Accepted Manuscript

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

This is a PDF file of an unedited manuscript that has been accepted for publication. Asa service to our customers we are providing this early version of the manuscript. Themanuscript will undergo copyediting, typesetting, and review of the resulting proof beforeit is published in its final form. Please note that during the production process errors maybe discovered which could affect the content, and all legal disclaimers that apply to thejournal pertain.

ACCEP

TED M

ANUSC

RIPT

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: kayodeadebanji@gmail.com; adebayoolajide2003@yahoo.co.uk

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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.

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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)-

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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).

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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).

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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-

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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-

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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)

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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,

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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.

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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.

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

References

Abdullah, W.H., 1999. Organic facies variations in the Triassic shallow marine and deep marine shale

of central Spitsbergen, Svalbard. Mar. Petrol. Geol. 16, 467–481.

Adegoke, A.K., Abdullah, W.H., Hakimi, M.H., Sarki Yandoka, B.M., 2014. Geochemical

characterisation of Fika Formation in the Chad (Bornu) Basin, northeastern Nigeria:

Implications for depositional environment and tectonic setting. Appl. Geochem. 43, 1–12.

Adegoke, A.K., Abdullah, W.H., Hakimi, M.H., 2015. Geochemical and petrographic

characterisation of organic matter from the Upper Cretaceous Fika shale succession in the

Chad (Bornu) Basin, northeastern Nigeria: Origin and hydrocarbon generation potential. Mar.

Petrol. Geol. 61, 95–110.

Adekola, S.A., Akinlua, A., Mangelsdorf, K., 2012. Organic geochemical evaluation of Cretaceous

shale samples from the Orange Basin, South Africa. Appl. Geochem. 27, 1633–1642.

Agagu, O.K., Ekweozor, C.M., 1982. Source rock characteristics of Santonian shales in the Anambra

syncline, southern Nigeria. J. Min. Geol. 19, 52–61.

Akaegbobi, I.M., Schmitt, M., 1998. Organic facies, hydrocarbon source potential and the

reconstruction of depositional paleoenvironment of the Campano-Maastrichtian Nkporo Shale

in the Cretaceous Anambra Basin, Nigeria. NAPE Bull. 13, 1–19.

Akinlua, A., Ajayi, T.R., Adeleke, B.B., 2007. Organic and inorganic geochemistry of northwestern

Niger Delta oils. Geochem. J. 41, 271–281.

Alalade, B., Tyson, R.V., 2010. Hydrocarbon potential of the Late Cretaceous Gongila and Fika

Formations, Bornu (Chad) Basin, NE Nigeria. J. Petrol. Geol. 33, 339–354.

Alias, F.L., Abdullah, W.H., Hakimi, M.H., Azhar, M.H., Kugler, R.L., 2012. Organic geochemical

characteristics and depositional environment of the Tertiary Tanjong Formation coals in the

Pinangah area, onshore Sabah, Malaysia. Int. J. Coal Geol. 104, 9–12.

Aquino Neto, F.R., Trendel, J.M., Restle, A., Connan, J., Albrecht, P., 1983. Occurrence and formation

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

of tricyclic and tetracyclic terpanes in sediments and petroleum. In: Bjorøy, M. (Ed.), Advances

in Org. Geochem. 1981. Wiley, New Jersey, pp. 659–667.

Awad, M.Z., 1994. Stratigraphic, palynological and paleoecological studies in the East-Central Sudan

(Khartoum and Kosti Basins), Late Jurassic to Mid Tertiary: Berliner Geowissenschaftlich

Abhadlungen A, 161, 1–163.

Benkhelil, J., 1989. The origin and evolution of the Cretaceous Benue Trough, Nigeria. J. Afr. Earth

Sci. 8, 251–282.

Bordenave, M.L., Espitalié, L., Leplat, P., Oudin, J.L., Vandenbroucke, M., 1993. Screening

techniques for source rock evaluation. In: Bordenave, M.L. (Ed.), Applied Petroleum

Geochemistry Paris: Editions Technip, 217–278.

Chandra, K., Mishra, C.S., Samanta, U., Anita Gupta, Mehrotra, K.L., 1994. Correlation of

different maturity parameters in the Ahmedabad–Mehsana block of the Cambay basin. Org.

Geochem. 21, 313–321.

Cranwell, P.A., Eglinton, G., Robinson, N.,1987. Lipids of aquatic organisms as potential contributors

to lacustrine sediments-II. Org. Geochem. 11, 513–527.

Dapple, E.C., 1974. Sandstone types and their Associated depositional environments. J. Sed. Petrol. 13,

412–431.

DembickiJr.H., 2009. Three common source rock evaluation errors made by geologists during

prospect or play appraisals. AAPG Bull. 93, 341–356.

Dembicki, Jr.H., Horsfield, B., Ho, T.T.Y., 1983. Source rock evaluation by pyrolysis-gas

chromatography. AAPG Bull. 67, 1094–1103.

Didyk, B.M., Simoneit, B.R.T., Brassell, S.C., Eglinton, G., 1978. Organic geochemical indicators of

palaeoenvironmental conditions of sedimentation. Nature 272, 216–222.

Eglinton G, and Hamilton R.G., 1967. Leaf epicuticular waxes. Science 156, 1322–1344.

Eglinton, T.I., Sinninghe Damsté, J.S., Kohnen, M.E.L., de Leeuw, J.W., 1990. Rapid estimation

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

of the organic sulphur content of kerogens, coals and asphaltenes by pyrolysis gas

chromatography. Fuel 69, 1394–1404.

Ehinola, O.A., Sonibare, O.O., Falode, O.A., Awofala, B.O., 2005. Hydrocarbon potential and

thermal maturity of Nkporo Shale from Lower Benue Trough, Nigeria. J. Appl. Sci. 5, 689–

695.

Ekine, A.S., Onuoha, K.M., 2008. Burial history analysis and subsidence in the Anambra Basin,

Nigeria. J. Phys. 20, 145–154.

Ekweozor, C.M., Gormly, J.R., 1983. Petroleum geochemistry of Late Cretaceous and Early Tertiary

shales penetrated by Akukwa-2 well in the Anambra Basin, southern Nigeria: J. Petrol. Geol. 6,

207–216.

El Diasty, W.Sh., Moldowan, J.M., 2012. Application of biological markers in the recognition of the

geochemical characteristics of some crude oils from Abu Gharadig Basin, north Western Desert

– Egypt. Mar. Petrol. Geol. 35, 28–40.

Giraud, A., 1970. Application of pyrolysis and gas chromatography to geochemical characterization of

kerogen in sedimentary rocks. AAPG Bull. 54, 439–451.

Hakimi, M.H., Abdullah, W.H., Shalaby, M.R., 2011. Organic geochemical characteristics and

depositional environments of the Jurassic shales in the Masila Basin of Eastern Yemen.

GeoArabia 16, 47–64.

Hakimi, M.H., Abdullah, W.H., 2013. Organic geochemical characteristics and oil generating potential

of the Upper Jurassic Safer shale sediments in the Marib-Shabowah Basin, western Yemen. Org.

Geochem. 54, 115–124.

Hakimi, M.H., Abdullah, W.H, Shalaby, M.R., Alramisy, G.A., 2014. Geochemistry and organic

petrology study of Kimmeridgian organic rich shales in the Marib-Shabowah Basin,

Yemen: Origin and implication for depositional environments and oil-generation potential.

Mar. Petrol. Geol. 50, 185–201.

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

Hartwig, A., di Primio, R., Anka, Z., Horsfield, B., 2012. Source rock characteristics and

compositional kinetic models of Cretaceous organic rich black shales offshore southwestern

Africa. Org. Geochem. 51, 17–34.

Horsfield, B., 1989. Practical criteria for classifying kerogens: Some observations from pyrolysis-

gas chromatography. Geochim. Cosmochim.Acta 53, 891–901.

Hossain, Z.M., Sampei, Y., Roser, B.P., 2009. Characterization of organic matter and depositional

environment of Tertiary mudstones from the Sylhet Basin, Bangladesh. Org. Geochem. 40, 743–

754.

Huang, W.Y., Meinschein, W.G., 1979. Sterols as ecological indicators. Geochim. CosmochimActa.

43, 739–745.

Kogbe, C.A., 1989. The Cretaceous and Paleogene sediments of southern Nigeria. Geology of Nigeria,

2nd

Edition, Rock View, Nigeria. pp. 325–334.

Large, D.J., Gize, A.P.,1996. Pristane/phytane ratios in the mineralized Kupferschiefer of the Fore-

Sudetic Monocline, southwest Poland. Ore Geol. Rev.11, 89–103.

Larter, S.R., Douglas, A.G., 1980. A pyrolysis-gas chromatographic method for kerogen typing. Phys.

Chem. Earth 12, 579–583.

Larter, S.R., 1984. Application of analytical pyrolysis techniques to kerogen characterization and

fossil fuel exploration/exploitation. In: Voorhees, K. (Ed.), Analytical pyrolysis: Methods

and Application. London: Butterworths, 212–275.

Mackenzie, A.S., Patience, R.L., Maxwell, J.R., Vandenbroucke, M., Durand, B., 1980. Molecular

parameters of maturation in the Toarcian shales, Paris Basin, France-I. Changes in the

configurations of acyclic isoprenoid alkanes, steranes, and triterpanes. Geochimi. Cosmochim.

Acta 44, 1709–1721.

Marynowsky, L., Narkiewicz, M., Grelowski, C., 2000. Biomarkers as environmental indicators in a

carbonate complex, example from the Middle to Upper Devonian, Holy Cross Mountains,

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

Poland. Sediment. Geol. 137, 187–212.

May, F.E., 1977. Functional morphology, paleoecology and systematic of Dinogymnium tests.

Palynol.1, 103–121.

Meyers, P.A., Snowdon, L.R., 1993. Types and maturity of organic matter accumulated during Early

Cretaceous subsidence of the Ex-mouth Plateau, Northwest Australia margin. AAPG Studies in

Geology 37, 119–130.

Murray, A.P., Sosrowidjojo, I.B., Alexander, R., Kagi, R.I., Norgate, C.M., Summons, R.E., 1997.

Oleananes in oils and sediments: evidence of marine influence during early diagenesis?

Geochim. Cosmochim. Acta. 61, 1261–1276.

Mustapha, K.A., Abdullah, W.H., 2013. Petroleum source rock evaluation of the Sebahat and

Ganduman Formations, Dent Peninsula, Eastern Sabah, Malaysia. J. Asian Earth Sci. 76, 346–

355.

Nwajide, C.S., 1990. Cretaceous sedimentation and paleogeography of the Central Benue Trough.

In: Ofoegbu, C.O., (Ed.), The Benue Trough: Structure and Evolution. International

Monograph Series, Braunschweig, pp. 19–38.

Nwajide, C.S., Reijers, T.J.A., 1996. Sequence Architecture in outcrops: Example from Anambra

Basin, Nigeria. NAPE Bull. 11, 23–33.

Obaje, N.G., Wehner, H., Scheeder, G., Abubakar, M.B., Jauro, A., 2004. Hydrocarbon prospectivity

of Nigeria’s inland basins: from the viewpoint of organic geochemistry and organic petrology.

AAPG Bull. 88, 325–353.

Ojo, O.J, Akande, S.O., 2004. Palynological and palaeoenvironmental studies of Gombe Formation,

Gongola Basin, Nigeria, J. Min. Geol. 40, 143–149.

Olabode, S.O., 2014. Soft sediment deformation structures in the Maastrichtian Ajali Formation

Western Flank of Anambra Basin, Southern Nigeria. J. Afr. Earth Sci. 89, 16–30.

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

Oloto, I.N., 1989. Maastrichtian dinoflagellates cyst assemblage from the Nkporo Shale on the Benin

flank of the Niger Delta. Rev. Paleobot. Palynol. 57, 173–186.

Ourisson, G., Albrecht, P., Rohmer, M., 1979. Hopanoids: palaeochemistry and biochemistry of a

group of natural products. Pure App. Chem. 51, 709–729.

Peters, K.E., 1986. Guidelines for evaluating petroleum source rocks using programmed pyrolysis.

AAPG Bull. 70, 318–329.

Peters, K.E., Moldowan, J.M., 1993. The Biomarker Guide: Interpreting Molecular Fossils in

Petroleum and Ancient Sediments. Prentice-Hall, Inc. Englewood Cliffs, New Jersey, 363pp.

Peters, K.E., Cassa, M.R., 1994. Applied source rock geochemistry. In: Magoon, L.B., Dow, G.W.

(Eds.), The petroleum system – from source to trap. AAPG Mem. 60, 93–120.

Peters, K.E., Walters, C.C., Moldowan, J.M., 2005. The Biomarker Guide: Biomarkers and Isotopes in

Petroleum Exploration and Earth History, 2nd

Edition, volume 2. Cambridge University Press,

Cambridge, United Kingdom, 1155pp.

Petters, S.W.,1978. Mid-Cretaceous paleoenvironment and biostratigraphy of the Benue Trough,

Nigeria. GSA Bull.89, 151–154.

Philp, R.P., 1985. Fossil fuel biomarkers: applications and spectra. Elsevier Science Publishers B.V,

Amsterdam, 294pp.

Philp, R.P., Gilbert, T.D., 1986. Biomarker distributions in Australian oils predominantly derived from

terrigenous source material. Org. Geochem.10, 73–84.

Powell, T.G., McKirdy, D.M., 1973. Relationship between ratio of pristane to phytane, crude oil

composition and geological environment in Australia. Nature 243, 37–39.

Qiuhua, Y., Zhigang, W., Youjun, T.,2011. Geochemical characteristics of Ordovician crude oils in the

northwest of the Tahe oil field, Tarim Basin. Chinese J. Geochem. 30, 93–98.

Reyment, R.A., 1965. Aspects of the Geology of Nigeria. University Press, Ibadan, Nigeria. 145p.

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

Reyment, R.A., Dingle, R.V., 1987. Paleogeography of Africa during Cretaceous period. Palaeogeogr.

Palaeoclimatol. Palaeoecol.59, 93–116.

Rull, V., 1997. Oligo-Miocene palynology of the Rio Chama sequence (Western Venezuela), with

comments on fossil algae as paleoenvironmental indicators. Palynol.21, 213–229.

Sachse, V.F., Littke, R., Heim, S., Kluth, O., Schober, J., Boutib, L., Jabour, H., Perssen, F., Sindern,

S., 2011. Petroleum source rocks of the Tarfaya Basin and adjacent areas, Morocco. Org.

Geochem. 42, 209–227.

Scalan, R.S., Smith, J.E., 1970. An improved measure of the odd-even predominance in the normal

alkanes of sediment extracts and petroleum. Geochim. Cosmochim. Acta. 34, 611–620.

Seifert, W.K., Moldowan, J.M., 1978. Applications of steranes, terpanes and monoaromatics to the

maturation, migration and source of crude oils. Geochim. Cosmochim. Acta. 42, 77–95.

Seifert, W.K., Moldowan, J.M., 1980. The effect of thermal stress on source-rock quality as measured

by hopane stereochemistry. Phys. Chem. Earth. 12, 229–237.

Seifert, W.K., Moldowan, J.M., 1986. Use of biological markers in petroleum exploration. In: (Johns,

R.B. (Ed.), Methods in Geochemistry and Geophysics 24, 261–290.

Sinninghe Damsté, J.S., Kenig F., Koopmans, M.P., Koster, J., Schouten, S., Hayes, J.M., de Leeuw, J.

W., 1995. Evidence for gammacerane as an indicator of water column stratification. Geochim.

Cosmochim. Acta. 59, 1895–1900.

Schrank, E., 1989. Non-marine Cretaceous correlations in Egypt and northern Sudan. Palynolgical and

Paleobotanical Evidence, p. 87–89.

Taylor, G.H., Teichmüller, M., Davis, A., Diessel, C.F.K., Littke, R., Robert, P., 1998. Organic

petrology. Gebr, Borntraeger, Berlin, 704 pp.

Ten Haven, H.L., de Leeuw, J.W., Rullkötter, J., Sinninghe Damsté, J.S., Scheck, P.A., Palmer, S.E.,

Zumberge, J.E., Fleet, A.J., Kelts, K., Talbot, M.R., 1988. Application of biological markers in

the recognition of palaeohypersaline environments. In: AJ Fleet, K Kelts, MR Talbot (eds)

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

Lacustrine Petroleum Source Rocks. Geological Society Special Publication 40, Blackwell,

London, pp 123–130.

Traverse, A., 1988. Paleopalynology. Unwin Hyman, London. Sydney. Wellington. 600pp.

Tyson, R.V., 1995. Sedimentary organic matter–organic facies and palynofacies. London, Chapman &

Hall, 615pp.

Unomah, G.I., Ekweozor, C.M., 1993. Petroleum source rock assessment of the Campanian Nkporo

Shale, Lower Benue Trough, Nigeria. NAPE Bull. 8, 172–186.

Vadjda-Santivanez, V., 1998. Cretaceous palynofloras from Southern Scandinavia. Lund Publications

in Geology 1, 35, 1–24.

Van Aarssen, B.G.K., Hessels, J.K.C., Abbink, O.A., de Leeuw, J.W., 1992. The occurrence of polyc-

yclic sesqui-, tri-, and oligo-terpenoids derived from a resinous polymeric cardinene in crude

oils from southeast Asia. Geochim. Cosmochim. Acta. 56, 1231–1246.

Van Koeverden, J.H., Karlsen, D.A., Backer-Owe, K., 2011. Carboniferous non-marine source rocks

from Spitsbergen and Bjørnøya: comparison with the Western Arctic. J. Petrol. Geol. 34, 53–66.

Volk, H., George, S.C., Middleton, H., Schofield, S., 2005. Geochemical comparison of fluid inclusion

and present-day oil accumulations in the Papuan Foreland – evidence for previously

unrecognised petroleum source rocks. Org. Geochem. 36, 29–51.

Volkman, J.K., 1986. A review of sterol biomarkers for marine and terrigenous organic matter. Org.

Geochem. 9, 83–89.

Waples, D.W., Machihara, T., 1991. Biomarkers for Geologists: A Practical Guide to the Application

of Steranes and Triterpanes in Petroleum Geology. AAPG Methods in Exploration 9, Tulsa,

Oklahoma.

Whiteman, A.J., 1982. Nigeria: Its Petroleum Geology, Resources and Potential, 1 and 2. London:

Graham and Trotman.

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

(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

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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).

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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).

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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).

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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).

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

Fig. 2: Simplified composite lithology of Akukwa-2 well, Anambra Basin, showing sampled points.

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

Fig. 3: Stratigraphic distribution chart of palynomorphs in the Akukwa-2 well.

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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.

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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.

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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).

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

Fig. 7: Mass fragmentograms m/z 85, m/z 191 and m/z 217 of saturated hydrocarbons of two studied

Akukwa-2 sediment extracts.

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

Fig. 8: Cross-plot of S2 pyrolysis yield versus TOC showing kerogen quality in the analysed samples.

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

Fig. 9: Plot of hydrogen index (HI) versus pyrolysis Tmax for the analysed samples, showing kerogen

quality and thermal maturity stage.

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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).

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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).

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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).

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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).

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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).

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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).

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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)

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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

ACCEPTED MANUSCRIPT

ACCEP

TED M

ANUSC

RIPT

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.

ACCEPTED MANUSCRIPT