Sedimentology and Geochemistry of Nigerian Strata

Sedimentology and Geochemical Evaluation of Campano-Maastrichtian Sediments, Anambra Basin, Nigeria.
IJGM
Sedimentology and Geochemical Evaluation of
Campano-Maastrichtian Sediments, Anambra Basin,
Nigeria.
1Uzoegbu MU* and2 Okon OS
1Department of Geology, Michael Okpara University of Agriculture, Umudike, Abia State, Nigeria.
2Department of Geology, Federal University, Oye-Ekiti, Ekiti State, Nigeria.
The Cretaceous sediments in the Anambra Basin (SE Nigeria) consist of a cyclic succession of
coals, carbonaceous shales, silty shales, siltstones and sandstones interpreted as deltaic
deposits. Statistics reveals a graphic mean range from 1.5 to 2.8, sorting range from 0.45 to 1.58,
skewness range from -0.58 to 0.32 and kurtosis between 0.38 and 2 for the Ajali Sandstone. From
these results, the sandstones in the area are dominated by medium to coarse grains, poorly to
moderately sorted, coarse skewed and very platykurtic sediments. Further sedimentological
evaluation in six localities indicates fluvial-flood plain-marginally marine facies for the Mamu and
Nsukka Formations and marine for the Nkporo and Enugu Shales. The geochemical evaluations
show that total organic carbon (TOC) (8.95wt%) of the samples constitutes that of good to
excellent source rock with oil, oil/gas, gas prones for kerogen types I, II/III, III indicated by Rock-
Eval S2/S3 (9.13). The high oxygen index (OI) (42.61 mgCO2g-1
TOC) suggest deposition in a shallow
marine environment. The Tmax (430o
C), indicate the immaturity to onset of maturity of these source
rocks. Potential reservoir units occur in the fluvial sandstones of the Ajali Formation and in the
marginal marine and flood plain sandstones of the Mamu Formation. The shales and claystones
of the Nsukka and Imo Formations may provide regional seals.
Key words: Sedimentology, Geochemistry, Kerogen, Petroleum system, Depositional environment, Cretaceous,
Anambra Basin.
INTRODUCTION
The Anambra Basin became the site of major deposition
following the Santonian folding in southeastern Nigeria
(Fig. 1). Compressional uplift of the Lower Benue Trough
succession (Albian to Coniacian) along a NE-SW axis was
accompanied by tectonic inversion and downwarping of
the Anambra platform. Estimates of total sediment
thickness in the Anambra Basin from gravity
measurements range from 1000-4500m (Ladipo et al.,
1992), out of which between 3000 and 3500m were
deposited during the late Cretaceous (Upper Campanian
to Maastrichtian). Traditionally coal petrographic studies
are mainly used for determining coal quality, coking
properties and composition, paleodepositional
environment, or coal ranking (Taylor et al., 1998). Recently
multi-analytical approach to coal petrography analysis
uses SEM-EDS, microprobe, Rock-Eval 6 pyrolysis,
solvent extract, and gas chromatography - Mass
Spectrometry (GC – MS) (i.e biomarkers), hydrous
pyrolysis (e.g Fowler et al., 1991; Taylor et al., 1998;
Petersen, 2002; Walker and Mastalerz, 2004) were
reviewed.
*Corresponding author: Uzoegbu M Uche. Department
of Geology, Michael Okpara University of Agriculture,
Umudike, Abia State, Nigeria. Email:
mu.uzoegbu@mouau.edu.ng GSM: 08030715958; Co-
Author Email: otobong.okon@fuoye.edu.ng
International Journal Geology and Mining
Vol. 3(2), pp. 110-127, September, 2017. © www.premierpublishers.org. ISSN: XXXX-XXXX
Research Article

Uzoegbu and Okon 111
Fig. 1: Generalised geological map of the SE Nigeria (boxed areas of inset) showing the location of the coal deposits. Numbers
indicate Cretaceous and Tertiary formations shown as follows 1. Asu River Group; 2. Odikpani Formation; 3. Eze-Aku Shale; 4.
Awgu Shale; 5. Enugu/Nkporo Shale; 6. Mamu Formation; 7. Ajali Sandstone; 8. Nsukka Formation; 9. Imo Shale; 10. Ameki
Formation and 11. Ogwashi-Asaba Formation (modified from Akande et al., 2007)
According to Nwajide (2005) sedimentation in the
Anambra Basin was dominantly terrigenous resulting in up
to 3000m thick shale (60%), sands (40%) and limestone
(<1%). Onuoha (2005) identified three hydrostratigraphic
units in the Anambra Basin that includes;
1. Quaternary deposits and sandy horizons of the Ameki
Formation
2. The Ajali Sandstone, the sandy horizon of the
overlying Nsukka Formation, and the
3. Sandy beds in the Awgu, Nkporo and the lower Mamu
Formations.
The first hydrostratigraphic unit is very shallow
(approximately not more than 500m deep) to form viable
reservoir.
Ladipo et al. (1992) and Nwajide (2005) inferred that the
Mamu and Nsukka Formations are probably delta front
sand bars. Ajali Sandstone on the other hand was
attributed to fluvial deposition (Agagu, 1985) characterized
by large channels containing lithic fill of fining upward
pebbly sandstones (Nwajide, 2005). It is also related to the
development of shallow marine subtidal sand bars
(Ladipo, 1985; Ladipo et al., 1992). These potential
reservoir sands are mostly laterally extensive and may
reach local thickness of up to 5015ft (>1000m) where
stacked. This paper deals in delineation and
characterization of sedimentology and biomarker
geochemistry evaluation of the basin.
GEOLOGY OF THE AREA
The infilling of the Anambra and Afikpo basins started
during the early Campanian to the early Paleocene
(Danian) under two major eustatic cycles; the more
pronounced Nkporo transgression and the less active
Nsukka transgression with the Anambra basin showing the
most complete stratigraphic sections (Fig. 2). These
cycles are also found in the Afikpo syncline SE of the
Abakaliki anticlinorium and the Dahomey embayment,
west of the Ilesha basement spur, although both are
incomplete (Murat, 1972).
The first cycle which took place during the Lower
Campanian to the Maastrichian started with the deposition
of the Nkporo shale whose lateral (age) equivalents are

Int. J. Geol. Min. 112
Fig. 2: The Stratigraphy of the Anambra Basin Southeastern Nigeria (After, Ladipo, 1988 and Akande et al., 1992;
Modified in Uzoegbu et al., 2013b).
the Enugu shale and Owelli sandstone (Fig. 2). This is the
basal unit of the Campano-Maastrichian transgression and
comprises of dark mudstone, gray, fissile friable shales
with thin beds of marl, sandy shale and limestone overlying
an angular unconformity (Reyment, 1965). The regressive
phase was marked with the development of a large offlap
complex, starting with the paralic sequence of the Mamu
formation (Lower coal measure) overlying the Nkporo
shale (Reyment, 1965). It is thought to be lower
Maastrichian in age with a basal part that contains thin
marine intercalations, while the coal bearing part consist of
fresh water and low salinity sandstones, shale, mudstone
and sandy shales with coal seams occurring at several
levels (Simpson, 1955).
The Mamu formation is overlain by the continental
sequence of the Ajali sandstone. This sandstone unit has
received several names such as false bedded sandstone
(Tattam, 1944), basal sandstone (Simpson, 1955) etc. iIts
present name was given by Reyment (1965) after
establishing its type locality at the Ajali river. Virtually all
exposures of the formation are characterized by a lateritic
profile at the top. It was deposited during the regressive
phase of the Campano-Maastrichian transgression and is
age Maastrichian.
The Ajali sandstone is overlain conformably by the Nsukka
Formation (Upper coal measures), and it consists of
alternating succession of gray sandy shales, sandstones,
plant bearing beds and thin beds of coal (Reyment, 1965).
Thin bands of marine limestone heralded the return of
marine sedimentation at the top of the formation. These
dark shales and the intensely bioturbated sandstones are
well exposed at Ihube, along the Enugu – Port Harcourt
expressway. The age range of the formation is late
Maastrichian to Danian based on the fossil record. This
formation bears the K/T boundary which is described by
Reyment (1965) as a period of transition in Nigeria. Mbuk
et al. (1985) identified this boundary in the Nsukka
Formation in Ozu Abam area of Abia State.
MATERIALS AND METHODS
Intensive field study covered a total of five localities from
where samples were taken. Eleven representative
sandstone samples were retrieved from the field survey
along road cut at Milliken Hills at approximately 100m apart
from each location.
In the laboratory, the samples were later disaggregated
and dried for at least 24 hours in an oven at 600 C to
remove the moisture before analysis. Afterwards, sieve
analysis was carried out for each of the samples. Lumped
samples were disintegrated so that the sieve analysis
result can be authentic. Sieving technique is applied to
separating the grains of various size-classes, as proposed
by (Ingram, 1971). British Standards were employed with
AGE
SEDIMENTARY
SEQUENCE
LITHOLOGY DESCRIPTION DEPOSITIONAL
ENVIRONMENT
REMARKS
ANKPA
SUB-
BASIN
ONITSHA
SUB-
BASIN
MIOCENE
OLIGOCENE
OGWASHI-
ASABA FM.
Lignites, peats,
Intercalations of
Sandstones &
shales
Estuarine
(off shore bars;
Intertidal flats)
Liginites
EOCENE AMEKE NANKA
FM. SAND
Clays,shales,
Sandstones
& beds of grits
Unconformity
Subtidal, intertidal
flats, shallow marine
PALEOCENE
IMO SHALE Clays, shales
& siltstones
Marine
MAASTRICHTIAN
Clays, shales, thin
sandstones & coal
seams
Coarse sandstones,
Lenticular shales,
beds of grits &
Pebbls.
Clays, shales,
carbonaceous
shale, sandy shale
& coal seams
NSUKKA FM.
AJALI SST.
MAMU FM.
? Estuarine
Subtidal, shallow
marine
Estuarine/ off-shore
bars/ tidal flats/
chernier ridges
CAMPANIAN
ENUGU/
NKPORO SHALE Clays & shales Marine
CONIACIAN-
SANTONIAN
AWGU SHALE
TURONIAN EZEAKU SHALE
Clays &
shales Marine
ODUKPANI FM.
CENOMANIAN
ALBIAN
L. PALEOZOIC
ASU RIVER GP.
B A S E M E N T C O M P L E X
Sub-
bituminous
Sub-
bituminous
3rd Marine
cycle
Unconformity
2nd Marine
cycle
Unconformity
1st Marine
cycle
Unconformity
NODEPOSITION
(? MINOR
REGRESSION
REGRESSION
(Continued
Transgression
Due to geoidal
Sea level rise)
TRANSGRESSION
(Geoidal sea level
Rise plus crustal
Movement)
Coal
Rank
~
~ ~
~
~
~ ~
~ ~
~ ~
~
~
~
~
~
~
~ ~
~
~ ~
~
~ ~
~ ~
~ ~
~ ~
~
~
~ ~
~~
~
~
~
~
~
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~ ~ ~
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
.
. .
. ..
... .
... .

a sieve set in the order of mesh sizes: 2 mm, 1mm, 500
µm, 250 µm, 150 µm and 75 µm respectively. The sieves
were arranged in such a way that the one with the highest
opening was placed at the top while the one with the
smallest opening was placed at the bottom with the base
pan at the base. The dried samples were placed at the top
sieve, covered up and placed on a mechanical shaker. The
amplifier was used to operate the shaker at a medium
frequency. The sieve analysis was carried for about five
minutes while checking at intervals. After the sieve
analyses have been completed, the sediment in each
sieve was weighed and recorded. These procedures were
carried out for each of the eleven samples.
Thin sections of representative samples of sandstones
were prepared for petrographic studies of the minerals and
textures of the grains using a polarizing microscope. Both
friable and consolidated samples were used. The friable
sandstone samples were initially impregnated prior to
cutting. The impregnation helped to harden the samples.
The highly consolidated samples were thoroughly washed
with water. The samples were each mounted with polished
slide on a glass slide using Canada balsam. The mounted
sample was again ground on a lap wheel with a coarse
abrasive and was later washed with water. These was
followed by manual grinding with sludge of fine abrasive
on a glass plate until the slide was fine or thin enough for
individual mineral identification. The slide was then
thoroughly washed with water and was allowed to dry
before covering with a cover slip. A total of eleven
representative samples of sandstones were cut into thin
sections. The prepared slides were viewed under plane
polarized light (PPL) and cross polarized light (CPL) using
a petrological microscope to obtain information on the
lithology, fabric, texture and mineralogy. The stage of the
microscope was rotated continuously to attain different
views of the slides. The petrographic studies enabled the
identification of various mineral contents as
photomicrograph of each slide were taken under plane
polarized light and crossed nicols to ascertain their
compositional features.
For geochemical analyses, a total of four bulk shale
samples (about 30-50 g) were used. The samples were
washed using water/organic solvent to remove the dirty
and sands on them. The washed samples were kept in the
oven for 24 hours to dry at temperature of 40°C. The dried
samples were crushed by mortar and pestle. After that,
each of these crushed samples was divided into two equal
parts. Half of the crushed samples in each case was
packaged in a plastic bag and the remaining half was
pulverized by vibratory disc mill Model RS 100 to <50µm
size.
RESULTS AND DISCUSSIONS
The sedimentological characteristics of the Anambra
Basin fill are based on field descriptions of lithofacies at
outcrop sections comprising road-cuts, mining pits and
stream valleys at six localities in the northwestern and
southeastern portion of the basin, namely Lokpauku/Leru,
Onyeama, Okpara, Milikin Hills, Okaba and Ezimo (Fig. 1).
The Campano-Maastrichtian Enugu Shale with an
approximate thickness of 150 m has its type section
exposed at Imilike Agu. Simpson (1954) and Reyment
(1965) described the Enugu Shale from its area in Asata
and Obweeti stream valley within Enugu area. The Enugu
Shale which underlies the plain east of the Enugu
escarpment in the mapped area covers about 40% of the
area. It consists of predominantly soft, dark grey blue
carbonaceous shales and sandy shale which alternate
with siltstones and occasional bands of mudstones
intercalation.
In the study area, the lithologic unit of the Enugu Shale can
be subdivided into three main subunits as (a) Sandy shale,
(b) Grey shale and (c) Carbonaceous shale. An exposure
of this subunit is recognized along the channel of the Ekulu
River bank and Leru in Lokpauku. It comprises of dark soft
carbonaceous shales which are thinly laminated (Fig. 3).
The shales are dark blue with nodular concretions and
potholes at the base with intercalation of clay which
alternate with siltstone (Akhirevbulu and Ogunbajo, 2011).
At the top it is weathered to ferruginized sandstone. Along
Opi new road which is about 3km outside the studied
area,a good exposure of this subunit was recognized.
The type sections of this subunit in the study area were
recognized along the channels of Ofianzu and Isiogene-
Onu streams at Mbu and at Eha-ndiagu along the channels
of Omeme and Iyi-akwa streams. The grey shales
alternate with mudstones and fine sandstones. The grey
shale is commonly found overlying the carbonaceous
shale in most of the outcrops studied, and also has nodular
concretions. This shale gradually graded into sandy shale
upwards and in some places it is weathered to reddish
brown coarse sandstone or ferruginized sandstone and
the top of which is capped with laterites.
The grey shales are characteristically fissile and split into
thin flexible flakes of various sizes. The colours of dark
carbonaceous shale and grey shale are due to the quantity
of organic matter or oxidation state of iron in the rock
(Pettijohn, 1975; Ojo and Akande, 2009, Valkarelo and
Bhattacharya, 2009; Uchebo, 2010; Obaje et al., 2011;
Omali et al., 2011; Uzoegbu et al., 2013a). In thin section,
the carbonaceous Enugu/Nkporo shales are a mixture of
silt-size quartz and clay minerals which appear to be
floating in dense mudstone matrix (Plate). Quartz grains
are more abundant than clay minerals and shales are
generally silty. The fine quartz crystals are sub-angular to
subrounded and scattered within the clay matrix. Dark
minerals which are colourless under crossed polar are
present and carbonaceous matter mainly plant fragments
are observed.

Table 1: Lithostratigraphic succession established in the studied area (modified from De Swardt and Casey, 1963).
Age Formation Member Thickness (m) Descriptions
Maastrichtian Ajali
Sandstone
Ajali
Sandstone 10
Medium to coarse grained, poorly consolidated, friable with whitish
mudstone bands at various horizons. The sandstone is typically
cross-bedded.
Maastrichtian Mamu Mamu
5
2
Medium to fine grained, Friable white sandstone.
Alternation of grey shales, and sandy shale which are thinly
laminated and occasionally carbonaceous shale with coal seams
at various horizons in rhythemic manner.
Campano -
Maastrichtian
Enugu
Shale Enugu
1.5
Soft dark grey to bluish carbonaceous shales. Nodular concretions
are present in these shales and the shales are thinly laminated
with mudstones.
Fig. 3: Lithologic profile of the Enugu/Nkporo Shales.
The sandy shale forms the upper part of the grey shale in
the study area and consists of alternating thinly laminated
shales, very fine sandstones which are whitish to brownish
in colour with thin bands of clay intercalations (Plate). The
top of this sandy shale is weathered to reddish brown and
is capped with laterites. Varve structures are common to
the sandy shales. Well exposures of this subunit are
recognized along the new road cut at Mbu, Ogboduaba
and along the channel of Ofianzu stream. The sandy shale
contains plant debris and bioturbation structures in some
places. Microscopic studies show that sandstone consists
of subangular to subrounded quartz floating in the matrix
(Plate). The grains are equant to subequant. The modal
analysis of the framework elements shows; Quartz (85%),
Matrix (10%) and Opaque (5%). The quartz grains contain
monocrystalline quartz (95%) and polycrystalline quartz
(5%). The sandstone is quartz arenite (Pettijohn et al.,
1972; Ojo and Akande, 2009, Valkarelo and Bhattacharya,
2009; Uchebo, 2010; Obaje et al., 2011; Omali et al., 2011;
Uzoegbu et al., 2013a).
The occurrence of plant remains, concretions, shale casts,
varve structures, thin lamination and freshwater
associated with these subunits of the Enugu/Nkporo
Shales suggests deposition under shallow marine
conditions. The gradation from carbonaceous shale to fine
and siltstone also suggests open or shallow marine
environment.Mamu Formation is subdivided into lower and
upper members due to the characteristics of the sediments
(Lower Mamu Member). The lower Mamu covers about
30% of the studied area and three subunits of this
formation are recognized; Carbonaceous shale, Grey
shale and Sandy shale (Fig. 4).
MAASTRICHTIAN
MAMU
ϕϕϕ
6.40
5.20
3.40
0.50
7.60
11.60
12.00
AGE FORMATION THICKNESS
(Meters)
LITHOLOGIC SECTION DESCRIPTION
Overburden
Reddish ironstone capping
Ferruginized thinly
Laminated sandstone
Grey mudstone
Massively bedded fine
Grained sandstone
Yellowish-brown massively
Bedded siltstone
Parallel laminated dark
Grey shale
Coal bed with dull lustre
Fig. 4: Lithologic profile of the Mamu Formation.
This subunit has well outcrops at the sources of Igbogbo
stream, Iyi-Agu Orba and along channel Iyi-Vava at
Anyazulu Umabor. The shale is dark thinly laminated with
occasional thin bands of clay intercalations towards the
upper part (Akhirevbulu and Ogunbajo, 2011). Each
lamination is about 2cm. Varve structures and concretions
are found in some of places. The shale is commonly found
in very deep valley of the streams at the lower slope of

Table 2: Seive analysis for white sandstone of Mamu Formation.
Phi (ø) Wt. Retained on Seive Corrected wt. Cumulative wt. Cumulative wt. % Weight (%)
0.0-5.0
0.5-1.0
1.0-1.5
1.5-2.0
2.0-2.5
2.5-3.0
3.0-3.5
3.5-4.0
0.6
2.4
3.2
2.9
4.0
8.3
19.8
12.5
0.6
2.4
3.2
2.9
4.0
8.4
20.1
12.7
0.6
3.0
6.2
9.1
13.2
21.6
41.7
154.3
1.1
5.6
11.5
16.8
24.2
39.7
76.7
100.0
1.1
4.5
5.3
5.9
7.4
15.5
37.0
23.3
Seive loss
Total wt.
0.7
53.7
Enugu escarpment. In some places this subunit delineates
the contact between the Enugu Shale and lower Mamu
Formation.The grey shale consists of thinly laminated dark
grey shale alternating with mudstone, and fine sandstone
and gradually graded into sandy shale at upper part. The
grey sandy shale is highly fissible and contains plant
impressions and bioturbation structures. The sandy shale
at the upper part of the grey shale alternates with fine
sandstone and siltstone and occasionally poorly laminated
in some places. The sandy shale is highly weathered to
reddish brown and the sandstone is ferruginized. The
upper part is commonly capped with laterites. In the
studied area the grey sandy shale has excellent exposures
at Omeme stream and Ugene stream (Upper Mamu
Member). The upper Mamu Formation covers about 10%
of the studied area and is made of only one unit. In the
studied area this unit is not well exposed but it has an
excellent outcrop along Opi road cut. In this unit the main
rock type is whitish to grey sandstone. The sandstone is
very fine grained, friable and thinly laminated with thin
bands of mudstone intercalation. Each lamination is about
1cm thick. Towards the base the lamination increases in
thickness for about 3cm and the sandstones became
coarser. These grades upwards to medium and fine
grained. This shows fining upward sequence (Ojo and
Akande, 2009, Valkarelo and Bhattacharya, 2009;
Uchebo, 2010; Obaje et al., 2011; Omali et al., 2011;
Uzoegbu et al., 2013a). This unit is highly burrowed and
bioturbation structures which are evidence of ichnofossils
were found.
In general, the coal-bearing sections in these locations
consist of cyclic successions of coal, carbonaceous
shales, heteroliths (sandy shales and shaly sandstones),
siltstones and bioturbated sandstones. The cyclothems
are well exposed at the Onyeama section, and are
interpreted as deposits typical of a deltaic setting (Akande
et al., 2007). At Onyeama, the exposed succession shows
a basal coal seam (Fig. 5) 1.2m thick, overlain by
carbonaceous and parallel-laminated grey shale. This is
overlain by a heterolithic unit of grey shale with sandstone
streaks, which grades upward into fine-grained
sandstones. Sieve analysis of the sandstone shows that it
is negatively skewed; leptokurtic in grain size distribution
and with mean size between 2.50-3.40 ø (Table 2) and the
histogram plot show a unimodal distribution (Fig. 5). The
abundance of carbonaceous shale in the study area and
coal seams in the Mamu Formation suggests non-marine
or swamp deposits.
Weight(%)
0
10
20
30
40
50
-1 0 1 2 3 4 5 Phi (ø)
Mode 4.0-4.5ø
Fig. 5: Histogram plot for white sandstone of Mamu
Formation.
The regressive phase in the second sedimentary fill up of
Anambra Basin is marked by the development of paralic
sequence of Mamu Formation which is overlain by
continental sequence of the Ajali Sandstones (Reyment,
1965; Murat, 1970).
The Nsukka Formation (Upper Coal Measures)
conformably overlies the Ajali Sandstone Formation and
occurs from the north of Awka to the upper Ankpa sub-
basin. The lithology is mainly interbedded shales,
siltstones, sands and thin coal seams (Fig. 6), which have
become lateritized in many places where they
characteristically form resistant capping on mesas and
buttes.
The formation is diachronous, spanning upper
Maastrichtian into Danian. Depositional environment has
been suggested to be similar in many ways to the Mamu
Formation (Lower Coal Measures) i.e
transitional/shoreline, mud flat and swamps, deposited
during a largely regressive phase. The result of sieve
analysis for Nsukka Formation is found in Table 3.

Table 3: Sieve analysis for Nsukka Formation.
Phi (ø) Wt. Retained on Seive Corrected wt. Cumulative wt. Cumulative wt. % Weight (%)
˃ 2.0
-2.0-1.5
-1.5-1.0
-1.0-0.5
-0.5-0.0
0.0-0.5
0.5-1.0
1.0-1.5
1.5-2.0
2.0-2.5
2.5-3.0
3.0-3.5
3.5-4.0
4.0-4.5
˃ 4.5
0.6
0.5
2.2
3.8
6.4
9.8
14.2
8.8
4.7
1.9
1.2
1.7
0.7
0.6
0.5
2.2
3.8
6.5
9.9
14.3
8.9
4.7
1.9
1.2
0.7
0.7
0.6
1.1
3.3
7.1
13.6
23.5
37.7
46.6
51.3
53.2
54.4
55.1
55.8
1.0
2.0
5.9
12.8
24.4
42.1
67.6
83.6
92.0
95.3
97.4
98.7
100.0
1.0
1.0
3.9
6.9
11.6
17.7
25.5
15.9
8.4
3.4
2.1
1.3
1.3
Weight(%)
0
5
10
15
20
25
-2 -1 0 1 2 3 5
Phi (ø)
Mode 1.5-2.0ø
4
30
Fig. 6: Histogram plot for Nsukka Formation.
The type section described above is similar in lithology to
upper part of the 50 m thick sequence described for the
Nsukka Formation at Oji River by Offodile (2002). The
Akpugo Eze section has thicker shale beds. The lower part
of the section is comprised of shales, sandy clay, medium
to coarse blackish sands and brownish red sandy clay,
blackish shales, medium to coarse brownish sands and
lateritic sand at the top. This part of the sequence (50 m)
is considered in this study to be a part of the Ajali
Formation which underlies the Nsukka Formation. Sieve
analysis of the sandstone shows that it is negatively
skewed; leptokurtic in grain size distribution and with mean
size between 2.50-3.40 ø (Table 3) and the histogram plot
show a bimodal distribution (Fig. 6).
The upper part of the Ajali Formation is composed of
coarse sandstones with thin lenticular shales, beds of grit
and pebbly gravel (Ladipo et al., 1992). This upper section
is considered to be part of the Nsukka Formation. The
abrupt contact between the Nsukka and the Ajali
Formations in the section is drawn at the top of thin
lenticular shales, beds of grit and pebbly gravel at 50 m
because the Nsukka Formation does not contain beds of
grit and pebbly gravel (Fig. 3). The Awgu section is
composed of coarse pebbly blackish sand (Offodile, 2002)
and at Ezimo it composed of shale, lateritic sand and thin
coal seams (Uzoegbu, 2010) and is part of Nsukka
Formation. The Nsukka Formation is different from the
overlying Imo Shale which consists of intercalations of
bluish to greyish clays and deep bluish marine shales
(Ladipo et al., 1992; Offodile, 2002). It occurs, as highlands
at Ebenebe and Ugwuoba in Enugu and Anambra States
(Ladipo et al., 1992). With some similarity and comprising
views about the Mamu and Nsukka Formations a
lectostratotype was proposed to provide a standard for the
definition and recognition of the Nsukka Formation
(Uzoegbu et al., 2013).
The lectostratotype does not include upper and lower
boundaries of the formation which are however, well
illustrated by Federal Department of Water Resources,
Enugu and Unomah and Ekweozor (1993) in Fig. 7.
Simpson (1954), Reyment (1965), Adeleye (1975)
described the Ajali Sandstone from its type areas along
Ajali river and Nkpologu in Nsukka area. It occupies about
15% of the studied area and is found on the western
margin of the area. This is the youngest formation and it
overlies the Mamu Formation. It has well exposed
outcrops at Milikin Hills in the study area and along Opi
road cut. At Milikin Hills this outcrop has a well developed
fascinating cave system. The sandstone is
characteristically friable poorly consolidated, white to
pinkish colour, iron stained with thin bands of white
mudstones. At Milikin Hills the sandstone is typically cross-
bedded on small scale (Fig. 8). The grains grade from
medium to fine grain upwards and the beddings are

214
AGE FORMATION THICKNESS
(m)
LITHOLOGY
50
NSUKKA
MAASTRICHTIAN ϕϕϕ
214
200
150
100
9.50
DESCRIPTION
Overburden
Greyish siltstone
Medium – coarse brownish sand
Brownish red sandy clay
Blackish shales
Coarse pebbly blackish sand
Hard blackish shales
Brownish white sand
Siltstones
Greyish black coal
Sand blackish coarse grained
Shale blackish
Sand coarse
Shale blackish, organic soft
Sandy clay, greenish siltstone
Sand medium – coarse blackish
Shales, soft blackish grey
Sand medium – coarse blackish
Sand, blackish, medium – coarse
Greyish black coal
ᵹ
ᵹ
Fig. 7: Lithologic profile of Nsukka Formation.
massive at the base and became thinly laminated
upwards.
Sieve analysis of the sandstone shows that it is well sorted
and skewness is positive and ranges from nearly
symmetrical to leptokurtic grain size distribution (Table 3).
Histogram plots show bimodal distribution between 1.0-
1.50 ø (Fig. 9). The mechanical analysis of coarse
sandstone shows that the sandstone is poorly sorted
negatively skewed leptokurtic and unimodal to slightly
bimodal in some locations.
The ichnofossils such as burrow fills, trails and preserved
leaf impressions are recognized (Fig. 8). The characteristic
reticulate venation pattern of some of the leaf impressions
suggests that they are typical dicot plant (Rao et al., 1981).
Plant fragments are found in the mudstone or claystone
interbedded with the sandstone. Bioturbation structures
are commonly found as ichnofossils (Ojo, 2009). Banerjee
(1979) recognized the presence of plant fragments in thinly
laminated shale and mudstones in Ajali sandstone at
Nkpologu southwest of Nsukka and these can be
correlated with those of the studied area in Nsukka
southeast.
The microscopic studies show that the dominant mineral is
medium grained quartz crystals with a small amount of
matrix and few opaque minerals probably iron oxide (Ojo
and Akande, 2009, Valkarelo and Bhattacharya, 2009;
Uchebo, 2010; Obaje et al., 2011; Omali et al., 2011;
Uzoegbu et al., 2013a). The matrix is made of silt-size
quartz and no feldspar is found (Plate). The modal analysis
of the framework elements shows; quartz (95%), matrix
(10%) and opaque (4%).
Monocrystalline quartz = 95%
Polcrystalline quartz = 5%

Table 4: Percentile values for grain size analysis.
Sample locations 5% 16% 25% 50% 75% 84% 95%
L1 (M) 0.1 2 2.4 2.85 3.15 3.6
L1 (B) 1.95 2.15 2.25 2.45 2.7 2.95 3.6
L2 (M) -0.3 1.9 2.35 2.95 3.4 3.55 3.75
L2 (B) -0.9 0.1 2.2 2.85 3.2 3.65
L3 (T) -0.2 2 2.2 2.5 3.1 3.3 3.7
L3(M) -0.6 1.9 2.2 2.5 3.1 3.3 3.7
L3 (B) 1 2 2.15 2.4 2.6 2.7 3.2
L4 (T) 1.95 2.15 2.25 2.55 2.95 3.25 3.65
L4 (B) 0.5 2.05 2.2 2.5 2.9 3.2 3.7
L5 (T) -0.8 2.6 3.35 3.55 3.8
L5 (B) -0.6 0.9 1.9 2.95 3.4 3.55 3.8
Fig. 8: Lithologic profile of Ajali Sandstone.
The grains are equant to subequant and are predominantly
subangular to subrounded (Table 4). Many large
polycrystalline quartz grains have been found to break
along individual grain boundaries, indicating a textural
inversion (Hoque and Ezepue, 1977). They therefore
concluded that significant portion of the finer sand and silt
fraction could come from fragmentation of large
polycrystalline quartz in a dynamic environment.
Cross bedding is the dominant sedimentary structure of
the formation. It is associated with reactivation surfaces,
mud drapes, tidal bundles, backflow ripple channels cut
Age FM THICK
NESS (m)
LITHOLOGY DESCRIPTION
4.0
1.5
AJALISANDSTONE
1.0
2.3
1.0
1.5
MAASTRICHTIAN
1.0
1.3
Thinly laminated very fine
sandstone friable white on
some beds and reddish in some
units due to ferruginization.
Well foliated friable bed with
reddish to pinkish colouration.
Pinkish friable cross bedded
strata.
Friable and brownish well bedded
unit.
Highly burrowed and cross
bedded unit.
Friable and ferruginized well
Bedded unit.
Reddish friable cross bedded
unit.
Friable brownish fine
grained bed.

Table 5: Calculated values for grain size parameters for Ajali sandstones.
Sample locations Graphic mean Standard deviation Skewness Kurtosis
L1 (M) 1.88
(Medium grain)
1.31
(Poorly sorted)
-0.42
(Very coarse skewed) 1.74 (Very leptokurtic)
L1 (B) 2.52 (Fine grain) 0.45 (Well sorted) 0.32 (Very fine skewed) 1.50 (Leptokurtic)
L2 (M) 2.8 (Fine grain) 1.03 (poorly sorted) -0.44 (Very coarse skewed) 1.58 (Very Leptokurtic)
L2 (B) 1.5 (medium grain) 1.58 (poorly sorted) -0.36 (Very coarse skewed) 0.54 (Very platykurtic)
L3 (T) 2.6 (Fine grain) 0.92 (Moderately sorted) -0.08 (near symmetrical) 1.78 (Very Leptokurtic)
L3(M) 2.57 (Fine grain) 1.00 (Moderately sorted) -0.15 (Coarse skewed) 1.96 (Very Leptokurtic)
L3 (B) 2.37 (Fine grain) 0.51 (Moderately well sorted) -0.21 (Coarse skewed) 2.00 (Very Leptokurtic)
L4 (T) 2.65 (Fine grain) 0.53 (Moderately well sorted) 0.28 (Fine skewed) 1.0 (Mesokurtic)
L4 (B) 2.58 (Fine grain) 0.77 (Moderately sorted) -0.02 (Near symmetrical) 1.87 (Very Leptokurtic)
L5 (T) 2.05 (Fine grain) 1.46 (Poorly sorted) -0.42 (Very coarse skewed) 0.38 (Very platykurtic)
L5 (B) 2.47 (Fine grain) 1.33 (Poorly sorted) -0.58 (Very coarse skewed) 1.20 Leptokurtic)
and fills, lateral accretion surfaces, as well as Skolithos
and Ophiomorpha ichnogenera (Ladipo et al., 1992).
These structures characterized the formation over the
entire basin, and suggest tidal origin within a shallow
marine environment. Paleocurrent trends across the basin
suggest a depositional environment similar to the southern
part of the North Sea, which is characterized by helicoidal
tidal currents and dominated by large-scale sand waves
(Dike, 1976a).
In the studied area Ajali is characterized by presence of
fine to medium grained sandstone, small scale cross-beds,
leaf impressions, burrow-fills and absence of calcareous
and carbonaceous matters. These suggest a continental
environment or fluviatile environment. Grove (1951),
Reyment (1965) suggest that Ajali was deposited in a
continental environment. Hoque and Ezepue (1977) gave
a detailed account of petrology of Ajali sandstones and
confirmed the earlier views as regards its depositional
environment viz continental (fluvio-deltaic).
Based on the uniqueness of the Ajali Sandstones more
detailed analytical work was done on sieve results base on
Fork and Ward (1957) model ogive curves plotted for Figs.
8 and 9a-c. The field photos are cited on Plates. Table 4
shows the percentile values for grain size analysis. The
results of the various parameters including mean grain
size, sorting, kurtosis and skewness are presented in
Table 5. The result from the statistical data on grain size
distribution was used for bivariate plot which was used to
deduce the depositional environment of the sediments
(Figs. 10-11).
Fig. 9a: Grain Size curves for the Ajali sandstone samples
from Milikin Hills (Location 1 and 2). M- middle of
exposure; B – bottom of exposure.
Fig. 9b: Grain Size curves for Ajali sandstone samples
from Milikin Hills (Location 3). T – top of exposure; M-
middle of exposure; B – bottom of exposure.

Fig. 9c: Grain Size curves for Ajali sandstone samples
from Miliken Hill (Location 4 and 5). T – top of exposure; B
– bottom of exposure.
Fig. 10a: A graphical plot of individual sample weight
retained (%) versus particle size (L1, L2
and L3).
Fig. 10b: A graphical plot of individual sample weight
retained (%) versus particle size (L3, L4 and L5).
The histogram plot for the individual sample weights from
the study area reveals a unimodal frequency distribution
for the soils. Only sample location 5 (Top) reveals a
bimodal distribution pattern (Figs. 10a-b).
According to Folk (1980), the best graphic measure for
determining overall size of sediment is the Graphic Mean.
The graphic mean values (Mz) were used for the
classification of sediments in the study area as it describes
the average grain size of the sediments. According to Folk
& Ward (1957), the various classes of graphic mean are
as follows; Boulder (-12 to -8 phi), Cobble (-8 to -6ø),
Pebble (-6 to -2 ø), Granular (-2 to -1 ø), Very coarse
grained (-1 to 0.0 ø), Coarse grained (0.0 to 1.0 ø),
Medium grained (1.0 to 2.0 ø), Fine grained (2.0 to
3.0ø), Very fine grained (3.0 to 4.0ø), Coarse silt (4.0 to
5.0ø), Medium silt (5.0 to 6.0ø), Fine silt (6.0 to 7.0ø), Very
fine silt (7.0 to 8.0ø) and Clay (8.0ø and smaller). In the
study area, graphic mean ranges from 1.5 – 2.8ø (Table
5). The lowest graphic mean value is obtained from sample
L2 (B), while the peak value of graphic mean is associated
with sample L2 (T). Mean graphic mean value is 2.36 ø
(fine grained). The result of the graphic mean as shown in
Table 5 reveals that all the samples are fine to medium
grained sediments. The result suggests deposition in a
dominantly low energy environment (Folk, 1974; Eisema,
1981).
The standard deviation is the spread of the grain size
distribution with respect to the mean. Sorting is the most
useful grain size data parameter since it gives an indication
of the effectiveness of the depositional medium in
separating grains of different classes. According to Folk &
Ward (1957), the various classes of sorting are as follows;
< ø 0.35 (Very well sorted), ø 0.35 to ø 0.5 (Well sorted),
ø 0.50 to ø 0.71 (Moderately well sorted), ø 0.71 to ø 1.0
(Moderately sorted), ø 1.0 to ø 2.0 (Poorly sorted), ø 2.0 to
ø 4.0 (Very poorly sorted) and > ø 4.0 (Extremely poorly
sorted). For this study, the calculated values indicated a
sorting values range from 0.45 to 1.58 ø, mean of 0.99 ø
and defined poorly sorted to moderately sorted, with only
one sample at L1B (0.45) characterizing well sorted
sandstones (Table 5).
According to Friedman (1961), the various ranges of
sorting in sands indicate the various environments of
deposition of the sand and results from this analysis depict
a fluvial (river) and shallow marine shelf environment
(Table 5). These results are consistent with studies of
Adekoya et al. (2011) on sedimentological characteristics
of Ajali sandstone in the Benin flank of Anambra basin and
reported deposition in shallow marine (littoral) environment
as indicated by the shale facie as well as in fluvial
environments as indicated in the attributes of the overlying
tabular and ferruginous upper sandstone facies.
This is a reflection of the depositional process. It is simply
a measure of the symmetry of the distribution. Skewness
is useful in environmental diagnosis because it is directly
related to the fine and coarse tails of the size distribution,
and hence suggestive of energy of deposition. According
to Folk & Ward (1957), the various classes of skewness
are as follows; ø 1.0 to ø 0.3 (Very fine skewed), ø 0.3 to

ø 0.1 (Fine skewed), ø 0.1 to ø -0.1 (Near symmetrical), ø
-0.1 to ø -0.3 (Coarse-skewed) and ø -0.3 to ø -1.0 (Very
coarse skewed). The skewness values of the samples
from the study area ranges from -0.58 to 0.32 ø and mean
of -2.06 ø (Table5), thus indicating the presence of coarse
skewed near symmetrical to fine skewed in the population
of particles. The dominance of negative values indicating
skewness towards the coarser grain sizes, hence,
suggesting that the coarse admixture dominates and
therefore exceeds the fine components. Coarse skewed to
strongly coarse skewed are indicative of low energy
environments (Table 5).
This is a measure of the peakedness of the curves towards
the coarser grain sizes. According to Folk & Ward (1957),
the various classes of kurtosis are as follows; < ø 0.67
(Very Platykurtic), ø 0.67 to ø 0.90 (Platykurtic), ø 0.90 to
ø 1.11 (Mesokurtic), ø 1.11 to ø 1.50 (Leptokurtic), ø
1.50 to ø 3.00 (Very leptokurtic), and > ø 3.00 (Extremely
leptokurtic). In numerical terms, the range of kurtosis in the
area is between 0.28 and 2, and mean of ø 1.41 (Table 5).
If the tails are better sorted than the central portions, then
it is termed as platykurtic, whereas leptokurtic, if the central
portion is better sorted. The samples analyzed shows a
dominance of leptokurtic (9 samples) followed by
Platykurtic (1 sample) and mesokurtic (1 sample). The
results suggest a generally better sorting at the central
portion. This strongly suggests a fluvial environment,
confirming that the sands are river deposited (Adeyode et
al., 2011).
Plots of mean grain size (Mz) against standard deviation
(D) were plotted for Ajali Sandstones at Miliken Hills using
the methods of Miola and Weiser, (1968); and Folk, (1974).
The results of mean grain size against sorting after Miola
and Weiser, (1968), shows that deposition occurs in the
fluvial field (Fig. 10). The plot of mean grain size against
sorting (Folk, 1974) show that deposition was in a fluvial
environment with minor tidal influence (Fig. 11). The result
of multivariate analysis shows that Ajali Sandstones were
deposited in a fluvial environment with shallow marine
incursions (Table 7).
Fig. 11: Bivariate plot of Mean against Sorting for Ajali
sandstone exposed at Milikin Hills (After Miola and Weiser,
1968).
Fig. 12: Scatter plot of sorting versus graphic mean
(modified after Folk, 1974).
Table 7: Multivariate results of Ajali Sandstone from the
study area.
Sample
locations Results Interpretation
L1 (M) -6.25 Shallow marine
L1 (B) -9.69 Fluvial
L2 (M) -5.89 Shallow marine
L2 (B) -6.60 Shallow marine
L3 (T) -7.73 Fluvial
L3(M) -7.39 Shallow marine
L4 (T) -9.44 Fluvial
L4 (B) -8.03 Fluvial
L5 (T) -6.16 Shallow marine
The result of petrographic analysis for the Ajali sandstones
exposed at Milikin Hills is displayed in Table 8. The result
reveals the presence of quartz, plagioclase, orthoclase
feldspar and iron oxide as the only opaque mineral. The
quarts are fine grained and together with the iron oxides,
they constitute the cement.
Microscopic studies show that the dominant mineral is fine
grained quartz and Orthoclase feldspar crystals with a
small amount of clay matrix and few opaque minerals
probably iron oxide. The cement is made of fine grained
quartz and iron oxide. The grains of the sandstone range
from angular to subangular. They are mostly
monocrystalline quartz although some are polycrystalline.
Most of the grains are not in contact but a few show
straight, concavo-convex and sutured contacts.
Cementing materials are mainly silica in form of authigenic
quartz and iron-oxide coatings. The modal analysis of
framework element shows an average of monocrystalline

Table 8: Mineralogical Composition of the Ajali Sandstones from Thin Section.
Sample locations
Quartz (%) Feldspar (%) OTHERS
MQ PQ TQ KF PF TF CLAY IRON
L1 (M) 40 5 45 30 8 38 2 15
L1 (B) 30 - 30 35 15 50 - 20
L2 (M) 37 10 47 25 15 37 - 16
L2 (B) 32 5 37 25 15 40 5 18
L3 (T) 22 8 30 30 17 47 3 20
L3(M) 30 20 50 20 15 35 - 15
L3 (B) 20 16 26 30 15 45 5 14
L4 (T) 50 5 55 20 15 35 2 8
L4 (B) 40 - 40 30 13 38 5 10
L5 (T) 30 10 40 25 19 44 7 9
L5 (B) 28 5 33 15 30 45 10 12
Average 32.64 9.33 39.36 25.91 16.09 41.27 4.88 14.27
Note: MQ - Monocrystalline Quartz; PQ - Polycrystalline Quartz; TQ –Total Quartz; KF - Potassium Feldspar; PF - Plagioclase Feldspar;
TF - Total Feldspar.
quartz (32.56%), polycrystalline quartz (9.33%), plagioclase
(16.09%), potassium feldspar (25.91%), clay matrix (4.88%)
and opaque (14.27%).
Due to the dominance of monocrystalline quartz and
feldspars (Plagioclase and orthoclase), the sandstones
are sub-feldspathic arenites. Quartz is colourless under
plane polarized light, but white to dull white in colour under
crossed nicols. Plagioclase is gray and orthoclase is pink
to brownish colour. Iron oxide is generally dark under
crossed polarized light (Plate).
Plate: Photomicrograph of sandstones exposed at
Location 1. O – Orthoclase feldspar; Q – Quartz; P –
Plagioclase feldspar; F – Iron oxide (image on Left =
Location 1 middle; Right = Location 1 bottom).
Organic Geochemistry
The TOC content for the lithostratigraphic of the boreholes
NKP01, NKP02 and NKP03ranges from 1.25 to 55.07 wt.
% (mean 8.87 wt. %), 0.22 to 51.42 wt. % (mean 12.90 wt.
%) and 0.07to 7.47 wt. % (mean 2.00 wt. %) respectively
(Table 9). These TOC values show that the sediments
have comparable average TOC contents, which are
greater than the 0.5 wt. % threshold value required for a
potential source rock to generate hydrocarbons (Tissot
and Welte, 1984). There is no clear trend for the TOC-
values with depth.
The source rock quality of the coals and shales in the three
boreholes is confirmed by the pyrolysis-derived generative
potential (S1+S2) of selected samples (Table 9).
The hydrocarbon generative potential of boreholes
NKP01, NKP02 and NKP03 ranges from 1.81-295.28 mg/g
rock, 2.56-332.22 mg/g rock and 0.05-34.84 mg/g rock
respectively. Hydrogen index (HI) values for the studied
samples ranges from 60 to 527 mgHC/g TOC for borehole-
NKP01, 59 to 755 mgHC/g TOC and 40 to 444 mgHC/g
TOC for boreholes NKP02 and NKP03 respectively. These
values indicate a moderately good source rock with gas
and oil generating potential (> 2 mg/g; (Tissot and Welte,
1984).
The type of organic matter in sediments penetrated by the
three boreholes (NKP01, NKP02 and NKP03) was
assessed by Rock-Eval pyrolysis (Table 9). Most of the
studied rock units from the three wells are mainly of type
III with subordinate type II-III. The plots Rock-Eval S2
versus TOC (Fig. 13) are useful to compare the petroleum-
generative potential of source rocks (Langford and Blanc-
Valleron,1990; Peters, 1986). The slopes of lines radiating
from the origin in Figure 14 are directly related to hydrogen
index (HI). Hydrogen index values of greater than 600,
300-600, 200-300, 50-200 and less than 50 mg HC/g TOC
classifies organic matter as type I (very oil prone), type II
(oil prone), type III (gas prone) and type IV (inert)
respectively (Peters, 1986). The relationship between the
hydrogen indexes (HI) versus oxygen index (OI) (Fig. 14),
reveals kerogen of type III and mixed type II-III organic
matter which are predominantly gas prone. Plots of HI
versus Tmax (the maximum temperature of pyrolysis) (Fig.
15) and HI versus %Ro (Fig. 16), also shows that the
organic matter in the samples is mainly type III with
subordinate type II/III.
The below results are in agreement with the data obtained
by earlier workers Akaegbobi and Schmitt, 1998; Akande
et al., 2007; Shanmugam, 1985).

Fig. 13: Plots of Rock-Eval S2 versus TOC Fig.14: Plot of HI versus OI for the coal and shale units from
(Langford and Blanc-Valleron,1990). the Campano-Maastrichtian Formations
Fig. 15: Plot of HI versus Tmax for characterizationof the Fig. 16: Plot of HI versus %Ro.
organic matter for boreholes NKP01, NKP02 and
NKP03 from the Campano-Maastrichtian Formations.
Thermal maturity provides an indication of the maximum
paleotemperature reached by a source rock. The thermal
maturity of the shales and coals of the Anambra Basin
have been discussed by several authors (Akaegbobi and
Schmitt, 1998; Akaegbobi, et al., 2000; Unomah and
Ekweozor,1993). The degree of thermal maturity of the
shales and coals of the Maastrichtian Mamu Formation
was assessed by pyrolysis-derived indices, such as Rock-
Eval Tmax, production index and %Ro (Table 9).
According to Peters (1986), PI and Tmax values less than
about 0.1 and 435oC, respectively, indicate immature
organic matter while Tmax greater than 470oC points to the
wet-gas zone. The Tmax values of the coal and shale
samples in NKP01, NKP02, and NKP03 ranges from 424
to 441oC (mean 435oC), 432 to 441oC (mean 437oC) and
338 to 4437oC (mean 405oC) respectively. The %Ro
values range between 0.47 to 0.78 (NKP01), 0.62 to 0.78
(NKP02) and 0.49 to 1.00 (NKP03). Both values indicate
that the samples are thermally immature to marginally
mature with respect to petroleum generation. Plots of PI
versus Tmax (Fig. 17), PI versus %Ro (Fig. 18) also show
that the coal and shale sediments are partly within the oil
window.
The production index (PI) values > 0.1 (Table 9) observed
on few samples indicate possible impregnation, migration
oil or contamination by mud additives (Clementz, 1979).
Other samples with PI-values ranging from 0.02 to 0.06
show an expected result and free from any additives.
0.00
50.00
100.00
150.00
200.00
250.00
300.00
350.00
0.00 10.00 20.00 30.00 40.00 50.00 60.00
HI(mgHCg-1rock)
TOC (wt%)
NKP01
NKP02
NKP03
Type I
Oil Prone
Lacustrine
Type II
Oil Prone
Marine
Mixed Type II/III
Oil /Gas Prone
Type III
Gas Prone
Dry
Gas Prone
S2

Table 9: TOC and Rock-Eval pyrolysis results of the studied samples.
Fig. 17: Plot of PI againstTmax of the studied rock samples Fig. 18: Plot of PI versus%Ro.
from the Campano-Maastrichtian Formations
CONCLUSION
The sedimentological evaluations show that the basal
parts of the Ajali Sandstones were deposited under fluvial
environments characterized by debris flow, mass flow and
bed-load deposits, while the middle and upper parts were
characterized by fluvial channel environments. The basal
Mamu and Nsukka Formations are characterized by flood
plain deposits, the middle portion correspond to shallow
marine deposits while the upper portions are interpreted
as fluvial channel facies. Marine influences in the Nkporo/
Enugu Shales persisted into the lower Mamu Formation
with the remaining upper portions indicating flood-plain
deposits of fluvial origin.
The geochemical investigations show that potential source
rocks in the Anambra Basin are oil, oil/gas, and gas
prones. Potential reservoir units occur in the fluvial

sandstones of the Ajali Formation and in the marginal
marine and flood plain sandstones of the Mamu
Formation. The shales and claystones of the Nsukka and
Imo Formations may provide regional seals. Different trap
configurations are possible in the basin, ranging from traps
within uplifted blocks, traps in drapes and/or compacted
structures over deep horst to stratigraphic traps along
flanks of uplifted blocks.
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Accepted 28 August, 2017
Citation: Uzoegbu MU and Okon OS (2017).
Sedimentology and Geochemical Evaluation of Campano-
Maastrichtian Sediments, Anambra Basin,
Nigeria..International Journal Geology and Mining 3(2):
110-127.
Copyright: © 2017 Uzoegbu and Okon. 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.

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