (MFS) to minima at the next
overlying sandpackage, are interpreted as representing the fore
stepping, aggrading/prograding beds of the Highstand
SystemsTracts.TheNTG of theHST using Stieber estimation
ranges from0.55 ≤ NTG_Stieber< 0.8toNTG_Stieber>0.8 for the
best sand developed areas. They constitute mainly the sandy
lamination facies and the massive sand facies. The determination
of types of systems tracts and identification of systems tracts
associated with hydrocarbonreservoirs, seals and source rocks
are predicted by sequence stratigraphy. The morphology and
importance ofreservoirs and seals vary greatly between systems
tracts. The development of excellent reservoir sands and
sealsarise from shales of the upper Transgressive Systems Tract
(TST) enveloping sands on the outer shelfcharacterized by the
HST. The sand of the HST are observed to have good
petrophysical properties. In the NkporoFormation,the alternation
of Highstand Systems Tracts andTransgressive Systems Tract
sands and shales respectively provides a union of reservoir and
seal rocks that areessential for hydrocarbon accumulation and
stratigraphic trapping(see Figs 11-12, Table-4).
Fig.11:Sequence stratigraphic interpretation of Nkporo Formation section in
Well_3
SEQUENCE WELL_4 WELL_3 SYSTEM TRACTS CHRONOSTRATIGRAPHIC SURFACE
3200 MFS 70.49ma
3340-3200 TST
3100 SB 70.04ma
3200-3100 HST
3100-2830 LST
2720 MFS 69.75
2300-Base 2830-2720 TST
2170 2410 SB 69.42
2300-2170 2720-2410 HST
2100 2310 MFS 69.14
2170-2100 2410-2310 TST
1740 2225 SB 68.864
1
2
Table-4: Summary/Correlation of Well Log Based Sequence Stratigraphic Analysis of the Anambra Basin
3
Scientific Research Journal (SCIRJ), Volume V, Issue II, February 2017 9 ISSN 2201-2796
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Fig.12: Sequence stratigraphic interpretation of Nkporo Formation section in
Well_4
Fig.13: Petrophysical evaluation and NTG_Facies Definition forNkporo
Formation in Well_4
Fig.14: Petrophysical evaluation and NTG_Facies Definition forNkporo
Formation in Well_4
Fig.15: Petrophysical evaluation and NTG_Facies Definition forNkporo
Formation in Well_3
Scientific Research Journal (SCIRJ), Volume V, Issue II, February 2017 10 ISSN 2201-2796
Fig.16: Petrophysical evaluation and NTG_Facies Definition forNkporo
Formation in Well_3.
Conclusions
Architectural facies elements of the deltasystems identified in
this study include: foreshore, backshore and shoreface deposits
and channel fills (tidally influenced estuarine and fluvial
channels). The stacking patterns range from progradational,
retrogradational and aggradational patterns giving rise to
Highstand System Tracts (HST), Transgressive System Tracts
(TST) and Low Stand System(LST) -Prograding Wedge
Complex (PWC).
The reduced petrophysical properties of Nkporo Formation
observed in Well-3 drilled at Igbariam area compared to the
observation in Well-4 at Alo seem to suggest that compaction
was experienced more at Igbariam area than Alo area based on
the compaction trends of the sonic and density logs with depth.
The variation in reservoir qualities are in response to the
variation in facies architecture, stacking patterns of the
stratigraphic sequences and depositional environment.
A marginal marine to shallow marine depositional model for the
Nkporo formation in the southern Anambra formation has been
proposed by this study based on evidence from biostratigraphy,
log signatures, lithofacies analysis and multivariate discriminant
analysis.
The facies have been classified into four based on their reservoir
quality using the NTG Stieber facies method. These facies
include the :shale facies (NTG_Stieber<0.3), the shaly
lamination facies(0.3 ≤ NTG_Stieber< 0.55), sandy lamination
facies(0.55 ≤ NTG_Stieber< 0.8) and the massive sand facies
(0.8 ≤ NTG_Stieber).
In the Nkporo Formation, the alternation of Highstand Systems
Tracts (HST) andTransgressive Systems Tract (TST) sands and
shales respectively provides a union of reservoir and seal rocks
that areessential for hydrocarbon accumulation and stratigraphic
trapping based on their contrast in petrophysical properties . This
fact is very important for exploration strategies.
Recommendations.
There is need to integrate more wells from different parts of
the basin into the study so as to have a more regional view
of the facies variation, the stratigraphic sequences and the
effects on the reservoir quality. This will be of great
importance in future exploration strategies and placement of
infill wells.
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