Lithofacies Analysis and Petroleum Potentials of Parts of Ikom-Mamfe Embayment, South-Eastern Nigeria

Lithofacies Analysis and Petroleum Potentials of Parts of Ikom-Mamfe Embayment, South-Eastern Nigeria
IJGM
Lithofacies Analysis and Petroleum Potentials of Parts
of Ikom-Mamfe Embayment, South-Eastern Nigeria
1Nse Udo Essien, 2Ama Otele, *3Emmanuel Etim Okon
1,3
Department of Geology, University of Calabar, Calabar, Cross River State, Nigeria
2
Department of Petroleum Engineering/Geoscience Technology, Federal Polytechnic Ekowe, Bayelsa State,
Nigeria
An integrated study involving detailed lithofacies analysis and source rock evaluation were
carried out to reconstruct the paleoenvironment and assess the petroleum potentials of the Ikom–
Mamfe embayment, southeastern Nigeria. Sedimentological field mapping involving detailed
description of lithologic characteristics and facies characterisation was carried out. Geochemical
studies were carried out to determine the quantity of organic matter total organic carbon (TOC),
soluble organic matter (SOM), the organic matter quality (organic matter type) and level of
maturity. Results show that the dominant vertical succession of the various lithofacies indicate a
general finning upward succession with basal massive pebbly sandstone, medium to coarse
grained sandstones with intercalation of shale and mudstones. Seven lithofacies A to G, were
identified. These include: conglomerates, massive pebbly sandstone, trough cross-bedded
sandstone, planar cross-bedded sandstone, shale/mudstone facies. These facies were compared
with established standard facies association for determining paleoenvironment of deposition. The
facies analysis carried out pointed to fluvial (alluvial – braided) depositional system as the
environment of deposition. TOC values range from 0.05 – 4.13 wt% indicating poor to excellent
and SOM range from 200 – 6000 ppm indicating also poor to excellent. The amount of pyrolizable
carbon derived as S1 and S2 peaks suggested that the source rocks possess organic matter
capable of generating hydrocarbons. Hydrogen and oxygen indices (HI and OI) ranged from 0.24
to 656 and 0.53 to 61.90 mg/gTOC respectively. Analyses of the evaluated source rock shows that
the hydrocarbon potential of the study area is lean and typically of a reworked terrestrial deposit
of fluvial depositional system.
Keywords: Petroleum potentials, organic matter, mudstone facies, cross bedding, fluvial system
INTRODUCTION
Sedimentary environment of deposition is determined by
detailed study of the characteristics of the sediments.
These are hinged upon by the different environmental
factors upon which the sediments from their provenance to
their depositional site have being through e.g. physical,
biological and chemical factors and/or a combination of all
these (Tucker 2003; Okon 2015; Essien and Okon 2016;
Quasim et al 2017). It is also important to note that post-
depositional modifications (diagenesis and tectonic
adjustments, in some cases) leave imprints on the final
rock exposed for study, therefore care is to be taken to
properly decipher the depositional setting amidst these
factors. The Mamfe basin straddles the border between
south – eastern Nigeria and Western Cameroon. It
extends from the Southern Benue Trough, where it is
bounded at the NE and SW by the Bamenda and Oban
Massifs of about 500m elevation (Figure 1) to the south.
*Corresponding Author: Emmanuel Etim Okon,
Department of Geology, University of Calabar, Calabar,
Cross River State, Nigeria. E-mail:
etyboy911@yahoo.com; Tel: +234 8030841340
International Journal Geology and Mining
Vol. 4(2), pp. 190-201, August, 2018. © www.premierpublishers.org. ISSN: 3019-8261
Research Article

Essien et al. 191
Figure 1: Map of Cross River State showing the Ikom-Mamfe Embayment, (inset: Map of Nigeria showing Cross River State
Previous studies show that the Cameroon sector has
received more attention, notably Dumort, 1968; Eben
1984; Eyong 2003; Eyong et al,2013; Eseme et al, 2002;
Hell et al, 2000; Kangkolo, 2002; Ndougsa et al, 2004;
Nouayou 2005; Tabod, 2008, Tokam et al, 2010; Njoh et
al 2015; Bassey et al 2013, Abolo 2008; Ajonina, 2016
while the Nigerian sector has attracted relatively few
studies like those of Olade 1975, Fairhead and Okereke
1987, 1988, Fairhead et al. 1991; Petters et al,1987;
Reyment 1965, Bassey 2012; Oden 2012; Bassey et al
2013; Obi et al 2013; Oden et al, 2015 which lead to
paucity of geological information. Studies involving facies
analysis and stratigraphic subdivision of sedimentary
basin are commonly carried out to subdivide the basin infill
to connote their age relationship and depositional
environment. When this is established, further studies on
such platform may yield success into associated economic
potentials of the basin. For the Ikom-Mamfe embayment,
only few biostratigraphic studies and been carried out and
these studies where available are centred on outcrop
exposures and few road cuts.
In spite of these studies, not so much centred on the
sedimentological and stratigraphic framework/
development of the embayment. This has informed the
need for this research paper, essentially to investigate the
lithofacies and assess the petroleum potentials of parts of
the embayment. To achieve this goal, detailed geological
mapping was carried out to document the various
geological features (rock types, lithology, grain size
distribution/facies trend, sedimentary structures, etc.), also
geochemical analysis of the mud rock facies within the
study area to assess their petroleum potential.

Int. J. Geol. Min. 192
Geology of the study area
The Mamfe Basin is regarded is a side rift/half graben on
the eastern flank of the Southern Benue Trough, Nigeria.
It is bordered to the north by the Bamenda Massif and to
the south by the Oban Massif. It extends into the south-
western part of the Republic Cameroon to the east where
it is bordered by uplift of the Cameroon Volcanic Line
(CVL). Based on some of its notable structural styles, for
example, its fold axis parallel to basin axis and attendant
magmatic intrusions, it is regarded as being similar to
those of the Benue Trough and suggests that both basins
have a closely related tectonic framework and geodynamic
evolution (Ajonina et al., 2001). Thus, the basin is
generally regarded as one of the three sub basins that
make up the Southern Benue Trough. These sub basins
from east to west are Mamfe, Abakaliki, and Anambra
Basins.
The age of the Mamfe basin has been a subject of debate
since studies begun within the basin largely due to paucity
of biostratigraphic controls and complete lack/insufficiency
of exploratory well in the basin. However, judging from the
sedimentary fills of the WCARS which range in age from
Neocomian to Recent and consists mostly of siliciclastics
and volcaniclastics sediments of predominantly
Cretaceous age, deposited in fluvial, lacustrine, marine,
and deltaic settings. Ajonina (2016) proposed a
Neocomian age for the basin (Figure 2).
Figure 2: Stratigraphic modification chart of the Ikom-Mamfe Embayment (Modified from Petters and Ekweozor 1982,
Petters et al 1987, Bassey et al 2013 and Ajonina 2016).
Reyment (1965) described the Mamfe Formation as thickly
folded and faulted series of massive arkosic sandstones
and grits having intercalations of marlstones, arenaceous
limestones and shale. He erected the type section at the
bank of the Cross River near Mamfe Town, Cameroon
Republic. The lithostratigraphic subdivision of the Mamfe
embayment is composed of sediments that range in age
from Aptian – Albian with thickness of about 4000m
(Petters et al, 1987), collectively referred to as Mamfe
Formation (Reyment, 1965; Dumort, 1968; Whiteman,
1982). The sedimentary package lies unconformably on
the Pan-African crystalline basement complex composed
predominantly of gneisses, schists and granitoids.
The formation is dominantly made up of sandstone, shale,
mudstone, conglomerate and limestone/marble units
constituting the basal unit of the Asu Rive Group (Petters
et al, 1987). Petters et al, (1987) further described the
Mamfe Formation sediments as coarse grained to pebbly,
finning upward sequence with well-developed cross
bedding and contains plant remains. The mud rocks
alternate with medium grained sandstones in the upper
part of the formation and often the sandstone pinch out in
the grey friable shales. Earlier attempts to subdivide the
sedimentary rocks of the Mamfe Formation, Abolo (2008)
proposed a three-member subdivision: Etuko/Okoyong
Member, Nfaitok Member and Manyu Member. Bassey et

Essien et al. 193
al. (2013), rather preferred a two-member subdivision: the
Manyu and Kesham members respectively. Due to these
plaguing inconsistencies in stratigraphic nomenclature,
Ajonina (2016) suggested an upgrade of the members to
formation status and presented the following: an older
Etuko Formation overlain by Mamfe Group (represented
by Nfiatok, Manyu and Okoyong Formations) and
overlying Ikom-Munaya Formation. However, because
these formations are not fully consistent with the
stratigraphic code for erecting formations, it is best to stick
to the Mamfe Formation until adequate data set and well
controls across the basin is available to constrain proper
subdivision if necessary. The occurrence of brine springs
in the Ikom-Mamfe embayment, suggesting a possible
marine influence during deposition was reported by
Petters et al, 1987 but stated the fluviatile origin of the
formation was overwhelming.
MATERIALS AND METHODS
Detailed field mapping was carried out across the study
area using a topographic map (part of UGEP Sheet 314)
as a guide. A total of 15 outcrops were visited and 53
samples collected for laboratory analyses. All the outcrop
and river sections were measured and their lithologic
characteristics were documented. This facilitated the
erection of vertical sedimentary section, taking into
account the colour, grain size, sedimentary structures,
bedding forms/types, bed thicknesses and nature of their
contacts. The lithostratigraphic facies analysis of this study
was carried out from general field observations comprising
description of rock types in the study area, with emphasis
on the texture, colour, bed thickness, grain shape and
sizes, sorting, grain size trends and sedimentary
structures. On the basis of the above, lithologic log
sections were constructed for outcrops from different
locations (Figure 3).
Figure 3: Map of the study area showing the various lithologic sections
Miall (1978) facies model was instrumental in the erection
of the facies succession for the study area (Table 1). The
comparison was based on similar lithologic characteristics
and the information obtained, was put together to interpret
the depositional environment of the study area.
Rock samples were collected at representative units for
further laboratory characterization/analyses, especially the
mudrocks (shale and mudstones). These were described
and subjected to geochemical analysis (Total Organic
Carbon – TOC, Soluble Organic Matter - SOM and Rock
Eval Pyrolysis) to evaluate the organic richness, maturity
and hydrocarbon generative potentials.
A total of 15 shale samples were subjected to TOC
analysis using the Walkley-Black titration method. This
involved the protocol referred to as “wet oxidation method”
where by 0.5g of the sample is pulverized and subjected
to chronic oxidation. After heating, excess potassium
dichromate (K2Cr207) was titrated against ferrous
ammonium sulphate. The difference between the initial
and residual potassium dichromate is a measure of the
organic content of the sample. This is calculated using the
relation:
%TOC = 10 * (1 – T/S) F

Table 1: Facies and characteristic structure of sediments and depositional environment (After Miall, 1978)
Gmm Matrix - supported, massive
gravel
Weak grading Plastic debris flow (high-strength, viscous)
Gmg Matrix - supported gravel Inverse to normal grading Pseudoplastic debris flow (low strength,
viscous)
Gci Clast - supported gravel Inverse grading Clast - rich debris flow (high strength), or
pseudoplastic debris flow (low strength)
Gcm Clast - supported massive
gravel
- Pseudoplastic debris flow (inertial bedload,
turbulent flow)
Gh Clast - supported, crudely
bedded gravel
Horizontal bedding,
imbrication
Longitudinal bedforms, lag deposit, sieve
deposits
Gt Gravel, stratified Trough cross beds Minor channel fills
Gp Gravel, stratified Planar cross beds Traverse bedforms, deltaic growths from older
bar remnants
St Sand, fine to very coarse,
maybe pebbly
Solitary or grouped, trough
cross-beds
Sinuous-crested and linguoid (3-D) dunes
Sp Sand, fine to very coarse,
maybe pebbly
Solitary or grouped, planar
cross-beds
Traverse and linguoid bedforms (2-D dunes)
Sr Sand, very fine to coarse Ripple cross-lamination Ripples (lower flow regime)
Sh Sand, very fine to coarse, may
be pebbly
Horizontal lamination, parting
or streaming lineation
Plane-bed flow (critical flow)
Sl Sand, very fine to coarse, may
be pebbly
low angle (15) cross-beds Scour fills, humpback or washed-out dunes,
antidunes
Ss Sand, fine to very coarse, may
be pebbly
Broad, shallow scours Scour fills
Sm Sand, fine to coarse massive, or faint lamination Sediment-gravity flow deposits
Fl Sand, silt, mud Fine lamination, very small
ripples
Overbank, abandoned channel, or waning
flood deposits
Fsm Silt, mud Massive Backswamp or abandoned channel deposits
Fm Mud, silt Massive, desiccation cracks Overbank, abandoned channel, or drape
deposits
Fr Mud, silt Massive, roots, bioturbation Root bed, incipient soil
C Coal, carbonaceous mud Plant, mud films Vegetated swamp deposits
P Paleosol carbonate (calcite,
siderite)
Pedogenic features: nodules,
filaments
Soil with chemical precipitation
where: %TOC = Percentage Total Organic Carbon
T = Sample Titration (Vol of ferrous ammonium sulphate
us for titration)
S = Standard or Blank titration
F = Factor derived from the relation
= {[1.0 x 12] / 4000 x 1.72 x 100/w}
And w = weight of the samples in grams;
1.0N = Normality of K2Cr207; 10 = Vol of K2Cr207
The final results were compared with the chart of Peters,
1986. Soluble organic matter was determined for the
shales using axiotrophic mixture of acetone and methanol
(ratio 9:1). Refluxing technique whereby the solvent is
made to be in prolonged contact with the sample such that
a complete extraction is established. The setup was left to
stand for 24 hours. After complete extraction, the weight of
the extract was determined and use to calculate the SOM
as shown below:
SOM = weight of extract/weight of sample x 1000000.
Using the data generated from the TOC and SOM
analysis, 8 samples were selected and subjected to Rock
Eval pyrolysis (Figure 4). This analysis was carried out at
Weatherford Laboratories, Texas, USA.
Figure 4: Schematic diagram of Rock-Eval pyrolysis
process (adapted from Okon, 2011)

Essien et al. 195
RESULTS
Lithostratigraphic Facies Analysis
The observations made in the field with emphasis on the
variations in colour, rock types, texture, bed thicknesses,
grain shape and sizes, sorting and sedimentary structures
formed the basis for the lithofacies inference which
facilitated the identification of the various depositional
environments. Comparing the lithofacies based on similar
characteristics with that of Miall (1978), seven (7)
lithofacies were identified as follows: conglomerate facies
(Gmm), massive pebbly sandstone facies (Sm), trough
cross-bedded sandstone facies (St), planar cross-bedded
sandstone facies (Sp), fine to coarse grain sandstone
facies (Sh) and mudstone/shale facies (Fl).
The codes used here were drawn from their similarities to
the Miall (1978) facies characterization.
Conglomerate Facies (Gmm)
This consist of brown, poorly sorted matrix supported
polymictic conglomerate exposed around Obubra
constitute conglomerate facies. It is analogous to the Miall
(1978) facies code Gmm interpreted as plastic debris flow
deposit. Its boulder to pebble sized quartz clasts are
thought to be products from redeposition (second cycle
sedimentation) into the part of Southern Benue Trough
(Afikpo/Mamfe Basin) resulting from the uplifted Abakaliki
Anticlinorium following the Santonian event.
Several cycles of deposition were mapped and the
average thickness of each bed was approximately 1.2m
(Figure 5). The overall thickness of this unit is
approximately 10m thick in exposed sections. This facies
succession was not observed in all sections studied but
notably occur overlying the black shale facies, where
observed.
Figure 5:. Vertical section of the polymict conglomerate
Massive pebbly sandstone facies (Sm)
The pebbly sandstone facies consist of a light grey to white
coloured arkosic sandstone observed almost throughout
the study area. It occurs at the basal section of the
sediment pile and generally lack well defined satisfaction.
This lack of clearly defined stratification has been
attributed to the absence of traction transport, whereby
sediments were subjected to rapid deposition. The
characteristics suggest is dumped very rapidly without
subsequent reworking to form a homogenous mass. A
crude fining – upward succession was however observed
based on sediment grain size, in the upper part but capped
with a thin conglomerate layer (lag deposit) and scour
under surface before the beginning of another cycle. The
massive pebbly sandstone faces (facies A) of the study is
similar to Miall’s (1978) facies (Sm) which is interpreted as
sediment gravity flow, with occasional lag and scour
surfaces.
Trough cross bedded sandstone facies (St)
The trough cross bedded sandstone facies is
characterized by dark-grey to brown, medium to coarse
grain sandstone (Facies B). The cross beds are stacked
on top of each other, resulting in multi-story pattern of
cosets. This is comparable with Miall (1978) facies code
(St) interpreted as sinuous-crested and linguoid dune
sands in channel deposit. The characteristic suggests it
resulted from a unidirectional traction current migration of
sinuous crested (ripple marked) sands (Cant and Walker,
1976, Allen, 1970).
The planer cross bedded sandstone facies in the study
area is reddish, brown to light brown in colour, medium to
coarse grain (Facies C), generally exhibit a finning –
upward succession (Figure 6). The facies are similar to
facies code (Sp) of Miall, 1978. Such characteristics are
typical of transverse and linguoid bedforms in braided
stream sub-environments.
Figure 6: Planer cross bedded sandstone facies (SP)

Table 2: Result of the rock eval pyrolysis of Mamfe Formation sediments
S/N Sample
No.
Sample
Type
Location Formation TOC
wt%
SOM
(ppm)
S1 S2 S3 S1+S2 Tmax HI OI S2/S3
1 IHR22A Outcrop Ihurumektet Mamfe Fm 0.05 300
2 IHR 22B Outcrop Ihurumektet Mamfe Fm 0.41 200
3 IHR 22C Outcrop Ihurumektet Mamfe Fm 0.52 200
4 IHR22D Outcrop Ihurumektet Mamfe Fm 0.16 1000 0.03 1.05 0.04 1.08 541 656 25 26.3
5 IHR22E Outcrop Ihurumektet Mamfe Fm 0.57 300
6 IY23A Outcrop Iyametet Mamfe Fm 0.77 300
7 IY23B Outcrop Iyametet Mamfe Fm 0.52 1000 0.01 0.01 0.22 0.02 327 1.92 42.3 0.0
8 IY23C Outcrop Iyametet Mamfe Fm 0.06 1000
9 OH 1A Outcrop Ohana Mamfe Fm 0.41 200
10 OH 2 Outcrop Ohana Mamfe Fm 0.21 200 0.01 0.02 0.13 0.03 386 9.52 61.90 0.2
11 OCH 7 Outcrop Ochon Mamfe Fm 0.36 300 0.01 0.01 0.12 0.02 -1 2.78 33.33 0.1
12 ND 13A Outcrop Nde Mamfe Fm 0.91 1000 0.01 0.01 0.04 0.02 376 1.09 4.39 0.3
13 OKHI5B Outcrop Okagha Mamfe Fm 1.87 6000 0.01 0.01 0.01 0.02 -1 0.53 0.53 1.0
14 AJ21_A Outcrop Ajasor Mamfe Fm 4.13 1000 0.01 0.01 0.06 0.02 -1 0.24 1.45 0.2
15 IK 17B Outcrop Ikom Mamfe Fm 0.61 2000 0.02 0.01 0.07 0.03 405 1.64 11.48 0.1
Fine - coarse grained sands (sh)
The fine - medium and medium - coarse grained
sandstone facies of this study occurs as mainly brown to
occasionally light-brown in colour (Facies D), exhibiting a
finning - upward trend. the fine – coarse grained sandstone
facies can be compared to the facies code (sh) of Miall
(1978). It is characterized by plane bed condition as they
transit from one grain size to another with occasional crude
laminations defined mostly by colour variations is
interpreted as channel sands facies.
Mudstone - shale facies (FI)
The mudstone-shale facies (E and F) of the studying area
is dark to grey in colour (Figure 7). Individual occurrence
of these facies is up to 0.5m thick and is occasionally
sandy (fine size). This is analogous to facies code (FI) of
Miall (1978). In all the locations that the mudstone-shale
facies occur, it appears gradational and non-erosional with
facies (A) and facies (D). Its texture (silt-clay size grains)
suggests accumulation under suspension and this points
to abandoned, waning flood-phase deposits.
Figure 7: Typical highly fissile shale facies
Organic Geochemistry
The results of Total Organic Carbon, TOC, Soluble
Organic Matter, SOM, and Rock - Eval pyrolysis from the
study area are presented in Table 2. The results show that
values for TOC range from 0.05 - 4.13wt% with a mean
value of 0.07wt%. The minimum threshold value of 0.5wt%
established for clastic sedimentary rocks, wherewith they
may generate hydrocarbon (Tissot and Welte, 1984) and
source quantitative scale of Peters (1986) was adopted in
this study. Using this as a basis for classification, the
source rocks range from poor source rock through fair
source rock to very good source rock potential (Table 2).
Samples collected especially from Okagha and Ajasor
have good and very good source potentials respectively.
Soluble organic matter in rocks are also used to
characterized the organic matter quantity Phillipi (1965),
prepared a cut off scale for charactering source rocks
(Table 3), and using the scale, some of the values indicate
fair to very good concentration of soluble organic matter
(SOM) in the samples. Organic richness alone may not be
sufficient for assessing the petroleum generation potential
of any given source rock. Temperature plays an important
role for the source to attain maturity. In this study, the
type/quality of organic source rock and the maturation
status of the potential source rock was based on the
hydrogen Index (HI), Oxygen Index (OI) and Tmax
obtained from the rock – eval pyrolysis respectively.
The rock-eval analysis data allows the derivative of HI
which is roughly the equivalent of H/C and OI that is also
roughly the equivalent of the O/C (Espitalie et al, 1977,
Peters, 1986). The HI and OI data are commonly plotted
on a modified Van Krevelen diagram (Tissot and Welte,
1984) which clearly demarcates Type 1 – 1V Kerogen).

Essien et al. 197
Table 3: Soluble Organic Matter Interpretation for source
rock potential (after Phillipi, 1965)
SOM (ppm) Description of Source Potential
50
150
500
1500
5000
>5000
Very Poor
Poor
Fair
Good
Very Good
Excellent
DISCUSSIONS
Facies and facies sequence (depositional
environments interpretation)
The study of the distribution and characteristics of
sedimentary facies present aid in the identification of
certain unique characteristics present in modern
depositional settings, which when extended to their
ancient counterpart facilitates interpretation of depositional
environment (Middleton, 1978, Quasim et al., 2017). Two
facies associations were observed namely: A-B-C and E-
F (Figure 8) within the study area. Notably, each of the
association depict well defined erosional base
characterized by scour surface and succession that
generally fines upward. The significance of scour surface
forming the base of individual sedimentation units is
logically starting point for reconstructing the history of
sedimentation (Visher, 1972).
Essentially, within this study area, the facies succession
starts with the massive pebbly sandstone facies (A) at the
base, followed by trough and planner cross bedded
sandstone facies (B) and (C), medium to coarse grained
sandstone facies (D), shale/mudstone facies (E and F),
and finally the massive conglomeritic sandstone facies
(G). From the lithofacies model (Figure 8) the Mamfe
Formation has a finning upward succession typical of
fluviatile origin. Visher (1972) suggested that the
characteristics produced by uni-directional flow must be
present before a fluvial interpretation can be considered
and it was observed in the medium - grained trough/planar
cross bedded sandstone facies (B and C). A very important
aspect to note is the 5.5m thick deposit of polymict
conglomerates with size varying from granules through
cobbles and boulders near Obubra (Loc 6) and was
interpreted to be of alluvial origin. Ascribing alluvial origin
to the deposit is based on the thickness of the deposit
which is regarded as debris flow deposit. The facies (G) in
combination with the braided stream gives an overall
alluvial-braided stream fluvial depositional system for the
study area. Facies (D) is interpreted as active channel
deposit while facies (E and F) mudstone/shale facies are
typical overbank deposits.
The massive conglomerate (G) and fissile shales are
believed to been deposited unconformably on the Asu
River Group sediments in the Ikom-Mamfe Embayment
following the Santonian deformational episode and uplift of
the Abakaliki Anticlinorium (Southern Benue Trough)
giving rise to sediments in their second cycle of deposition
(Figure 8).
Based on the observed characteristics, with the Miall
(1978) model as a guide, the Mamfe Formation is likened
to the Donjek type of braided river depositional profile
because the sedimentation cycles are marked by definite
boundaries with thin conglomerate layer and scour
surfaces with a general finning – upward successions. This
facies succession is comparable with sediments of the
Lower Member of the Guanajuato Conglomerate (Puy-
Alquiza et al 2017) interpreted to have been deposited in
alluvial fans setting associated with braided fluvial system.
For hydrocarbon accumulations to be substantial in the
Ikom-Mamfe basin, the understanding of the environment
of deposition vis-a viz the migration dynamics need to be
understood correctly. Facies analysis point to fluvial
setting for the sedimentary pile, however there is evidence
of some marine incursion at some point and the post
depositional tectonics of the basin have produced
structure that lends positive support to the potential
presence of hydrocarbon in the basin. Although the
quantity of organic matter from this study reflect lean
organic matter, there are sporadic increases in organic
matter from place to place (especially around Okagha and
Ajasor). The plot of Hydrogen Index (HI) against Oxygen
Index (OI) show that all samples, but one sample (IHR
22D) plotted in the Type III-IV kerogen field (Figure 9),
suggesting sources of organic matter from a fluvial
environment. Although reports of lacustrine sediments
exist in the literature (Njoh et al.,2015; Abolo, 2008) in the
Cameroon sector of the Mamfe basin, that may have been
responsible for the characteristics of the sediment that
plotted in the Type II field.

Figure 8. Correlation Panel for the lithologic sections from west to east of Ikom-Mamfe Embayment in the study area..

Essien et al. 199
Figure 9: Plot of Hydrogen index (HI) vs Oxygen (OI)
(After Tissot and Welte,1984)
It is important to note that since samples from this study
were collected mainly from outcrops, there is a possibility
of occurrence of lacustrine sediments which maybe deeply
buried and inaccessible, more so, the outcrop samples
may have been subjected to some degree of weathering,
even though deliberate efforts were made in the field to
collect the most unaltered samples.
Hydrocarbon Source Potential (SP)
The hydrocarbon source potential (SP) of the samples
range from 0.02 - 1.08mg HC/grock with an average value
of 0.16mg HC/grock (Table 2). From the criteria used by
Tissot and Welte, (1984) and Dymann et al (1996), the
samples fall below the pre-requisite for hydrocarbon oil
source rocks. Therefore, the yield potential of the present
in the rock suggest little or no oil source potential.
To further buttress this, a bivariate cross plot of S2 against
TOC, this plot showed that the kerogen has lean organic
matter character except one sample that plotted in the oil
prone field (Figure 10). Further assessment based on
thermal maturity using pyrolysis method (Tmax) show a
range from 327 to 541°C. This implies that the sediments
are thermally immature (Peter and Cassa,1984; Dow
1977; Peter, 1986), except one sample (IHR 22D) which
attained a Tmax value of 541°C and falls above 470°C,
indicating that the kerogen has reached the post mature
stage. It is also very important to point out the occurrence
of intrusives in close proximity to samples used in this
study, and this could have influenced the high Tmax value.
Figure 10: Plot of S2 vs TOC of samples from the study
area (after Tissot and Welte,1984)
SUMMARY AND CONCLUSION
The facies analysis and petroleum generative potential of
parts of Ikom-Mamfe embayment were investigated with
the purpose of carrying out paleo-environmental
reconstruction and assessment of the petroleum potential.
The study strongly points to the fact that the sediments
were deposited in a fluvial depositional environment. The
lithofacies generated facilitated paleo-environmental
inference. The various facies encountered range from
conglomerate / massive pebbly sandstone, trough/planar
cross bedded sandstone, fine-medium grained sandstone
and mudstone/shale facies, all showing unidirectional flow
characteristics and exhibiting essentially fining upward
succession. This is typical of sediments belonging to
alluvial-braided fluvial depositional system.
Also, evaluation of the kerogen for hydrocarbon source
potential revealed that the TOC and SOM were poor to
good. The organic matter was mostly derived from
different sources including continental and possibly
lacustrine environments. The thermal history of the
analysed samples seems not to have supported
hydrocarbon accumulation in the basin. This position can
be further strengthened when subsurface samples from
deep wells are made available for study in this basin.
As a concluding remark, it is important to note that outcrop
samples alone were used for this study, and may not, on
their own, be adequate to provide a deep insight on a
detailed stratigraphic sequence and petroleum generating
potential of the basin. It is recommended that where
possible, a drilling campaign where drill core samples or
even ditch cuttings could be analysed for the entire vertical
section of the basin (from its shallowest parts to the
deepest parts) is carried out. This would go a long way to
adequately constrain the age, depositional history and
much more reliably access the petroleum source
potentials.

ACKNOWLEDGEMENTS
I will like to express my profound gratitude to the
anonymous reviewers whose critical review comments
have greatly improved the quality of this manuscript.
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Accepted 28 June 2018
Citation: Essien NU, Otele A, Okon EE (2018). Lithofacies
Analysis and Petroleum Potentials of Parts of Ikom-Mamfe
Embayment, South-Eastern Nigeria. International Journal
Geology and Mining 4(2): 190-201.
Copyright: © 2018 Essien et al. This is an open-access
article distributed under the terms of the Creative
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provided the original author and source are cited.

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