Organic geochemical analyses were performed on shale samples belonging to the Cretaceous sediments from Asu River Group in the Afikpo syncline, Southeastern Nigeria. Shale sediments were taken on a traverse from Okigwe - Amaseri to Afikpo. Plots of hydrogen index (HI) versus Tmax and oxygen index (OI) respectively classified the shale organic matter as Type III - IV kerogen. Tmax values between 349°C and 454ºC indicate organic matters are thermally immature to marginally mature. The range of HI values from 3.95 - 47.98mgHC/gTOC and S1 + S2 yields from 0.37 - 18.92mgHC/g of rock, suggests shale potential to generate oil and gas. The value of 1.09 to 14.70wt% TOC with an average of 5.60 wt% shows good to excellent source potential. The OI values range from 6.00 - 39.83mgHC/gTOC suggests contribution from terrestrial organic matter poor in hydroxyl groups.

1. Organic Geochemical Evaluation of Cretaceous Sediments from Asu River Group in the Afikpo Basin, Southeastern Nigeria
Organic Geochemical Evaluation of Cretaceous Sediments
from Asu River Group in the Afikpo Basin, Southeastern
Nigeria
Omontese O. Soni1, Imasuen I. Okpeseyi2, *Uzoegbu M. Uche3
1Department of Geology, Faculty of Physical Sciences, University of Benin, Benin-City, P.M.B 1154, Nigeria
2Centre of Excellence/Department of Geology, Faculty of Physical Sciences, University of Benin, Benin-City P.M.B 1154,
Nigeria
3Department of Geology, Faculty of Science, University of Port Harcourt, P.M.B. 5323, Port Harcourt, Nigeria.
Organic geochemical analyses were performed on shale samples belonging to the Cretaceous
sediments from Asu River Group in the Afikpo syncline, Southeastern Nigeria. Shale sediments
were taken on a traverse from Okigwe - Amaseri to Afikpo. Plots of hydrogen index (HI) versus
Tmax and oxygen index (OI) respectively classified the shale organic matter as Type III - IV
kerogen. Tmax values between 349°C and 454ºC indicate organic matters are thermally immature
to marginally mature. The range of HI values from 3.95 - 47.98mgHC/gTOC and S1 + S2 yields from
0.37 - 18.92mgHC/g of rock, suggests shale potential to generate oil and gas. The value of 1.09 to
14.70wt% TOC with an average of 5.60 wt% shows good to excellent source potential. The OI
values range from 6.00 - 39.83mgHC/gTOC suggests contribution from terrestrial organic matter
poor in hydroxyl groups.
Keywords: Total Organic Carbon, Pyrolysable Hydrocarbon Temperature, Organic matter, Hydrogen Index, Oxygen
Index, Rock-Eval pyrolysis, Shale
INTRODUCTION
The prime constituent of the petroleum system theory is
petroleum source rocks (Magoon and Dow, 1994). As
necessary prerequisite, these source rocks reflect the
preceding requirements for oil and gas generation. A
good-quality source rock is an inestimable prerequisite for
any petroleum system. Notwithstanding, typical petroleum
systems must consist of matured source rocks, migration
pathways, reservoir rocks, traps, seals or cap rock,
provided the relative timing and geothermal temperature of
generation is reached. (Obaje et al., 2004).
The Afikpo Basin is part of the Southern Benue Trough, an
Inland intra-continental sedimentary basin originating from
the lower Benue Trough (Obaje et al., 2004) with NE-SW
trending towards the Niger Delta. According to Kulke
(1995), petroleum exploration in this basin was provoked
as a result of existence of surface seeps dating back to the
early 1930s. Nevertheless, due to the Santonian inversion
and prevalence of continental sediments, including
discoveries in the prolific Niger-Delta, the exploration and
exploitation for petroleum has been ongoing in this area
(Ekweozor and Gormly, 1983; Doust and Omatsola, 1990;
Haack et al., 2000; Ugwueze, 2015).
This region constitutes part of southern extension in
Anambra Basin, referred to as Afikpo sub-basin. It is
delimited by latitude 5o55ʹ to 6o00ʹN and longitudes 7o51ʹ
to 8o00ʹE, situated within Okigwe – Afikpo areas of
southeastern Nigeria. Accessibility to the study area is
through the Port-harcourt - Enugu express way, via
Umuahia - Bende to Ohafia - and Okigwe - Amaseri to
Afikpo Fig. 1). An access route to the study area also
includes - a network of major, secondary, minor roads and
footpaths which help in interconnecting the inhabitants of
these areas.
*Corresponding Author: Uzoegbu M. Uche; Department
of Geology, Faculty of Physical Sciences, University of
Benin, Benin-City, P.M.B 1154, Nigeria. Email:
uche.uzoegbu@uniport.edu.ng, Tel: +2348030715958
Research Article
Vol. 5(2), pp. 269-274, June, 2019. © www.premierpublishers.org. ISSN: 3019-8261
International Journal of Geology and Mining

2. Organic Geochemical Evaluation of Cretaceous Sediments from Asu River Group in the Afikpo Basin, Southeastern Nigeria
Omontese et al. 270
The first marine transgression in Nigeria occurred during
the middle Albian. Albian sediments not mentioned and
unvarying comprise Asu River Group and its equivalents
(Ojoh, 1990). Ukaegbu and Akpabio (2009) have
differentiated the Albian sediments, northeast Afikpo
Basin as consisting of alternating shale, siltstone with
occurrence of sandstone, maximum thickness of 1000m
rich in ammonites as well as foraminifera, radiolarian and
pollens. Also present in the shales are traces of elobiceras
and monticeras ammonites (Ojo, 1999). This research
aims to characterize the quality, maturity and palaeo
environment of organic matter in the shale samples from
the Afikpo Basin using modern techniques of organic
geochemistry like Rock-Eval pyrolysis.
Fig.1: Geology of Afikpo sub-Basin and study locations (modified from Microsoft Encarta, 2009)
MATERIALS AND METHODS
A total of 12 outcrop shale samples were obtained from the
Asu River Group at Amenu and Amauro localities in Albian
age of the Afikpo Basin. Care was taking to avoid
weathered portions of the outcrop and to obtain material
sufficient for various geochemical analyses. The samples
were hard, thickly laminated but not fissile, with texture
indicative of low permeability. In the laboratory, the
samples were reshaped using a rotating steel cutter to
eliminate surface that could be affected by alteration.
Chips were cut from the samples and dried in an oven at
105oC for 24 hours. The dried sample was pulverized in a
rotating disc mill to yield about 50 g of sample for analytical
geochemistry. The total organic carbon (TOC) and
inorganic carbon (TIC) contents were determined using
Leco CS 200 carbon analyzer by combustion of 100 mg of
sample up to 1600oC, with a thermal gradient of 160oC min-
1; the resulting CO2 was quantified by an Infrared detector.
The sample with known TOC was analyzed using a Rock-
Eval 6, yielding parameters commonly used in source rock
characterization, flame ionization detection (FID) for
hydrocarbons thermal conductivity detection (TCD) for
CO2.
RESULTS AND DISCUSSION
Asu River Group represents the oldest unit of the
Cretaceous sequences in the Afikpo Basin. Table 1 shows
12 shale samples and their organic geochemistry. There's
minute carbon and organic materials in samples are
present around the benchmark for potential rocks.
In Amauro locality, maximum TOC content (14.70wt%)
persist at benchmark of 1.09-18.24wt%, an average
5.60wt%. Pyrolysable hydrocarbon temperature values of
the samples from the Asu River Group ranges from 349-
454oC with an average of 406oC. Hydrogen index (HI) is
between 3.95 and 47.98mgHC/gTOC (Table1) with an
average of 23.17mgHC/gTOC.

3. Organic Geochemical Evaluation of Cretaceous Sediments from Asu River Group in the Afikpo Basin, Southeastern Nigeria
Int. J. Geol. Min. 271
Organic Matter Quality
According to Tissot and Welte (1984), Bordenave (1993)
and Hunt et al. (2002), TOC is very fundamental in source
rock evaluation, at a range of 0.5-1wt%. In this study, we
found an average of 4.23 wt%, which indicates good
source rock. High TOC of 4.45 wt% was obtained in Mamfe
basin and this value exceeds the threshold for oil
generation (Eseme et al., 2006).
It is not worthy that high TOC only cannot account for good
hydrocarbon formation. that coaly-seams can have a very
high TOC of about 50% but cannot produce hydrocarbon
unless it is pregnant with abundant liptinitic content, a
veritable proof of maceritic importance. This is very true of
shale components in delta regions especially the Niger-
Delta region of Nigeria where TOC is low, (about 1wt%)
and produce abundant hydrocarbons because there exists
appreciable quantum of deposits with much lipids.
However, it is important to note that more TOC promises
better criteria for preserving organic content when
deposited.
Table 1: Organic geochemistry analysis for Cretaceous shaly sediments of the Asu River Group.
Pyrosable hydrocarbon (S2) vs.TOC graph (Fig. 2) has
been suggested to be the most accurate way of finding the
correct average-value of HI and oil and gas adsorptive
capacity of sediments. This was put forward by Langford
and Blanc-Valleron in (1990) by means of regression
equation. They noted that HI in most cases, may be less
than the true average HI of the sample due to the
hydrocarbons adsorptive capacity of the source rock
matrix (Espitalie et al., 1985) and that using the regression
equation derived from the S2 vs. TOC graph (Fig. 2 )
automatically correct HI for this effect.
The average HI of the shale samples, from the S2 vs. TOC
plots is very reliable (correlation coefficient is 0.89 and has
indicated a value of 23.17 which is still 0-50mgHC/gTOC
and below (Peters, 1986), hence supporting the
predominant of type 4 with associated type 3 organic
materials in Asu River Group, Afikpo Basin. It is suggestive
of a reduction-oxidizing criterion but more of 02influence
favouring oxidation. High oxygen index (OI) of 20.84
mgCO2 g-1TOC suggests high contributory continental
organic materials poor in hydroxyl groups and oxygen-
prone conditions, supported by Tissot and Welte (1984).
The kerogen content of 1.10 mgHC g-1rock is fair, showing
S2/S3of 1.71 indicating gas dominated organic materials is
consistent with its Tmax of 349 to 454oC, indicative of
immaturity to early maturity. S1/TOC is 0.29, which indicate
early generation of petroleum. The HI is slightly below 50
mgHCg-1TOC for Type III - IV kerogens at the immature
stage. Type IV which is mostly inert was obtained in this
area of Afikpo Basin (Fig. 3). The OI is high, suggestive of
extreme vegetative inclusions and undisputed deposits in
O2province, supported by Uzoegbu and Ikwuagwu (2016).
HI of 41.20mgHCg-1TOC results to type III - IV kerogens at
immaturity to early maturity stage. The real presence of
gas indicate the kerogen cannot be of type II, showing
S2/S3>5. Tmax is indicative of presence of HI originating
from complete combustion of type III - IV, proving previous
HI from 600mgHCg-1TOC to 850mgHCg-1TOC (Lafargue
et al., 1998).
Maturity Indicators
HI vs. Tmax diagram (Fig. 4) classifies the shales organic
matter in Asu River shales as type IV (inert) kerogen
(Akande et al., 2007) with some samples slightly above the
threshold (430oC) stage.
According to Bordenave et al. (1993), this is most
especially important for similar source rocks of dissimilar
quality, as its then referred to transformation ratio. Hunt
(1996) proposed a PI of 0.06 - 0.96 for sediments around
the hydrocarbon window-show. In this research, the
production index (PI) at 0.41 corresponds with Tmax 405oC

4. Organic Geochemical Evaluation of Cretaceous Sediments from Asu River Group in the Afikpo Basin, Southeastern Nigeria
Omontese et al. 272
Fig. 2: A diagram of S2 versus TOC of shale samples from
Asu River Group with calculated average hydrogen indices
(Av. HI) [After Langford and Blanc-Valleron, 1990].
Fig. 3: Kerogen type from modified van Krevalen diagram
(After Peters, 1986).
on maturity stage. This phenomenon is further extended to
the moderately flourishing organic materials at Tmax 430oC,
getting to 430-435oC, considering type III non-mature
sediments with very little sulphuric content (Bordenave,
1993., Hunt, 1996).
Consequently, PI suggested by Ruullkӧtter et al. (1988)
cannot be harmed by being driven out, therefore it is useful
as a tool for transformative organic materials due to
generative capacity at 0.55%Ro for source rocks of type II.
This was supported by study of Leythaeuser et al. (1980).
To further strengthen its usefulness, Rullkӧtter et al.,
1988referred to a weigh balancing system showing the
reach of Posidonia shale 'TR' in north Germany at 30%,
given the Ro at 0.68%. Various maturity parameters
indicate non-matured to slightly matured settings for
hydrocarbon expulsion. The current HI of 23.17 mgHCg-
1TOC is believed to support intense heating change for
organic materials as a result of adjustments in the HI
pyrolysis programme from 600mgHCg-1TOC to
850mgHCg-1TOC. This is exemplary of type III kerogens
(Lafargue et al., 1998). The soluble organic matter also
referred to as the extract yield was determined.
Soluble organic matter (SOM)vs. TOC (Fig. 5) diagram
indicates that no migration of oil has taken place. This was
initially initiated by Landis et al. (1984) and extended to the
works of Jovancicevic et al. (2002). S1 + S2 vs TOC
supports and characterizes the shale samples from the
Afikpo Basin as good to excellent source rocks with TOC
and S1 + S2 above 1.0wt% and 5.0mg/g respectively(Fig.
6). Four samples with TOC greater than 0.6wt% were
derived from shaly carbonaceous samples. This is also
supported by the report of Beka et al. (2007) from their
investigations on shaly facies of gas prone sequences in
the Afikpo Basin based on the values of TOC (1.09-
18.24wt%) and soluble organic matter (SOM) (190-
2900ppm) which are indicative of good to excellent and
adequate source potential. Udofia and Akaegbobi (2007)
also investigated the Maastrichtian sediments around
Enugu escarpment of the Anambra Basin which revealed
the exceeding minimum threshold TOC value (0.65-
1.82wt%) for sediment samples and (18.35-19.12wt%) for
coal samples. Thermal maturity was confirmed by plotting
the profiles of Tmax vs TOC showing that almost all the
samples did not attain to “oil window” (430ºC) except few
sediments. HI vs Tmax diagram also supports this
statement which determines the immaturity status of the
entire sample except few samples (Fig. 4).
Fig. 4: Tmax versus HI of shale samples for Asu River
Group describing organic matter quality (After Langford et
al., 1990).

5. Organic Geochemical Evaluation of Cretaceous Sediments from Asu River Group in the Afikpo Basin, Southeastern Nigeria
Int. J. Geol. Min. 273
Fig. 5: SOM vs TOC showing the characterization of
organic matter for Asu River Group (based
on works of Landais et al., 1984).
Fig. 6: (S1 + S2) vs. TOC of shale samples for Asu River
Group indicating the quality of different source rocks, (After
Langford et al., 1990).
CONCLUSIONS
Organic geochemical analyses have revealed the
petroleum potential of the Cretaceous shale sediments in
the Afikpo Basin as immature to marginally matured.
Parameter values favour oxidation of organic matter with
average oxygen indexes, all of which suggest donation
from mostly terrestrial organic matter poor in hydroxl
groups. This is further suggestive of extreme vegetative
inclusions and undisputed deposits in O2provinces. The
result from the shale analyses from Amenu and Amauro
localities suggest source rocks of Afikpo Syncline as gas-
oriented more of type III kerogen.
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Accepted 12 June 2019
Citation: Omontese OS, Imasuen IO, Uzoegbu MU
(2019). Organic Geochemical Evaluation of Cretaceous
Sediments from Asu River Group in the Afikpo Basin,
Southeastern Nigeria. International Journal of Geology
and Mining 5(2): 269-274.
Copyright: © 2019: Omontese et al. This is an open-
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