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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

[email protected]

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

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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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© 2017, Scientific Research Journal

(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 – 4 1.90503 Shallow marine

N – 5 -9.45023 Fluvial


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


Offshore Deposits

Marine Fan


KeyMeandering Rivers

Delta Plain

Deltaic


Offshore Deposits

Marine Fan




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


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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



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


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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


Formation in Well_4


Formation in Well_3




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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