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Paleoenvironmental reconstruction and hydrocarbon potentials ofUpper Cretaceous sediments in the Anambra Basin, southeasternNigeria
Olajide Femi Adebayo, Adebanji Kayode Adegoke, KhairulAzlan Mustapha, Mutiu Adesina Adeleye, Amos OkechukwuAgbaji, Nor Syazwani Zainal Abidin
PII: S0166-5162(17)31079-0DOI: doi:10.1016/j.coal.2018.04.007Reference: COGEL 3001
To appear in: International Journal of Coal Geology
Received date: 31 December 2017Revised date: 7 April 2018Accepted date: 15 April 2018
Please cite this article as: Olajide Femi Adebayo, Adebanji Kayode Adegoke, KhairulAzlan Mustapha, Mutiu Adesina Adeleye, Amos Okechukwu Agbaji, Nor SyazwaniZainal Abidin , Paleoenvironmental reconstruction and hydrocarbon potentials of UpperCretaceous sediments in the Anambra Basin, southeastern Nigeria. The address for thecorresponding author was captured as affiliation for all authors. Please check ifappropriate. Cogel(2018), doi:10.1016/j.coal.2018.04.007
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Paleoenvironmental reconstruction and hydrocarbon potentials of Upper Cretaceous sediments
in the Anambra Basin, southeastern Nigeria.
Olajide Femi Adebayo1; Adebanji Kayode Adegoke
1*; Khairul Azlan Mustapha
2; Mutiu Adesina
Adeleye3; Amos Okechukwu Agbaji
1; Nor Syazwani Zainal Abidin
2,4
1. Department of Geology, Faculty of Science, Ekiti State University, P.M.B. 5363, Ado-Ekiti, Nigeria.
2. Department of Geology, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia.
3.Department of Geology, Faculty of Science, University of Ibadan, Ibadan, Nigeria.
4.Department of Geoscience and Petroleum Engineering, University Technology Petronas, 31750 Tronoh Perak, Malaysia.
*Corresponding author: [email protected]; [email protected]
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Abstract
Palynological, organic petrographic, and organic geochemical analyses of the Campanian-
Maastrichtian sediments in Akukwa-2 well were carried out to infer their paleoenvironments, origin of
the organic matter, and hydrocarbon generation potentials. The TOC values of the analysed sediments
range from 0.27 – 3.02 wt. %, while the S2 pyrolysis yield range from 0.55 to 3.35mg HC/g rock. This
indicates that the Nkporo and Mamu sediments possess fair source generative potential. The samples
contain Type III-II and Type III kerogen as shown by the present-day HI values between 58 and 292
mg HC/g TOC and pyrolysis-GC data. The organic matter within the sediments is also likely to
generate mainly gas. This is in agreement with the petrographic observations, which revealed that the
analysed shale samples contain abundant vitrinite macerals, apart from bituminite, alginite, cutinite,
and resinite. Also, the sediments are immature to early mature in terms of hydrocarbon generation as
indicated by vitrinite reflectance, biomarker maturity, and pyrolysis Tmax data. Biomarker distribution
ratios, palynomorphs assemblage, and organic petrographic observations further point out that the
organic materials within the sediments were of mixed aquatic and terrigenous origin and were
deposited under suboxic paleodepositional conditions. Based on sedimentological, palynological, and
biomarker characteristics, the environment of deposition of the analysed sediments was inferred to be a
relatively quiet, shallow marine with fluvial incursion, most especially at the upper part of the intervals
studied and consequently, it is a delta associated depositional environment with a fluviatile influence.
The sediments are therefore suggested to be deposited in a paleogeographic setting close to vegetation
source.
Keywords: Palynology; Biomarker distribution; Organic matter; Source rocks; Suboxic
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1.0 Introduction
The ever–increasing demand for hydrocarbons and decreasing yields from existing oil fields especially
in the Niger Delta Basin, as well as the need to increase the nation’s oil and gas reserve, call for an
intensified exploration in Nigeria’s sedimentary basins. Anambra Basin is one of Nigeria’s inland
sedimentary basins, where petroleum exploration activities are presently taking place. It is a nearly
triangular shaped depression covering about 3000 km2 and containing 6000 m thick Cretaceous and
Tertiary sediments (Ekine and Onuoha, 2008). The basin is situated west of the lower Benue Trough
and is bounded to the south by the Niger Delta Basin hinge line. It harbours one of the largest deposits
of lignite and sub-bituminous coal in Nigeria. In addition to coal and lignite, the basin could be next to
the Niger Delta Basin in terms of hydrocarbon potential.
Several workers had previously published some reports on Anambra Basin, but most of these studies
were limited to the hydrocarbon generative potential and thermal maturity of organic matter within the
sediments of the basin (e.g. Agagu and Ekweozor, 1982; Ekweozor and Gormly, 1983; Unomah and
Ekweozor, 1993; Akaegbobi and Schmitt, 1998; Obaje et al. 2004; Ehinola et al. 2005). There have not
been many detailed organic geochemical investigations of the origin and depositional setting of the
organic matter within these sediments. In this study, biomarker ratios were extensively used for the
characterisation of depositional setting and source input of organic matter within the Upper Cretaceous
(Campanian – Maastrichtian) Nkporo and Mamu sediments penetrated by the Akukwa-2 well drilled in
the north of central Anambra Basin (Peters and Moldowan, 1993; Peters et al., 2005; Adegoke et al.,
2014; Fig. 1). The molecular geochemistry was integrated with organic petrology and palynology.
Consequently, this study aims to provide insights into the geology and source rock potential of the
sediments within the basin, needed for further exploration.
2.0 Geologic setting
The Anambra Basin (Fig. 1) is one of the Cretaceous sedimentary basins of Nigeria. The geological
evolution of the southern sedimentary basins in the country began during the Lower Cretaceous with
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the formation of the Benue Trough as a failed arm of the rift triple junction, which is linked to the
splitting of the South American and African plates (Obaje et al., 2004). The Anambra Basin came into
existence during the Santonian tectonic episode, which affected the entire Benue Trough. The event
produced several synclines and anticlines (Benkhelil, 1989; Obaje et al., 2004). The depression formed
the accommodation space for about 6000 m thick Cretaceous and Tertiary sediments (Benkhelil, 1989).
The basin became tilted westwards during the Maastrichtian, leading to the development of vegetated
swamps, and a broad delta fan (Benkhelil, 1989).
The oldest sediment in the basin is the Nkporo Group, which comprises Nkporo Shale, Owelli
Sandstone and Enugu Shale, and was deposited in the Late Campanian (Reyment, 1965; Nwajide,
1990; Nwajide and Reijers, 1996). Mamu Formation (Lower Coal Measures), which was deposited in
the Early Maastrichtian, overlies the Nkporo Group (Kogbe, 1989). It consists of sandstone, shale,
siltstone, and coal seam. Mamu Formation is overlain by the Ajali Sandstone, which was deposited in
the Maastrichtian, and consists of unconsolidated coarse – fine grained siltstone and poorly cemented
mudstone (Reyment, 1965; Kogbe, 1989; Nwajide and Reijers, 1996). Ajali Sandstone is overlain by
the Nsukka Formation, which is conformably overlain by the Imo Shale. The Nsukka Formation
(Upper Coal Measures) is dated Maastrichtian–Danian, while the Imo Shale, which comprises shale
with ironstone and thin sandstone, with occasional higher land plants, is dated Paleocene (Reyment,
1965; Kogbe, 1989; Nwajide, 1990; Nwajide and Reijers, 1996). The youngest strata in the basin is the
Ameki Group, which is dated Eocene.
3.0 Materials and methods of study
A total of 138 ditch cuttings taken from Akukwa-2 well drilled in Anambra Basin, southeastern Nigeria
were used for this study. The samples, which are free from oil-based additives (drilling mud), were
supplied by the Nigerian Geological Survey Agency (NGSA). The sampling interval is between 1266 –
1740 m and covers mainly Mamu and Nkporo formations (Fig. 2). The samples were described
lithologically under a binocular microscope, while the percentage of shale content was also noted.
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Sample preparation for palynological analysis was by the standard method of Traverse (1988). Ten
grams of each sub-sample, crushed to about 0.5 cm, were soaked in 36 % hydrochloric acid (HCl) for
24 hours to dissolve the carbonates minerals, and subsequently treated with 48% hydrofluoric acid
(HF) for another 24 hours to remove the silicates. Sieving process with 5μm mesh followed this. The
recovered organic material was slightly oxidised using nitric acid (HNO3), followed by heavy mineral
separation using Zinc Chloride and Hydrochloric Acid (ZnCl2/HCl) solution (specific gravity 2.0). The
residue was then mounted on glass slides with glycerol in a ratio of 1:1 by volume. The frequency
counts of the palynomorphs (Tyson, 1995) present were determined for each sample and interpreted by
comparison with the previous literature. Photomicrographs of diagnostic species seen in the samples
were taken using Olympus binocular camera-equipped microscope at 1000x magnification (MODEL
CH30RF200) (see the plate).
Total organic carbon content test (TOC, wt.%) and Rock-Eval type pyrolysis were performed on forty-
four (44) samples using a LECO Olympus binocular camera Carbon-Analyzer IR 112 and Weatherford
Source Rock Analyzer-TPH/TOC (SRA) instruments, respectively. Fifteen (15) powdered samples
were Soxhlet extracted for 72 hours using an azeotropic mixture of dichloromethane (DCM) and
methanol (93:7). The extracts were fractionated by column chromatography on neutral alumina over
silica gel into three fractions (aliphatic hydrocarbon, aromatic hydrocarbon and, NSO - nitrogen,
sulphur and oxygen) compounds. Aliphatic fractions were eluted with petroleum ether (100 ml),
aromatic fractions with dichloromethane (100 ml), and NSO fractions with methanol (50 ml). Gas
chromatography-mass spectrometry (GC-MS) analysis of aliphatic hydrocarbon fractions was
performed on a HP V5975B MSD mass spectrometer with a gas chromatograph attached directly to the
ion source. Pyrolysis-gas chromatography (Py-GC) analysis was carried out on eight powdered samples
using a Double-Shot Pyrolyzer PY-2020iD from Frontier Laboratories Ltd. Also, petrographic
observations were performed on 12 whole rock samples under oil immersion in a reflected white light,
using a LEICA CTR 6000 photometry system equipped with a 50x oil immersion objective, a 546 nm
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filter and fluorescence illuminators. The samples were also observed under ultraviolet (UV) light. The
procedures for the polished blocks preparation and the vitrinite reflectance measurements follow the
guidelines published in Taylor et al. (1998). These procedures are based on the international standard
methods (ISO and ASTM). Mean random vitrinite reflectance (Ro %) was measured on the samples
using a Windows-based DISKUS Fossil software in the microscope prior calibration using standard
sapphire (0.589% refractive index) and immersion oil (ne = 1.518; 23 °C). The percentage of incident
light reflected from the vitrinite particles in the samples was measured in comparison to the known
standard of 0.589% (Table 1). Photomicrographs of the macerals observed in the samples were also
taken.
4.0 Results
4.1 Sedimentology
Lithologically, the studied section of Akukwa-2 well is composed mainly of dark to light grey, hard,
fissile shales, and sandy mudstones that contain few carbonaceous detritus and ferruginous material.
The basal part consists of uniform shales with minor sandstone unit. The shaly content of the sediment
reduces gradually upwards from shale to interbedded mudstone (Fig. 2). The included quartz grains
vary from fine to medium grained, angular to well-rounded, and moderately sorted.
4.2 Palynology
Moderately-rich and diverse assemblages of palynomorphs species were observed (see the chart, Fig.
3). The assemblage consists of pollen (38%), spores (33.8%), dinoflagellate cysts (9%),
algae/botryococcus (1.2%), and indeterminate species (unidentified palynomorphs) (18%). A total of
562 palynomorphs were counted and recorded, out of which 410 palynomorphs were identified. The
palynomorphs include Cyathidites minor, Cyathidites spp., Retidiporites magdaleneis, Monocolprites
marginatus, Longapertites marginatus, Longapertites spp., Cingulatisporites ornatus,
Loevigatosporites spp., Verrucatosporite spp., Hystrichorpheridium spp., Spiniferite spp., and
Pediastrum among others. The palynomorphs were fairly well-preserved in the analysed sediments (see
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the plate, Fig. 4). Due to the clastic nature of the sediments, some of the processed samples were barren
of palynomorphs.
4.3 Organic petrography
Petrographic observations indicated that majority of the analysed shale samples contain abundant
macerals (vitrinite, bituminite, alginite, cutinite, resinite), bitumen and inorganic components such as
quartz and framboidal pyrite (Fig. 5). Dull yellow to orange fluorescence of bituminite and alginite can
also be observed under ultraviolet light excitation (Fig. 5). The resinite macerals are amber coloured
with variable sizes and are believed to be associated with the higher vascular plant. Most of these
macerals are moderately well-preserved. Clustered framboidal pyrites are found and are commonly
associated with vitrinite macerals (Fig. 5). Additionally, the vitrinite reflectance (Ro) values,
measured on the vitrinites found in the shale samples, varied between 0.52 to 0.60% (Table 1).
4.4 Bulk geochemical parameters
TOC content and pyrolysis data with calculated parameters of the analysed Nkporo and Mamu samples
are shown in Table 1. Over 95% of the analysed samples have a total organic carbon (TOC) content
above 0.5 wt.%, which is the minimum threshold of a hydrocarbon source rock. The TOC values of
Nkporo and Mamu formations analysed range from 0.27–3.02 wt. % and 0.78–2.57 wt. %, respectively
(Table 1). The S2 pyrolysis yield and hydrogen index in the analysed Nkporo and Mamu samples range
from 0.55 to 3.35and 0.77 to 2.33 mg HC/g rock and 58 to 292 and 63 to 291 mg HC/g TOC,
respectively. Also, the Tmax values in the analysed Nkporo samples are in the range of 427–441 oC,
while the Tmax values in the Mamu samples range from 427–435 oC (Table 1). The Tmax values
increase with depth and are marginally higher in the Nkporo samples than in the Mamu samples. The
production index (PI) of Nkporo samples also range from 0.05 to 0.25, while the PI for the Mamu
samples are from 0.05 to 0.10 (Table 1).
4.5 Open-system-pyrolysis-gas chromatography
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The pyrolysis products of the analysed samples from Akukwa-2 well are dominated by a homologous
series of n-alkene/n-alkane doublets (Fig. 6). While the Py-GC pyrograms of some of the samples from
the Nkporo Formation display prominent n-alkane/n-alkene doublets in the low molecular weight end
(<n-C10) and in the high molecular weight end (>n-C15), the pyrograms of other samples, especially
from the Mamu Formation show prominent n-alkane/n-alkene doublets in the low molecular weight
end only. Also, there are copious quantities of alicyclic compounds such as naphthalenes and light
aromatic compounds (benzene, toluene, ethylbenzene and xylenes) as well as sulphur compounds
(mainly thiophenes) in the samples (Fig. 6). The relative abundance of n-octene (n-C8:1), m(+p)-xylene
and phenol, and that of o-xylene, 2,3-dimethyl-thiophene and n-C9:1 were also determined from the
pyrograms (Table 2). The “type index” (R) and the n-C8:1/xy ratio calculated from the Py-GC traces of
the studied Akukwa-2 well samples range from 0.9 to 2.4 and 0.4 to 1.2, respectively (Table 2).
4.6 Straight chain alkanes and isoprenoids
Mass chromatograms m/z 85 of aliphatic hydrocarbons of two analysed Akukwa-2 extracts are shown
in Figure 7. The chromatograms reveal that there is a preponderance of n-C15–n-C37 and isoprenoid
hydrocarbons in the saturated fractions. The n-alkane pattern in the analysed samples show mainly
bimodal distribution and a preponderance of medium molecular weight and long-chain alkanes in
almost all of the chromatograms (Fig. 7). Acyclic isoprenoids (pristane - C19 and phytane - C20) are
present in all of the analysed samples as shown by Pr/n-C17 and Ph/n-C18 ratios (Fig. 5; Table 3). The
Pr/n-C17 versus Ph/n-C18 ratios in the analysed Nkporo and Mamu samples range from 1.0 to 1.8 and
0.6 to 1.0, respectively. The pristane/phytane (Pr/Ph) ratios have been used to interpret the redox
conditions of the source rock depositional environments (Powell and McKirdy, 1973; Didyk et al.,
1978; Peters et al., 1993; Peters et al., 2005). The Pr/Ph ratios in the analysed samples are in the range
of 1.62–2.40 (Table 3).
Carbon preference index (CPI) of n-alkanes gives some information on the paleoredox conditions,
provenance, and thermal maturity of organic matter (Meyers and Snowdon, 1993). The CPI values for
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all the analysed Nkporo and Mamu samples, which were determined based on the formula proposed by
Peters and Moldowan (1993), range from 1.06 to 1.16 and 1.11 to 1.16, respectively (Table 3). The
improved odd – even predominance (OEP; Scalan and Smith, 1970) values also range from 0.95 to
1.09 and 0.97 to 1.08 in the Nkporo and Mamu samples, respectively. The amount of terrigenous
organic materials may also be determined by the waxiness index. The degree of waxiness in the studied
samples, which is expressed by the formula ∑(n-C21 – n-C31)/∑(n-C15 – n-C20), is based on the
presumption that aquatic organic materials contribute low molecular weight n-alkane components to
the oil or extracts, while the high molecular weight normal alkanes were thought to come from the
terrigenous organic materials (Peters et al., 2005). The waxiness index for the studied Nkporo and
Mamu samples range from 2.34 to 3.73 and 2.18 to 3.72, respectively (Table 3). The
Terrigenous/Aquatic ratios range from 1.19 to 2.21 and 1.19 to 2.03, respectively for the analysed
Nkporo and Mamu samples (Table 3). These ratios were used to indicate the relative amount of
terrigenous and aquatic organic matter input in source rock.
4.7 Terpanes
The terpane and the hopane distributions of the analysed Nkporo and Mamu samples were obtained
from m/z 191 mass fragmentograms as shown in Figure 7. The peaks on the m/z 191 ion
fragmentograms were identified using their retention times and the published literature (Philp, 1985;
Waples and Machihara, 1991; Sachse et al., 2011; Adegoke et al., 2014). The m/z 191 mass
chromatograms of the Nkporo and Mamu extracts show moderate amounts of tricyclic and pentacyclic
terpanes with low abundance of tetracyclic terpanes. The composition and distribution of hopanoids
biomarkers are the same in many of the samples studied and mostly comprise C27 to C35 17α,21β(H)-
hopanes with C29αβ and C30αβ hopanes as major compounds (Fig. 7). However, the amount of C29αβ
hopane is less than that of C30αβ hopane in many of the samples analysed, with C29/C30-hopane ratios
ranging from 0.76 to 1.04 (Table 4). C31-hopane predominates among the homohopanes (C31 – C35) in
all the analysed samples. Other compounds detected include 17β,21α(H)-moretane and 18α(H)-
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oleanane. The calculated C30-moretane/C30-hopane ratios for the samples range from 0.14 to 0.51,
while the oleanane/C30-hopane ratios (oleanane index) range from 0.03 to 0.74. In addition, the Ts/(Ts
+ Tm) ratios range from 0.10 to 0.50 (Table 4).
Gammacerane, which is widely considered as an indicator of salinity stratified water column, is present
in all of the samples analysed (Sinninghe Damsté et al. 1995; Ten Haven et al. 1988). Gammacerane
index for the extracts ranges between 0.04 and 0.18 while the C24/C23 and C22/C21 tricyclic terpane
values are in the range of 0.18 to 0.82 and 0.33 to 1.50, respectively (Table 4). C21/C23 tricyclic terpane
ratios also range from 0.12 to 0.43. In many of the samples, the abundance of C25 tricyclic terpane is
more than that of the C26 tricyclic terpane, with the value of C26/C25 ranging from 0.50 to 1.25 (Table 4;
Fig. 7). Furthermore, the C32 22S/(22S+22R) ratios for the analysed Nkporo and Mamu samples range
from 0.51 to 0.62 and 0.51 to 0.56, respectively.
4.8 Steranes
Sterane distributions of the analysed Nkporo and Mamu samples were obtained from m/z 217 mass
chromatograms (Fig. 7). The peaks on m/z 217 fingerprints were identified by their retention times and
the published literature (Philp, 1985; Volkman, 1986; Waples and Machihara, 1991; Sachse et al.,
2011; Adegoke et al., 2014). The peaks identified are named in the Appendix and the ratios obtained
are shown in Table 5. The relative proportions of each of the ‘regular’ steranes (C27, C28 and C29) in
samples vary, depending upon the type of organic matter input to the sediment. The C29 steranes (22.8–
54.1%) are relatively more abundant than the C27 (30.7–66.2%) and C28 (11.0–22.5%) steranes in the
analysed Nkporo and Mamu extracts (Table 5). The C29/C27 sterane ratios also range from 0.18 to 0.51
(Table 5). Other parameters calculated from the m/z 217 fingerprints are the C29 ββ/(ββ+αα) and the
20S/(20S + 20R) for C29 steranes. The values of ββ/(ββ+αα) and 20S/(20S+20R) for the analysed
Nkporo and Mamu sediments range between 0.21 and 0.53 and 0.34 to 0.63, respectively (Tables 5).
5.0 Discussions
5.1 Organic richness and kerogen type
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The quantity and type of organic matter within the Nkporo and Mamu sediments were evaluated using
the TOC contents and pyrolysis data such as the amount of free hydrocarbon (S1) and the remaining
hydrocarbon potential (S2) in the rock. These data were also used to assess the present-day hydrocarbon
generative potential of source rocks (Peters, 1986; Bordenave et al, 1993). The TOC contents show that
majority of the analysed Nkporo and Mamu samples can be regarded as source rock with fair to very
good petroleum generative potential (Peters and Cassa, 1994; Table 1). The S2 values (< 4 mg/g) in the
analysed samples also reveal that the rocks have fair generative potential. Samples from the Nkporo
Formation are organically richer and have better source rock quality than those from Mamu Formation,
as shown by the TOC and S2 values and the cross-plot of S2 versus TOC (Table 1; Fig. 8). This is in
agreement with Akaegbobi and Schmitt (1998), which concluded that the Nkporo sediments are likely
the main source rocks in the Anambra Basin.
The kerogen in the analysed samples was characterised using the pyrolysis data (Table 1). The
hydrogen index (HI) indicates that there is a preponderance of Type III organic matter within the
Nkporo and Mamu sediments. Also, the cross-plot of HI versus Tmax reveals that the organic matter in
the analysed samples is predominantly Type III-II and Type III (Fig. 9). About 75% of analysed
Nkporo samples have HI below 200 mg HC/g TOC and are gas prone, while the remaining 25%
possess HI above 200 mg HC/g TOC (Peters and Cassa, 1994). Mamu Formation shows a similar trend
with 85% of the samples being gas prone, while the remaining 15% are oil and gas prone, as they
possess HI above 200 mg HC/g TOC. This is in agreement with the petrographic observations, which
reveal that analysed shale samples contain abundant macerals (vitrinite), with a minor amount of
bituminite, alginite, cutinite, and resinite.
5.2 Molecular kerogen composition (Pyrolysis-GC)
The source rock quality (i.e. molecular kerogen type) and the type of hydrocarbons generated in the
samples were further characterised using open-system-pyrolysis-gas chromatography technique. This is
because Py-GC directly monitors specific chemical compounds in a kerogen pyrolysate and provides
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detailed insights into the macromolecular organic matter in terms of its structural moieties (Giraud,
1970; Larter and Douglas, 1980; Dembicki et al., 1983; Larter, 1984; Horsfield, 1989; Eglinton et al.,
1990; Dembicki, 2009). Usually, the type of kerogen depends on the amount of aliphatic, aromatic, and
phenolic components, of which the aliphatic carbon content is the most critical for the generation of
hydrocarbons.
The characterisation of kerogen in the analysed samples was enabled in terms of the chromatographic
“fingerprint”. Dembicki (2009) noted that Type I kerogens display abundant long-chained n-alkanes/n-
alkenes (>n-C15) and short-chained n-alkanes/n-alkenes (<n-C10) in their Py-GC traces, whereas Type
III gas-prone kerogens show the bulk of the pyrolysis products restricted to the low molecular weight
end (<n-C10) of the pyrograms. An intermediate situation is typical of Type II kerogens, while Type IV
kerogens are mostly inert and give little or no signal. Some of the studied samples yielded pyrograms
that are characteristic of Type III-II kerogen, while the others display pyrograms that are typical of
Type III kerogen (Dembicki et al., 1983; Dembicki, 2009; Fig. 6). These samples are probably
indicative of aromatic-rich with significant aliphatic compounds, and suggest a mixture of oil and gas
generation, but mainly gas.
Also, the numerical “type index” (R), determined as the peak height ratio of m(+p)-xylene and n-octene
(n-C8:1) in the pyrogram was used to evaluate the quality of organic matter in the analysed samples
(Larter and Douglas, 1980). The kerogen type is closely related to “type index”. Type I kerogens have
low type index (<0.4), Type II kerogens have values between 0.4 and 1.3, while Type III kerogens have
“R” values ranging from 1.3 to more than 20. The “type index” calculated from the Py-GC traces of the
studied Akukwa-2 well samples indicates that the kerogen type range from mixed Type III and Type II
kerogens [mixture of 75% Type III and 25% Type II kerogens, according to Dembicki (2009) to Type
III kerogens (Table 2). This is in agreement with the ternary plot of the relative abundance of o-xylene,
2,3-dimethyl-thiophene and n-C9:1 (Eglinton et al., 1990; Hartwig et al., 2012 (Fig. 10).
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Furthermore, semi-quantitative and qualitative analysis of the samples was carried out based on the
ratio of n-octene/m(+p)-xylene (n-C8:1/xy). The n-C8:1/xy ratio has been used as a measure of the
comparative abundance of aliphatic to aromatic hydrocarbons and also to determine hydrocarbon
generating potential (van Aarssen et al., 1992; Abdullah, 1999; Mustapha and Abdullah, 2013). These
authors noted that high n-C8:1/xy ratio of more than 1.0 is interpreted as possessing good hydrocarbon
generating potential (highly oil prone), whereas ratio below 1.0 is interpreted as being less oil prone
and more gas prone. The analysed samples display widely variable n-C8:1/xy ratios (0.4 to 1.2), which
indicate that they are more gas prone, which agrees with the bulk geochemical interpretation and
petrographic observations.
5.3 Maturity of organic matter
The maturity data include Tmax values, vitrinite reflectance data, production index (PI) and, biomarker
maturity ratios (Tables 1; 3; 4 and 5). The Tmax values and the PI in the samples indicate that the
sediments are immature to early mature for hydrocarbon generation (Peters and Moldowan, 1993;
Peters and Cassa, 1994). These data further show that Nkporo samples are marginally more mature
than Mamu samples. This agrees with the cross-plots of HI versus Tmax and the measured vitrinite
reflectance values (0.52 to 0.60%) (Fig. 9; Table 1). It is noteworthy that there is a good correlation
between Tmax and Ro, as indicated by a good correlation coefficient (r2 = 0.85), which could be as a
result of their regular increase with increasing maturity. It is also pertinent to note that though vitrinite
reflectance (Ro) is one of the most widely used methods for evaluation of thermal maturity of organic
matter. Ro measured on ditch cuttings rather than conventional core samples may only present a
conservative assessment of the organic maturation. This is due to the limitation of depth matching of
the samples and the problem of caving. The dull yellow to orange fluoresces of the alginite under UV
light excitation further suggests that the organic matter is immature to early mature for hydrocarbon
generation (Hakimi et al., 2014; Fig. 5).
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Also, biomarker maturity parameters were used to assess the thermal maturity of organic matter within
the sediments (Peters and Moldowan, 1993; Peters et al., 2005). Carbon preference index (CPI) and
odd-even predominance (OEP) values obtained from n-alkanes provide a rough assessment of organic
maturation. The CPI and OEP values in the samples indicate that the organic matter within the
sediments is immature to mature (Peters and Moldowan, 1993; Peters et al., 2005; Table 3). This is
supported by the cross-plot of Pristane/n-C17 and Phytane/n-C18 (Fig. 11) and other maturity–dependent
biomarker ratios as shown in Tables 4 and 5. These are moretane/hopane, C32 22S/(22S + 22R)
homohopane, Ts/(Ts + Tm), C29ββ/(ββ + αα) sterane, and C29 20S/(20S + 20R) sterane ratios
(Mackenzie et al., 1980; Waples and Machihara, 1991). According to Peters et al. (2005), 17α,21β(H)-
hopanes are more thermally stable than the 17β,21α(H)-moretanes, hence the abundance of the C30
moretanes to the corresponding hopanes decrease with increasing thermal maturity (Mackenzie et al.,
1980; Seifert and Moldowan, 1980; Peters et al., 2005). The C30 moretanes/C30 hopanes ratios for the
analysed samples (0.14 – 0.51) suggest that many of the studied samples are thermally mature for
generation of hydrocarbon. Furthermore, the C32-22S/(22S+22R) homohopane values, Ts/(Ts + Tm)
ratios and the dominance of S-isomers over the R-isomers among the homohopanes (C31 – C35) in many
of the extracts indicate the samples’ thermal maturity (immature to mature).
The C29 ββ/(ββ + αα) sterane and the C29-5α,14α,17α(H)-20S/(20S + 20R) sterane ratios obtained from
the m/z 217 ion mass fragmentograms were also used for the assessment of organic maturation in the
sediments (Table 5). These ratios are directly proportional to thermal maturity and suggest that most of
the organic matter within the sediments are within the oil window (Peters and Moldowan, 1993; Peters
et al., 2005). This is further supported by the cross-plot of two biomarker maturity parameters (Fig. 12).
5.4 Paleodepositional conditions and source input of organic matter
Biological marker distributions, petrographic, and palynological data were used to describe the
provenance and conditions of the depositional environment of organic matter within the analysed
Nkporo and Mamu sediments (Peters and Moldowan, 1993; Tyson, 1995; Peters et al., 2005). N-
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alkanes distribution provides evidence about source organisms, for instance, organic matter sourced
from algae has abundant short-chain n-alkanes <nC20, while long chain n-alkanes > nC27 are
preponderant in the land plant-derived organic matter (Eglinton and Hamilton, 1967; Cranwell et al.,
1987). The n-alkane pattern in the samples indicates that the organic materials within the sediments
were derived from mixed algal and land plant source input, with significant terrigenous source input
(Peters and Moldowan, 1993; Adegoke et al., 2014; Fig. 7; Table 3).
Pr/Ph ratios of oils and extracted organic matter (bitumen) within the oil-generative window have also
been used to indicate the bottom water conditions during accumulation of source sediments (Powell
and McKirdy, 1973; Didyk et al., 1978; Peters and Moldowan, 1993; Chandra et al., 1994; Large and
Gize, 1996; Adegoke et al., 2015). The Pr/Ph ratios in the analysed samples suggest that the Nkporo
and Mamu Formation sediments encountered in the Akukwa-2 well were deposited under
paleodepositional conditions that were mainly suboxic (Peters and Moldowan, 1993; Peters et al., 2005;
Hakimi et al., 2011; Adegoke et al., 2014; Table 3). The cross-plot of the relationship between
isoprenoids and n-alkanes (pristane/n-C17 versus phytane/n-C18), the CPI values calculated from the
chromatograms, and the moderately well-preserved nature of the observed macerals further support this
interpretation (Meyers and Snowdon, 1993; Peters and Moldowan, 1993; Peters et al., 2005; Akinlua et
al., 2007; van Koeverden et al., 2011; Table 3; Fig. 5; Fig. 11). Also, the degree of waxiness as
expressed by the waxiness index, the relatively high TAR values, the cross-plot of Pr/Ph ratio versus
waxiness index, and the abundance of vitrinite macerals in the samples show a dominance of land
plant-derived organic materials (Table 3; Figs. 5; 13).
Hopanoid biological markers are important for indicating bacteria-derived organic matter (Ourisson et
al., 1979). The provenance of organic materials in Nkporo and Mamu sediments has been indicated by
several tricyclic terpanes ratios (Peters et al., 2005; Adekola et al., 2012). Tricyclic and tetracyclic
terpanes, which are believed to have both marine and terrestrial sources, are abundant in all of the
analysed samples (Aquino Neto et al., 1983; Philp and Gilbert, 1986; Marynowski et al., 2000). The
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relatively low to moderate C22/C21 and C24/C23 tricyclic terpane values, the low C21/C23 tricyclic terpane
ratio (0.12 to 0.43) and the higher amount of C25 tricyclic terpane than C26 tricyclic terpane in the
samples are suggestive of a mixed aquatic and land-derived organic materials (Volk et al., 2005;
Qiuhua et al., 2011; Adekola et al., 2012; Table 4, Fig. 7). Furthermore, the high abundance of C23
tricyclic terpane in all of the analysed samples suggests suboxic environmental conditions during the
deposition of the sediments (Peters et al., 2005). Gammacerane is present in all of the studied extracts,
with gammacerane index in the range of 0.04 and 0.18. This suggests that there is moderate salinity
stratified water column and suboxic bottom water conditions at the time of accumulation of Nkporo
and Mamu sediments. The occurrence of 18α(H)-oleanane, a land plant-derived biological marker, in
all of the analysed samples further gives credence to the presence of substantial terrestrial-sourced
organic matter input in the sediments (Peters and Moldowan, 1993; Peters et al., 2005).
Huang and Meinschein (1979) proposed that the relative proportions of the C27, C28 and C29 regular
steranes in sediments might provide some insights into the paleoenvironment and provenance of
organic matter in the sediment. They suggested that a preponderance of C29 steranes, C28 steranes and
C27 steranes would indicate a significant land-plant, lacustrine algae and marine phytoplankton
contributions, respectively. Both C27 and C29 steranes are present in abundant quantities in the Nkporo
and Mamu samples (Table 5), reflecting a substantial contribution of marine and land–plant-derived
organic matter (Peters and Moldowan, 1993). This is supported by the ternary diagram of regular
sterane ratio, the cross-plot of pristane/phytane ratios versus steranes C27/(C27+C29) ratios, and the low
to moderate C29/C27 sterane ratios (0.18–0.51) in the studied samples (Huang and Meinschein, 1979;
Hossain et al., 2009; Table 5; Fig. 14; Fig. 15). The occurrence of framboidal pyrite in the studied
samples further indicate that the organic materials were accumulated under suboxic paleodepositional
conditions (Fig. 5).
Palynological observations were also used to assess the provenance and paleodepositional conditions of
organic matter within the sediments. The analysed sediments contain both terrestrial and marine-
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derived palynomorphs such as Proteacidites sigalii, Cingulatisporites ornatus, Buttinia andreevi,
Hystrichodium spp., Spinferites spp. and Hystricholcolpomas spp (see Fig. 3 and plate). The
palynomorphs were only moderately well-preserved, which suggests that they may have been partially
oxidised. The nature of these palynomorphs points out that the organic matter was deposited under
environmental conditions that were mainly suboxic (Tyson, 1995).
5.5 Hydrocarbon generation potential
The organic matter within Nkporo and Mamu sediments in the Akukwa-2 well section was
characterised using total organic carbon (TOC) contents, pyrolysis data, and biomarker distributions. It
was shown that the sediments contain mixture of Type III-II and Type III kerogens, with significant
land-plant derived organic materials expected to generate mainly gas and little oil as evidenced by
abundant vitrinite maceral. The S2 pyrolysis values also indicate that the rocks have fair hydrocarbon
generative potential (Peters and Cassa, 1994; Table 1). This is supported by the cross-plot of S2 versus
TOC (Fig. 8). Pyrolysis, biomarker maturity, and vitrinite reflectance data indicate that the analysed
sediments are immature to early mature for hydrocarbon generation. The presence of alginite, cutinite,
resinite, and bituminite suggests that hydrocarbon could be generated by these shale samples. The
significant occurrence of bitumen staining in many of the samples further shows that petroleum has
been generated (Fig. 5). Based on the pyrolysis data, Nkporo samples are found to be richer organically
than the Mamu samples, thereby substantiating Akaegbobi and Schmitt (1998) work that the Nkporo
Formation sediments are likely the main source rocks in the Anambra Basin.
5.6 Paleoenvironmental reconstruction
The sedimentological, palynological, and biomarker characteristics of the sediments enabled the
reconstruction of the depositional environments. Sedimentologically, the medium to fine-grained sandy
mudstone suggests fluctuating low to medium energy of deposition, while the coarsening upwards
nature of the sandy mudstone suggests that the basin was shallowing upwards in a prograding delta
(Dapple, 1974). More so, the presence of plant (e.g. reedlike vegetations like poaceae and cyberaceae)
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and carbonaceous materials (e.g. graminae cuticle and comminuted coal) within the samples is an
indication of the tidal environment (Dapple, 1974).
Environmental changes have also been found to be reflected by the palynological assemblage contained
in sediments (Oloto, 1989; Ojo and Akande, 2004). Palynologically, the occurrence of both terrestrial
and marine-derived palynomorphs at almost equal proportions between 1266 and 1461 m points to a
marginal marine environment (tidal and deltaic). Some of the continental palynomorphs, which
characterised the studied section are Retidiporites magdaleneis, Monocolprites marginatus,
Longapertites marginatus, Longapertites spp., Cingulatisporites ornatus, Psilatricolporites crassus,
Laevigatosporites crassus, Verrucatosporites spp., Buttinia andreevi and, Pachdermites diederixi.
Several marine palynomorphs such as Spiniferites spp. and Hystrichospheridium spp. are also present
in the analysed sediments. Dinoflagellates such as Andalusiella sp. are characteristic for near shore
marine environment, while the occurrence of chorate dinocysts such as Leioshaeridia sp. and
Subtilisphaera sp. are prevalent in open marine environment (May, 1977; Petters, 1978; Reyment and
Dingle, 1987; Awad, 1994; Vadja-Santinavez, 1998; Ojo and Akande, 2004). The presence of
Spiniferites spp. also suggests oceanic to neritic environment (May, 1977; Awad, 1994; Vadja-
Santinavez, 1998). Furthermore, the appearance of Botryococcus braunii in the sediments suggests the
influence of a brackish-water or lagoon environment, although they are also found in a fresh water
environment (Rull, 1997). At interval 1464 – 1742 m, terrestrially derived palynomorphs dominate,
suggesting a terrestrial environment with marine influence (Shrank, 1989). Therefore, the environment
of deposition is a relatively quiet, shallow marine tectonic setting with fluvial incursion especially at
the upper part of the intervals studied and consequently, it is a delta associated depositional
environment with a fluviatile influence.
Biomarker distributions were further used to reconstruct the paleoenvironments of the sediments
penetrated by Akukwa-2 well. According to Qiuhua et al. (2011), marine source rocks contain C21/C23
tricyclic terpane ratio that is less than 0.5. The ratio in the studied extracts is less than 0.5, and this
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indicates a marine environment deposition with terrestrial influence. The low to moderate C24/C23 and
C22/C21 tricyclic terpane values (Table 4, Fig. 7) in the studied samples also support this interpretation.
The C26/C25 tricyclic terpane ratios in the samples further shows deposition in marine environments
with significant terrestrial interference (Peters et al., 2005; Volk et al., 2005; Adekola et al., 2012;
Table 4; Fig. 7). Also, the presence of fluoresces bituminite in the samples under ultraviolet light
suggest a marine origin, although some are also terrigenous (Fig. 5). Alalade and Tyson (2010) and
Hakimi and Abdullah, (2013) had pointed out that amorphous organic matter (bituminite), which
fluoresces under ultraviolet light may indicate an aquatic origin and is usually of algal or other
phytoplanktonic origin. The terrestrial interference is indicated by the presence of significant amount of
vitrinite macerals (Fig. 5). Oleanane, a terrestrially-sourced biomarker that is present in the extracts,
has also been reported to indicate probable marine-influence (Murray et al., 1997; Alias et al., 2012).
Because of the presence of significant amount of terrestrially derived organic materials, the Nkporo and
Mamu sediments are therefore suggested to be deposited in palaeogeographic settings close to
vegetation source and consequently, it is a delta associated depositional environment with a fluviatile
influence.
6.0 Conclusions
The Upper Cretaceous sediments from the Akukwa-2 well in Anambra Basin, southeastern Nigeria
were characterised using a suite of palynological, organic geochemical (Rock-Eval-type pyrolysis, GC-
MS and open system pyrolysis-GC), and organic petrographic analyses (vitrinite reflectance
measurements). The pyrolysis data revealed that the Nkporo and Mamu sediments have fair petroleum
generation potential. Bulk geochemical and Pyrolysis-GC data indicate that there is a preponderance of
Type III-II and Type III organic matter within the analysed samples, which is mainly gas-prone. This is
in agreement with the petrographic observations, which revealed that analysed shale samples contain
abundant macerals (vitrinite, bituminite, alginite, cutinite, and resin). The Nkporo and Mamu sediments
are immature to early mature in terms of hydrocarbon generation as indicated by pyrolysis Tmax data,
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biomarker maturity ratios, and vitrinite reflectance values. Biomarker distribution ratios and
palynological observations also suggest that the organic matter within the sediments was derived from
mixed aquatic and terrigenous source input and deposited under suboxic paleodepositional conditions.
Based on sedimentological, palynological, and biomarker characteristics, the environment of deposition
of the analysed sediments was inferred to be a relatively quiet, shallow marine with fluvial incursion
especially at the upper part of the intervals studied and consequently, it is a delta associated
depositional environment with a fluviatile influence. The sediments are therefore suggested to be
deposited in a paleogeographic setting close to vegetation source.
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Acknowledgements
The authors are grateful to the Nigerian Geological Survey Agency (NGSA) and the Frontier
Exploration Services of the Nigerian National Petroleum Corporation (NNPC), for supplying the
samples for this research and Petroleum Geochemistry Laboratory, Department of Geology, University
of Malaya, Kuala Lumpur for analytical support. Partial funding of this work from the University of
Malaya Research Grant (Project Nos.: RF022B-2018 and J-21001-79012) is acknowledged. The
authors would also like to sincerely thank the Editor-in-Chief, Prof. Dr. Ralf Littke and the reviewers
for their useful comments that significantly improved this manuscript.
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Appendix
Peak assignments for alkane hydrocarbons in the gas chromatograms of the saturate fractions (I) in the
m/z 191 mass fragmentogram and (II) m/z 217 mass fragmentogram.
(I) Peak identity Compound Carbon no.
Fragmentogram
m/z 191
C21 C21 Tricyclic (Cheilanthane) 21
C22 C22 Tricyclic (Cheilanthane) 22
C23 C23 Tricyclic (Cheilanthane) 23
C24 C24 Tricyclic (Cheilanthane) 24
C24 C24 Tetracyclic 24
C25 C25 Tricyclic (Cheilanthane) 25
C26 C26 Tricyclic (Cheilanthane) 26
Ts 18α(H),22,29,30-trisnorneohopane 27
Tm 17α(H),22,29,30-trisnorhopane 27
C28αβ 17α(H),29,30-bisnorhopane 28
C29αβ 17α,(H)21β(H)-norhopane 29
C29Ts 18α(H),30-norneohopane 29
C29βα 17β(H),21α(H)-hopane (normoretane) 29
C30αβ 17α,(H),21β(H)-hopane 30
C30βα 17β(H),21α(H)-hopane (moretane) 30
ol 18α(H)-oleanane 30
Gammacerane Gammacerane 30
C31αβ
17α,(H),21β(H)-homohopane (22S)
(22S and 22R) 31
C31αβ
22S
17α(H),21β(H)-homohopane (22S)
31
C31αβ
22R
17α(H),21β(H)-homohopane (22R)
31
C32αβ
17α(H),21β(H)-homohopane
(22S and 22R) 32
C33αβ
17α(H),21β(H)-homohopane
(22S and 22R) 33
C34αβ
17α(H),21β(H)-homohopane
(22S and 22R) 34
C35αβ
17α(H),21β(H)-homohopane
(22S and 22R) 35
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(II) Peak identity Compound Carbon no.
Fragmentogram
m/z 217
C27ααα 20S 5α(H),14α(H),17α(H)-cholestane (20S) (sterane) 27
C27ααα 20R 5α(H),14α(H),17α(H)-cholestane (20R) (sterane) 27
C28ααα 20S 24-methyl-5α(H),14α(H),17α(H)-cholestane (20S) (sterane) 28
C28αββ 20R 24-methyl-5α(H),14β(H),17β(H)-cholestane (20R) (sterane) 28
C28αββ 20S 24-methyl-5α(H),14β (H),17β (H)-cholestane (20S) (sterane) 28
C28ααα 20R 24-methyl-5α(H),14α(H),17α(H)-cholestane (20R) (sterane) 28
C29ααα 20S 24-ethyl-5α(H),14α(H),17α(H)-cholestane (20S) (sterane) 29
C29αββ 20R 24-ethyl-5α(H),14β(H),17β(H)-cholestane (20R) (sterane) 29
C29αββ 20S 24-ethyl-5α(H),14β(H),17β(H)-cholestane (20S) (sterane) 29
C29ααα 20R 24-ethyl-5α(H),14α(H),17α(H)-cholestane (20R) (sterane) 29
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Table captions
Table 1: Bulk geochemical results of TOC content and pyrolysis analyses with calculated parameters
including measured vitrinite reflectance (%Ro) of the analysed samples.
Table 2: Peak height and abundance ratios of some compounds calculated from Py-GC pyrograms of
Akukwa-1 well samples.
Table 3: n-Alkanes and isoprenoids ratios of the studied samples.
Table 4: Hopane biomarker parameters calculated from m/z 191 mass chromatograms of the analysed
samples (see the Appendix for peak assignment).
Table 5: Sterane biomarker parameters calculated from m/z 217 mass chromatograms of the analysed
samples (see the Appendix for peak assignment).
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Figure captions
Fig. 1: Geological map of Nigeria, showing Anambra Basin and the location of the studied exploratory
well: Akukwa-2 (after Whiteman, 1982; Nwajide and Reijers, 1996; Olabode, 2014).
Fig. 2: Simplified composite lithology of Akukwa-2 well, Anambra Basin, showing the sampled
points.
Fig. 3: Stratigraphic distribution chart of palynomorphs in the Akukwa-2 well.
Fig. 4: Some of the palynomorphs recognized in the analysed samples.
Fig. 5: Photomicrographs of macerals from Mamu and Nkporo formations in Anambra Basin in
reflected white light (a, b, c, d, f, g, j, k, l) and in fluorescence mode incident light (e, h, i); (a).
Macerals (vitrinite) with pyrite assemblages in framboidal form; (b). Dispersed macerals with pyrite;
(c). Cellular texture of vitrinite maceral; (d). Vitrinite, cutinite and lamalginite associated with
framboidal pyrite; (e). Lamalginite under UV light; (f). Alginite and pyrite; (g). Shell fragment
associated with pyrite; (h). Dull yellow fluoresces of the shell fragment under UV light; (i). Dull yellow
fluoresces alginite under UV light; (j). Dispersed macerals (resinite and vitrinite) and pyrite; (k, l).
Dispersed macerals (vitrinite and alginite) and pyrite.
Fig. 6: Py-GC pyrograms of shale samples from the Akukwa-2 well in Anambra Basin which display
Kerogen Type III/II and Type III. The generation product is mixed oil/gas, but mainly gas. The n-
alkanes and n-alkenes doublets are represented by Cn (n is numeral and represents the carbon
numbers).
Fig. 7: Mass fragmentograms m/z 85, m/z 191 and m/z 217of saturated hydrocarbons of two studied
Akukwa-2 sediment extracts.
Fig. 8: Cross-plot of S2 pyrolysis yield versus TOC showing kerogen quality in the analysed samples.
Fig. 9: Plot of hydrogen index (HI) versus pyrolysis Tmax for the analysed samples, showing kerogen
quality and thermal maturity stage.
Fig. 10: A ternary plot of an aromatic compound (O-xylene), an n-alkane component (n-C9:1) and a
sulphur-compound (2,3-dimethyl-thiophene) identified in the pyrolysates, showing kerogen type
classification (adapted after Eglinton et al., 1990; Hartwig et al., 2012).
Fig. 11: Phytane to n-C18 alkane (Ph/n-C18) versus Pristane to n-C17 alkane (Pr/n-C17) showing
depositional conditions and type of organic matter of Nkporo and Mamu extracts (adapted from Peters
and Moldowan, 1993).
Fig. 12: Cross-plot of two biomarker parameters sensitive to thermal maturity of the analysed
sediments extracts which shows that most of the samples plot in the area of early oil window maturity
(modified from Peters and Moldowan, 1993).
Fig. 13: Cross-plot of waxiness versus pristane/phytane ratios indicating the depositional environment
conditions of the studied samples (adapted from El Diasty and Moldowan, 2012).
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Fig. 14: Ternary diagram of regular steranes (C27, C28 and C29) showing the relationship between
sterane compositions and organic matter input, shows that the analysed Nkporo and Mamu extracts are
composed of mixed marine/terrigenous organic matter (adapted from Huang and Meinschein, 1979).
Fig. 15: Cross-plots of C27/(C27 + C29) regular steranes versus pristane/phytane ratios show
paleodepositional conditions of organic matter (adapted from Hossain et al., 2009).
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Fig. 1: Geological map of Nigeria, showing Anambra Basin and the location of the studied exploratory
well: Akukwa-2 (after Whiteman, 1982; Nwajide and Reijers, 1996; Olabode, 2014).
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Fig. 2: Simplified composite lithology of Akukwa-2 well, Anambra Basin, showing sampled points.
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Fig. 3: Stratigraphic distribution chart of palynomorphs in the Akukwa-2 well.
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1. Monocolpites marginatus,
2. Psilatricolporites crassus,
3. Laevigatosporites crassus,
4. Verrucatosporites sp.
5. Ephedripites sp.
6. Charred Gramineae,
7. Botryococcus brauni,
8. Pachdermites diederixi,
9. Dinocyst indeterminate,
10. Buttinia andreevi,
11. Longapertites sp.
Fig. 4: Selected photomicrograph of the palynomorphs recognized in the analysed samples.
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Fig. 5: Photomicrographs of macerals from Mamu and Nkporo formations in Anambra Basin in
reflected white light (a, b, c, d, f, g, j, k, l) and in fluorescence mode incident light (e, h, i); (a).
Macerals (vitrinite) with pyrite assemblages in framboidal form; (b). Dispersed macerals with pyrite;
(c). Cellular texture of vitrinite maceral; (d). Vitrinite, cutinite and lamalginite associated with
framboidal pyrite; (e). Lamalginite under UV light; (f). Alginite and pyrite; (g). Shell fragment
associated with pyrite; (h). Dull yellow fluoresces of the shell fragment under UV light; (i). Dull yellow
fluoresces alginite under UV light; (j). Dispersed macerals (resinite and vitrinite) and pyrite; (k, l).
Dispersed macerals (vitrinite and alginite) and pyrite.
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Fig. 6: Py-GC pyrograms of shale samples from the Akukwa-2 well in Anambra Basin which display
Kerogen Type III/II and Type III. The generation product is mixed oil/gas, but mainly gas. The n-
alkanes and n-alkenes doublets are represented by Cn (n is numeral and represents the carbon
numbers).
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Fig. 7: Mass fragmentograms m/z 85, m/z 191 and m/z 217 of saturated hydrocarbons of two studied
Akukwa-2 sediment extracts.
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Fig. 8: Cross-plot of S2 pyrolysis yield versus TOC showing kerogen quality in the analysed samples.
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Fig. 9: Plot of hydrogen index (HI) versus pyrolysis Tmax for the analysed samples, showing kerogen
quality and thermal maturity stage.
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Fig. 10: A ternary plot of an aromatic compound (O-xylene), an n-alkane component (n-C9:1) and a
sulphur-compound (2,3-dimethyl-thiophene) identified in the pyrolysates, showing kerogen type
classification (adapted after Eglinton et al., 1990; Hartwig et al., 2012).
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Fig. 11: Phytane to n-C18 alkane (Ph/n-C18) versus Pristane to n-C17 alkane (Pr/n-C17) showing
depositional conditions and type of organic matter of Nkporo and Mamu extracts (adapted from Peters
and Moldowan, 1993).
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Fig. 12: Cross-plot of two biomarker parameters sensitive to thermal maturity of the analysed
sediments extracts which shows that most of the samples plot in the area of early oil window maturity
(modified from Peters and Moldowan, 1993).
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Fig. 13: Cross-plot of waxiness versus pristane/phytane ratios indicating the depositional environment
conditions of the studied samples (adapted from El Diasty and Moldowan, 2012).
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Fig. 14: Ternary diagram of regular steranes (C27, C28 and C29) showing the relationship between
sterane compositions and organic matter input, shows that the analysed Nkporo and Mamu extracts are
composed of mixed marine/terrigenous organic matter (adapted from Huang and Meinschein, 1979).
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Fig. 15: Cross-plots of C27/(C27 + C29) regular steranes versus pristane/phytane ratios show
paleodepositional conditions of organic matter (adapted from Hossain et al., 2009).
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Table 1: Bulk geochemical results of TOC content and pyrolysis analyses with calculated parameters
including measured vitrinite reflectance (%Ro) of the analysed samples.
Sample ID Depth
(m) Formation TOC S1 S2 Tmax HI PI Ro
AK-2-4180 1275 Mamu 0.99 0.07 1.15 435 116 0.06 0.52
AK-2-4210 1284 Mamu 0.81 0.06 0.97 432 120 0.06 N/A
AK-2-4220 1287 Mamu 0.92 0.11 1.13 433 123 0.1 N/A
AK-2-4230 1290 Mamu 0.91 0.06 0.98 432 108 0.06 N/A
AK-2-4240 1293 Mamu 1.36 0.08 1.17 433 86 0.06 N/A
AK-2-4250 1296 Mamu 1.91 0.08 1.21 431 63 0.06 N/A
AK-2-4260 1300 Mamu 2.22 0.12 1.93 428 287 0.06 N/A
AK-2-4300 1309 Mamu 1.59 0.08 1.42 430 89 0.05 0.55
AK-2-4310 1315 Mamu 0.78 0.08 1.04 432 133 0.07 N/A
AK-2-4320 1318 Mamu 1.34 0.08 1.33 432 99 0.06 N/A
AK-2-4340 1324 Mamu 2.57 0.14 2.33 427 291 0.06 N/A
AK-2-4360 1330 Mamu 1.52 0.11 1.11 432 73 0.09 N/A
AK-2-4370 1333 Mamu 1.48 0.11 1.41 429 95 0.07 N/A
AK-2-4420 1348 Nkporo 0.89 0.1 1.44 430 162 0.07 0.59
AK-2-4430 1351 Nkporo 0.81 0.14 0.82 434 101 0.15 N/A
AK-2-4440 1354 Nkporo 1.27 0.22 1.21 431 95 0.15 N/A
AK-2-4450 1357 Nkporo 0.68 0.14 0.93 433 137 0.13 N/A
AK-2-4460 1360 Nkporo 0.63 0.13 1.13 430 179 0.1 N/A
AK-2-4500 1373 Nkporo 1.8 0.41 1.36 427 76 0.23 0.56
AK-2-4510 1376 Nkporo 1.76 0.22 1.16 431 66 0.16 N/A
AK-2-4520 1379 Nkporo 1.88 0.26 1.73 430 292 0.13 N/A
AK-2-4590 1400 Nkporo 0.27 0.16 0.47 431 174 0.25 0.56
AK-2-4620 1410 Nkporo 1.92 0.15 1.45 428 76 0.09 N/A
AK-2-4670 1425 Nkporo 1.5 0.13 1.05 432 70 0.07 0.55
AK-2-4680 1428 Nkporo 1.52 0.2 1.2 428 79 0.14 N/A
AK-2-4730 1443 Nkporo 1.29 0.09 0.89 433 69 0.09 N/A
AK-2-4810 1467 Nkporo 1.07 0.05 0.93 429 87 0.05 N/A
AK-2-4830 1473 Nkporo 1.43 0.14 1.1 434 77 0.11 N/A
AK-2-4860 1482 Nkporo 1.16 0.17 0.95 433 82 0.15 N/A
AK-2-4870 1485 Nkporo 0.98 0.1 1.01 431 103 0.09 N/A
AK-2-4890 1492 Nkporo 1.67 0.14 1.24 431 74 0.1 N/A
AK-2-4900 1495 Nkporo 1.56 0.18 0.55 434 97 0.25 N/A
AK-2-4910 1498 Nkporo 0.4 0.07 0.65 433 163 0.1 N/A
AK-2-4930 1504 Nkporo 1.43 0.16 1.19 430 83 0.12 N/A
AK-2-4960 1513 Nkporo 0.95 0.15 1.61 431 169 0.09 N/A
AK-2-4970 1516 Nkporo 3.02 0.13 1.19 431 239 0.1 N/A
AK-2-5000 1525 Nkporo 1.42 0.07 1.02 429 72 0.06 N/A
AK-2-5030 1535 Nkporo 1.84 0.05 1.06 433 58 0.05 0.60
AK-2-5050 1541 Nkporo 1.62 0.12 1.36 431 84 0.08 0.57
AK-2-5210 1589 Nkporo 1.48 0.07 1.23 433 83 0.05 0.58
AK-2-5350 1632 Nkporo 2.3 0.13 2.36 437 203 0.05 0.57
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AK-2-5450 1663 Nkporo 1.95 0.18 1.82 434 93 0.09 N/A
AK-2-5580 1702 Nkporo 2.31 0.21 3.14 441 236 0.06 0.60
AK-2-5660 1726 Nkporo 2.38 0.23 3.35 439 241 0.06 0.59
Notes
S1 – Volatile hydrocarbon (HC) content, mg HC/g rock
S2 – Remaining HC generative potential, mg HC/g rock
HI – Hydrogen index = S2 x 100/TOC, mg HC/g TOC
TOC – Total organic carbon, wt.%
Tmax – Temperature at maximum of S2 peak oC
PI – Production index = S1/(S1 + S2)
Ro – Measured vitrinite reflectance
N/A – Not available
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Table 2: Peak height and abundance ratios of some compounds calculated from Py-GC pyrograms of
Akukwa-2 well samples.
Notes
2,3 DMT (%) – percent concentration of 2,3 dimethylthiopene in relation to O-xylene and n-C9:1
O-xylene (%) – percent concentration of O-xylene in relation to 2,3 dimethylthiopene and n-C9:1
n-C9:1(%) – percent concentration of n-C9:1 in relation to 2,3 dimethylthiopene and O-xylene
Type index (R) = m(+p)-xylene/n-octene
n-C8:1/xyl = n-octene (n-C8:1)/m+(p)-xylene
m(+p)-xyl. (%) = percent concentration of m(+p)-xylene in relation to n-octene (n-C8:1) and phenol
n-octene (n-C8:1) (%) = percent concentration of n-octene (n-C8:1) in relation to m(+p)-xylene and phenol
phenol (%) = percent concentration of phenol in relation to m(+p)-xylene and n-octene (n-C8:1)
Well
Sample
ID
Depth
(m) Formation
n-
C8:1
(%)
m(+p)-
Xylene
(%)
Phenol
(%)
2,3
DMT
(%)
O-
Xylene
(%)
n-C9:1
(%)
Type
Index
(R)
n-
C8:1/
xylene
Ak
uk
wa-1
AK-2-
4180 1275
Mamu 30.3
56.1 13.6 68.5 9.3 21.2
1.9 0.5
AK-2-
4490 1370
Nkporo 47.3
40.5 12.2 47.1 12.7 40.2
0.9 1.2
AK-2-
4830 1473
Nkporo 46.4
41.5 12.1 47.6 15.3 37.1
0.9 1.1
AK-2-
5030 1535
Nkporo 45.0
41.0 14.0 43.4 10.2 46.4
0.9 1.1
AK-2-
5210 1589
Nkporo 23.7
57.8 18.5 57.2 16.5 25.3
2.4 0.4
AK-2-
5350 1632
Nkporo 34.6
48.1 17.3 52.8 10.1 38.1
1.4 0.7
AK-2-
5580 1702
Nkporo 35.4
50.2 14.4 51.1 13.8 35.1
1.4 0.7
AK-2-
5660 1726
Nkporo 33.3
51.7 15.0 50.2 11.4 38.4
1.6 0.6
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Table 3: n-Alkanes and isoprenoids ratios of the studied samples.
Sample
ID
Dep
th
(m)
Format
ion
Pr/
Ph
Pr/
C17
Ph/
C18
Wa
x.
ind
ex
TA
R
C27/
C17
C
PI
OE
P
High
est
Peak
n-alkane
distribut
ion
n-
alkan
e
range
AK-2-
4180
127
5 Mamu 1.63 1.3 0.7 3.73
2.2
1 2.47
1.1
4
0.9
7
C18,C
27 Bimodal
C15 -
C37
AK-2-
4290
130
9 Mamu 1.86 1.2 0.8 2.36
1.1
9 1.08
1.1
1
1.0
5
C18,C
27 Bimodal
C15 -
C37
AK-2-
4410
134
5 Mamu 1.85 1.1 0.7 2.34
1.3
3 1.21
1.1
6
1.0
8
C18,C
27 Bimodal
C15 -
C38
AK-2-
4490
137
0 Nkporo 2.40 1.0 0.7 3.30
2.0
3 3.00
1.1
6
0.9
5
C21,C
27 Bimodal
C15 -
C37
AK-2-
4570
139
4 Nkporo 2.10 1.0 0.8 3.06
1.8
9 2.64
1.1
6
1.0
3
C25,C
29 Bimodal
C15 -
C37
AK-2-
4660
142
2 Nkporo 2.00 1.1 0.6 2.92
2.0
1 2.25
1.1
3
1.0
2
C18,C
27 Bimodal
C15 -
C38
AK-2-
4830
147
3 Nkporo 1.86 1.21 0.6 3.72
1.9
4 2.23
1.0
6
1.0
2
C18,C
27 Bimodal
C15 -
C37
AK-2-
5000
152
5 Nkporo 1.71 1.2 0.7 2.51
1.5
4 1.68
1.1
4
1.0
4
C18,C
21 Bimodal
C15 -
C38
AK-2-
5030
153
5 Nkporo 2.09 1.21 0.8 2.64
1.9
0 2.21
1.1
1
1.0
9
C20,C
25 Bimodal
C15 -
C37
AK-2-
5050
154
1 Nkporo 1.74 1.22 0.9 2.30
1.4
2 1.50
1.1
1
1.0
9
C18,C
25 Bimodal
C15 -
C37
AK-2-
5210
158
9 Nkporo 1.86 1.1 0.8 3.28
1.9
6 2.22
1.1
0
1.0
3
C18,C
24 Bimodal
C15 -
C36
AK-2-
5350
163
2 Nkporo 1.62 1.8 1.0 3.04
1.7
9 3.18
1.0
8
1.0
2
C21,C
24 Bimodal
C15 -
C38
AK-2-
5450
166
3 Nkporo 1.66 1.7 0.8 2.55
1.4
3 1.80
1.0
7
1.0
3
C19,C
24 Bimodal
C15 -
C37
AK-2-
5580
170
2 Nkporo 1.72 1.1 0.7 2.36
1.3
7 1.60
1.0
6
1.0
4
C20,C
25 Bimodal
C15 -
C37
AK-2-
5660
172
6 Nkporo 1.72 1.11 0.8 2.18
1.1
9 1.47
1.0
7
1.0
5
C20,C
23 Bimodal
C15 -
C37
Notes
Pr – Pristane
Ph – Phytane
Pr/Ph – Pristane / Phytane
Pr/n-C17 – Pristane/n-C17
Pr/n-C18 – Pristane/n-C18
CPI – Carbon preference index : {2(C23 + C25 + C27 + C29) / (C22 + 2[C24 + C26 + C28] + C30)} (Peters and Moldowan, 1993)
OEP – Improved odd : even predominance : (C21 + 6C23 + C25)/ (4C22 + 4C24) (Scalan and Smith,
TAR – Terrigenous/Aquatic ratio : (C27 + C29 + C31) / (C15 + C17 + C19) (Peters et al., 2005)
Waxiness index – ∑ (n-C21-n-C31)/∑ (n-C15-n-C20) (Peters et al., 2005)
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Table 4: Hopane biomarker parameters calculated from m/z 191 mass chromatograms of the analysed
samples (see the Appendix for peak assignment).
Maturity-
Dependent
Parameters
Source- input and paleodepositional conditions Parameters
Sa
mp
le
ID
De
pt
h
(m
)
For
mati
on
Ts/
(Ts
+T
m)
C30M
/C30
H
C32
22S/
(22S
+22
R)
T
m/
Ts
C29H
/C30
H
G
a/
C3
0
H
ol
/
C3
0
H
C21T
/C23
T
C22T
/C21
T
C24T
/C23
T
C26T
/C25
T
C24Te
t/C26
T
C31S
/C30
H
C31R
/C30
H
A
K-
2-
41
80
12
75 Ma
mu
0.1
3 0.42 0.51 7.1 0.89
0.
0
5
0.
0
7
0.22 0.75 0.44 0.50 1.33 0.43 0.34
A
K-
2-
42
90
13
09 Ma
mu
0.1
0 0.51 0.56
9.0
0 1.04
0.
1
8
0.
0
5
0.18 1.33 0.35 0.60 9.00 0.50 0.48
A
K-
2-
44
10
13
45 Ma
mu
0.1
8 0.46 0.55
4.5
0 0.86
0.
0
6
0.
2
9
0.15 1.50 0.38 1.25 5.67 0.43 0.31
A
K-
2-
44
90
13
70 Nkp
oro
0.1
4 0.36 0.58
6.4
2 0.97
0.
0
4
0.
0
6
0.43 0.33 0.71 0.67 5.00 0.39 0.26
A
K-
2-
45
70
13
94 Nkp
oro
0.1
2 0.38 0.51
7.4
0 0.87
0.
0
4
0.
0
7
0.12 1.00 0.35 0.75 3.00 0.39 0.30
A
K-
2-
46
60
14
22 Nkp
oro
0.5
0 0.14 0.60
1.0
0 0.81
0.
0
5
0.
0
7
0.33 0.50 0.44 0.66 0.25 0.25 0.15
A
K-
2-
48
30
14
73 Nkp
oro
0.1
7 0.40 0.57
5.0
6 1.00
0.
0
6
0.
0
7
0.21 0.67 0.47 1.00 3.00 0.36 0.28
A
K-
2-
50
00
15
25 Nkp
oro
0.1
8 0.37 0.59
4.4
3 0.96
0.
0
5
0.
0
7
0.14 1.00 0.38 0.50 2.67 0.37 0.27
A
K-
15
35
Nkp
oro
0.1
8 0.38 0.59
4.5
3 0.96
0.
0
0.
70.25 0.75 0.50 0.60 2.67 0.40 0.26
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2-
50
30
6 4
A
K-
2-
50
50
15
41 Nkp
oro
0.3
3 0.36 0.55
2.0
4 0.76
0.
1
2
0.
0
4
0.31 0.50 0.38 0.67 3.00 0.35 0.26
A
K-
2-
52
10
15
89 Nkp
oro
0.1
2 0.30 0.57
7.5
6 0.82
0.
0
4
0.
0
6
0.18 0.75 0.50 0.67 1.25 0.33 0.27
A
K-
2-
53
50
16
32 Nkp
oro
0.2
8 0.32 0.62
2.5
6 0.96
0.
0
7
0.
0
4
0.19 0.80 0.44 0.57 1.75 1.15 0.86
A
K-
2-
54
50
16
63 Nkp
oro
0.3
0 0.26 0.57
2.3
4 0.91
0.
0
5
0.
0
5
0.18 0.67 0.18 0.60 1.00 0.34 0.22
A
K-
2-
55
80
17
02 Nkp
oro
0.4
3 0.20 0.59
1.3
1 0.85
0.
0
6
0.
0
6
0.33 0.57 0.48 0.57 0.50 0.29 0.21
A
K-
2-
56
60
17
26 Nkp
oro
0.5
0 0.15 0.59
1.0
0 0.79
0.
0
4
0.
0
3
0.33 0.50 0.82 0.83 0.20 0.24 0.15
Notes
Ts/(Ts + Tm) = 18α(H)-22,29,30-Trisnorneohopane (Ts)/ [18α(H)-22,29,30-Trisnorneohopane (Ts) + 17α(H)-22,29,30-
Trisnorneohopane (Tm)]
C30M/C30H = 17β(H), 21α(H)-moretane/C3017α(H), 21β(H)-hopane
C32 22S/(22S + 22R) = C3217α(H), 21β(H)22S/[C3217α(H), 21β(H)22 (S + R)]
Tm/Ts = 17α(H)-22,29,30- Trisnorneohopane (Tm)/ 18α(H)-22,29,30-Trisnorneohopane (Ts)
C29H/C30H = C2917α(H), 21β(H)-hopane/ C3017α(H), 21β(H)-hopane
Ga/C30H =Gammacerane/ C3017α(H), 21β(H)-hopane
ol/C30H = 18α(H) + 18β(H)-oleananes/ C3017α(H), 21β(H)-hopane
C21T/C23T = ratio of C21 tricyclic terpane to C23 tricyclic terpane
C22T/C21T = ratio of C22 tricyclic terpane to C21 tricyclic terpane
C24T/C23T = ratio of C24 tricyclic terpane to C23 tricyclic terpane
C26T/C25T = ratio of C26 tricyclic terpane to C25 tricyclic terpane
C24Tet/C26T = ratio of C24 tetracyclic terpane to C26 tricyclic terpane
C31R/C30H = 17α(H), 21β(H)-homohopane (22R)/ C3017α(H), 21β(H)-hopane
C31S/C30H = 17α(H), 21β(H)-homohopane (22S)/ C3017α(H), 21β(H)-hopane
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Table 5: Sterane biomarker parameters calculated from m/z 217 mass chromatograms of the
analysed samples (see the Appendix for peak assignment).
Source input and paleodepositional conditions
parameters
Maturity-dependent
parameters
Sample ID Depth
(m)
Formation
C27-
ster
(%)
C28-
ster
(%)
C29-ster
(%)
C27-ster/
Ster(C27+C29)
Ster-C29
/ster C27
Ster C29
20S/
(20S +
20R)
Ster C29
ββ/
(ββ + αα)
AK-2-
4180
1275 Mamu 30.7 15.1 54.1 0.36 0.51 0.61 0.40
AK-2-
4290
1309 Mamu 32.6 16.3 51.1 0.39 0.18 0.45 0.53
AK-2-
4410
1345 Mamu 66.2 11.0 22.8 0.74 0.41 0.34 0.53
AK-2-
4490
1370 Nkporo 36.2 16.3 47.4 0.43 0.27 0.53 0.50
AK-2-
4570
1394 Nkporo 38.5 14.6 46.9 0.45 0.39 0.55 0.48
AK-2-
4660
1422 Nkporo 40.7 15.5 43.8 0.48 0.35 0.56 0.51
AK-2-
4830
1473 Nkporo 37.7 16.2 46.2 0.45 0.40 0.59 0.39
AK-2-
5000
1525 Nkporo 38.8 15.5 45.7 0.46 0.33 0.57 0.52
AK-2-
5030
1535 Nkporo 39.1 16.1 44.8 0.47 0.35 0.59 0.35
AK-2-
5050
1541 Nkporo 41.9 17.3 40.8 0.51 0.45 0.55 0.35
AK-2-
5210
1589 Nkporo 40.7 15.6 43.6 0.48 0.24 0.63 0.40
AK-2-
5350
1632 Nkporo 34.9 14.8 50.2 0.41 0.28 0.57 0.32
AK-2-
5450
1663 Nkporo 41.4 15.4 43.2 0.49 0.28 0.57 0.35
AK-2-
5580
1702 Nkporo 41.9 22.5 35.6 0.54 0.47 0.54 0.22
AK-2-
5660
1726 Nkporo 41.7 18.9 39.4 0.51 0.34 0.58 0.21
Notes
C27-ster (%) = percentage of C27 ααα-sterane 20R to sum of C27, C28, C29 ααα 20R steranes C28-ster (%) = percentage of C28 ααα-sterane 20R to sum of C27, C28, C29 ααα 20R steranes C29-ster (%) = percentage of C29 ααα-sterane 20R to sum of C27, C28, C29 ααα 20R steranes
Ster-C27/Ster-(C27+C29) = C27 ααα-sterane 20R/C27 ααα-sterane 20R/C29 ααα-sterane 20R
Ster-C29/Ster-C27 = C29 ααα-sterane 20R/C27 ααα-sterane 20R
Ster C29 20S/(20S+20R) = ratio of C29 ααα-sterane 20S/C29 ααα-sterane 20S + 20R
Ster C29 ββ/(ββ+αα) = ratio of C29 αββ-sterane 20S+20R/ C29 ααα + αββ-sterane 20S + 20R
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Highlights
Akukwa-2 sedimentary organic matter derived from aquatic algae and land plants.
The sediments were deposited under suboxic paleoenvironmental conditions.
The sediments generally have fair to very good hydrocarbon generative potential.
The sediments were deposited in shallow marine tectonic setting with fluvial incursion.
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