![m/z 184+198, 212, 226, 234, and 260 mass chromatograms showing the distribution of a dibenzothiophene and methyldibenzothiophenes, b dimethyldibenzothiophenes, c trimethyldibenzothiophenes, d benzo[b]naphthothiophene, and e phenyldibenzothiophenes in Niger Delta source rocks](https://www.researchgate.net/profile/Abiodun-Ogbesejana-2/publication/350333883/figure/fig1/AS:1010220817989632@1617866820309/m-z-184-198-212-226-234-and-260-mass-chromatograms-showing-the-distribution-of-a_Q320.jpg)
![m/z 184+198+212+226, 234, and 260 mass chromatograms showing the distribution of a dibenzothiophene, methyldibenzothiophenes, dimethyldibenzothiophenes, and trimethyldibenzothiophenes; b benzo[b]naphthothiophenes; and c phenyldibenzothiophenes in Niger Delta crude oils](https://www.researchgate.net/profile/Abiodun-Ogbesejana-2/publication/350333883/figure/fig2/AS:1010220822167552@1617866821005/m-z-184-198-212-226-234-and-260-mass-chromatograms-showing-the-distribution-of-a_Q320.jpg)
![The absolute concentrations of dibenzothiophenes, benzo[b]naphthothiophenes, and phenyldibenzothiophenes in Niger Delta crude oils](https://www.researchgate.net/profile/Abiodun-Ogbesejana-2/publication/350333883/figure/tbl1/AS:1010220826365953@1617866822177/The-absolute-concentrations-of-dibenzothiophenes-benzobnaphthothiophenes-and_Q320.jpg)
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ORIGINAL PAPER
Dibenzothiophenes, phenyldibenzothiophenes, and benzo[b]
naphthothiophenes in crude oils and source rock extracts
from the Niger Delta Basin, Nigeria, and their applications as source
facies, depositional environments, and thermal maturity indicators
Abiodun B. Ogbesejana
1
&Oluwasesan M. Bello
1
&Oluwadurotimi O. Akintade
2
&Oluwole Joshua Okunola
1
&
Paul M. Osamudiamen
3
&Kamaluddeen S. Kabo
1
&Tijjani Ali
1
&Uduma A. Uduma
1
Received: 13 August 2020 / Accepted: 2 March 2021
#Saudi Society for Geosciences 2021
Abstract
The distribution of dibenzothiophenes, phenyldibenzothiophenes, and benzo[b]naphthothiophenes and their geochemical sig-
nificance in the crude oils and source rocks from Niger Delta Basin have been investigated by gas chromatography-mass
spectrometry. The dimethyldibenzothiophenes, 4-phenyldibenzothiophene, and benzo[b]naphtho[2,1-d]thiophene were the most
abundant among the dibenzothiophene and its derivatives in the crude oils and rock samples. The phenyldibenzothiophene ratio-
1 and phenyldibenzothiophene ratio-2 values in the rock samples were in the rangesof 0.08 to 0.67 and 0.20 to 2.53, respectively,
while the 4-/1-methyldibenzothiophene, 4,6-/(1,4 + 1,6)-dimethyldibenzothiophene, 2,4,6-/(2,4,7 + 2,4,8)-
trimethyldibenzothiophene, phenyldibenzothiophene ratio-1, and phenyldibenzothiophene ratio-2 values in the crude oils ranged
from 1.52 to 5.82, 0.76 to 1.98, 0.77 to 1.22, 0.11 to 0.42, and 0.30 to 0.75, respectively. These values indicate source rocks and
crude oils with mixed input of terrestrial and marine organic matter and deposited in lacustrine-fluvial/deltaic environments
within immature to early mature stages. Four isomers of MDBT were also present in appreciable amounts in all the samples
studied. The distribution patterns of MDBTs were generally observed in the order 4-MDBT > 2+3-MDBT > 1-MDBT in the
crude oils and rock samples. The (1 + 4)-/(2+3)-methyldibenzothiophene ratio in the oils and rock samples ranged from 0.03 to
0.13 and 1.31 to 2.54, respectively. These values suggested source rocks with shale lithologies. A cross plot of (1+4)-/(2+3)-
methyldibenzothiophene versus pristane/phytane measured on rock samples and crude oils from Niger Delta was used to study
the influence of depositional environment and lithology on the distribution of the MDBT isomers. This cross plot clearly showed
that the source rocks and crude oils studied have shale lithologies and were distinguished into lacustrine and fluvial/deltaic/
freshwater environments and thus proposed in this study as a potential lithology and paleoenvironment indicator. This study
showed that dibenzothiophenes, phenyldibenzothiophenes, and benzo[b]naphthothiophenes were effective in determining the
origin, depositional environments, and thermal maturity status of crude oils and source rocks in the Niger Delta Basin, Nigeria.
Keywords Dibenzothiophenes .Phenyldibenzothiophenes .Benzo[b]naphthothiophenes .Crude oils .Source rocks .Gas
chromatography-mass spectrometry
Introduction
Polycyclic aromatic sulfur heterocycles (PASHs) are common
compounds in crude oils and sediment extracts. Sulfur-
containing compounds such as benzothiophenes (BTs),
dibenzothiophenes (DBTs), benzonaphthothiophenes
(BNTs), phenyldibenzothiophenes (PhDBTs), and their
alkylated homologues are commonly formed in crude oils as
thiophene moiety where sulfur is incorporated into polycyclic
aromatic hydrocarbons (PAHs) (Li et al. 2012). Their distri-
bution patterns and relative and absolute concentrations were
Responsible Editor: Santanu Banerjee
*Abiodun B. Ogbesejana
abiodunogbesejana@gmail.com
1
Department of Applied Chemistry, Federal University Dutsin-Ma, P.
M. B. 5001, Dutsin-Ma, Katsina State, Nigeria
2
Department of Chemical Sciences, Koladaisi University,
Ibadan, Oyo State, Nigeria
3
Department of Chemical and Food Sciences, College of Natural and
Applied Sciences, Bells University of Technology, Sango-Ota, Ogun
State, Nigeria
Arabian Journal of Geosciences (2021) 14:592
https://doi.org/10.1007/s12517-021-06872-3
Authors personal copy
commonly applied to suggest thermal maturity (Radke et al.
1986,1991;Radke1988; Budzinski et al. 1991; Radke and
Willsch 1994; Bao et al. 1996; Chakhmakhchev et al. 1997;
Santamaría-Orozco et al. 1998; Kruge 2000;Lietal.2013a,
2013b;Ogbesejanaetal.2019a), depositional conditions of
organic matter (Hughes 1984; Hughes et al. 1995; Jinggui
et al. 2005;Asifetal.2011;Lietal.2013c;Ogbesejana
et al. 2019a), kerogen forms in source rocks (Hughes 1984;
Connan et al. 1986; Budzinski et al. 1991; Radke et al. 1991;
Hughes et al. 1995;Wuetal.1995; Chakhmakhchev et al.
1997; Huang and Pearson 1999;Li2000; Li et al. 2013c;
Ogbesejana et al. 2019a), oil-source rock correlation (Fan
et al. 1990,1991; Jinggui et al. 2005), and pathways for oil
migration (Mössner et al. 1999; Hwang et al. 2002; Wang
et al. 2004;Yangetal.2005;Lietal.2008;Fangetal.
2016,2017;Yangetal.2016;Chenetal.2017).
DBT and two isomers of methyldibenzothiophene
(MDBT) were detected in crude oils in the 1980s (Hughes
1984;Connanetal.1986;Lietal.2014). In the 1990s, the
presence of dimethyldibenzothiophenes (DMDBTs) and
trimethyldibenzothiophenes (TMDBTs) in crude oils was
confirmed (Budzinski et al. 1991; Bao et al. 1996;Peters
et al. 2005;Lietal.2014). Some maturity parameters based
on the isomerization of sterane and hopane epimers are limited
to immature levels throughout the early stage of the oil win-
dow (ten Haven et al. 1986;Lietal.2013b). In addition, the
drastic decrease in hopanes and sterane concentrations at
higher thermal maturity stages can affect their applications
as thermal maturity indicators in the sediment extracts and
related oils (Radke et al. 1982; ten Haven et al. 1986;Li
et al. 2013b). Maturity indicators based on PAHs and
PASHs change consistently with increasing maturity for
source rocks and oils with moderate to high maturity and
depend on different thermodynamic stabilities in methyl
group positions (Radke 1988; Chakhmakhchev et al. 1997;
Santamaría-Orozco et al. 1998;Lietal.2013a,2013b;
Ogbesejana et al. 2019a). Thus, they are potential maturity
indicators and particularly useful for high to over-mature oils
and condensates (Li et al. 2013a). DBT-based maturity ratios,
such as 4-/1-methyldibenzothiophene (MDR), 2,4-/1,4-
dimethyldibenzothiophene (2,4-/1,4-DMDBT), 4,6-/1,4-
dimethyldibenzothiophene (4,6-/1,4-DMDBT), 4,6-/(1,4+
1,6)-dimethyldibenzothiophene (4,6-/(1,4+1,6)-DMDBT),
and 2,4,6-/(2,4,7+2,4,8)-trimethyldibenzothiophene (2,4,6-/
2,4,7+2,4,8)-TMDBT) ratios, were introduced and extensive-
ly used in organic geochemistry (Radke et al. 1982;Radke
1988; Chakhmakhchev et al. 1997;Lietal.2013a;
Santamaría-Orozco et al. 1998; Ogbesejana et al. 2019a).
Radke (1988) suggested that the MDR would depend on the
same chemical basis as the methylphenanthrene ratio (MPR),
i.e., a change in predominance from thermodynamically un-
stable to more stable isomers with increasing maturity (Li
et al. 2013b). The MDR was calibrated against Rm% (mean
vitrinite reflectance) with a high correlation in the 0.56–
1.32%(Rm) range (Radke 1988;Lietal.2013b). Therefore,
the MDR can also be defined as a maturity parameter analo-
gous to vitrinite reflectance, and the effect of organic facies
appears to be negligible (Li et al. 2013b). However, for type II
and III kerogens, MDR differences in the early maturation
stage (0.4–0.7 %Rm) indicate significantly differing matura-
tion trends (Radke et al. 1986;Lietal.2013b). The thermo-
dynamic stabilities of methyldibenzothiophene (MDBT) iso-
mers and the mechanisms of 1,2-methyl shift, methylation,
and demethylation were systematically studied by molecular
modeling and geochemical data from a suite of lacustrine
mudstone samples from the Liaohe Basin (East China). This
research revealed the possible reaction mechanisms for the
formation and occurrence of MDBT isomers in source rocks
(Yang et al. 2018). Furthermore, three isomers of
benzo[b]naphthothiophenes (BNTs) have been identified in
crude oils and applied to oil migration studies with parameters
such as benzo[b]naphtho[2,1-d]thiophene/
{benzo[b]naphtho[2,1-d]thiophene+benzo[b]naphtho[1,2-
d]thiophene} proposed as an oil migration indicator (Li et al.
2014;Fangetal.2016,2017).
It has been reported that by incorporating sulfur into biphe-
nyl, biphenyl and alkylated biphenyl can react to yield DBT
and alkylated DBTs (Kruge 2000;Lietal.2013b; Ogbesejana
et al. 2019a). Xia and Zhang (2002) synthesized a DBT series
from biphenyls in the presence of sulfur through simulation
experiments. Other simulation experiments (Asif et al. 2009;
Li et al. 2012) demonstrated that dibenzothiophene may be
formed by biphenyl and sulfur. Methyl-substituted biphenyls
also reacted to yield the corresponding methyl DBTs (Li et al.
2012). It has been proposed that the widespread distribution of
dibenzothiophene and alkylated DBTs in sediments and crude
oils is the result of a catalytic reaction on carbonaceous mate-
rial between biphenyl ring systems and surface-adsorbed sul-
fur (Asif et al. 2009;Lietal.2012). The geochemical rela-
tionship between the isomer distribution of methyl substituted
biphenyls and DBTs in crude oils and sediment extracts also
supported this mechanism. Laboratory experiments have
shown that the benzonaphthiophenes (BNTs) may be pro-
duced microbially from benzo[b]thiophenes with
Pseudomonas (Kropp et al. 1994;Lietal.2012). However,
the origin of BNTs in geological samples is still unclear.
Phenyl-substituted DBTs are important aromatic sulfur-
heterocyclic compounds in crude oils, ancient sedimentary
rocks, and coal tar and pitches (Meyer zu Reckendorf 1997,
2000;Marynowskietal.2002; Zhu et al. 2019). Several stud-
ies were performed to investigate the geochemical application
of PhDBTs and its formation. Marynowski et al. (2002)sug-
gested the 2-PhDBT/(2+1)-PhDBT ratio as a maturity param-
eter which is dependent on the relative abundances of
phenyldibenzothiophene isomers in sedimentary rocks of dif-
ferent thermal maturities (Zhu et al. 2019). Rospondek et al.
592 Page 2 of 22 Arab J Geosci (2021) 14:592
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(2007) evaluated whether the PhDBTs/(∑2-PhN[b]T+
2-(2-naphthyl)B[b]T+PhDBTs) ratio could be used as a
maturity indicator because it showed a good correlation
with vitrinite reflectance (%Ro) (Zhu et al. 2019).
Rospondek et al. (2008) assessed the relative thermody-
namic stability of phenyldibenzothiophene isomers and
investigated the composition of PhDBT equilibrium mix-
tures using laboratory maturation experiments, molecular
modeling, and geological evidence (Zhu et al. 2019).
Furthermore, Rospondek et al. (2008) suggested that 1-
PhDBT may be able to isomerize to 2-PhDBT by the
1,2-phenyl shift and cyclize to triphenyleno[1,12-
bcd]thiophene (Zhu et al. 2019). Recently, all four
phenyldibenzothiophene isomers in coals were unambig-
uously identified for the first time by co-injecting authen-
tic standards in GC-MS analyses and by comparing their
retention indices with those previously reported and eval-
uating their thermodynamic properties by means of quan-
tum chemical calculations for the purposes of applying
them for maturity assessment (Zhu et al. 2019).
Despite the enormous geochemical significance of
dibenzothiophenes, phenyldibenzothiophenes, and
benzo[b]naphthothiophenes in petroleum geochemistry, these
biomarkers have received less attention in crude oils and
source rocks from the Niger Delta Basin, Nigeria. However,
Ogbesejana et al. (2019a) reported dibenzothiophenes and
benzo[b]naphthothiophenes in the source rocks from Niger
Delta Basin, Nigeria, but phenyldibenzothiophenes are being
reported in Niger Delta source rocks for the first time in the
present study. Additionally, Sonibare et al. (2008) have reported
methyldibenzothiophenes in the Niger Delta crude oils, but to
date, there are no existing data on dimethyldibenzothiophenes,
trimethyldibenzothiophenes, phenyldibenzothiophenes, and
benzo[b]naphthothiophenes on Niger Delta crude oils to the best
of our knowledge. This paper studies the possibility of using
MDBT isomers as indicators for identifying depositional envi-
ronment and lithology on the basis of the distribution patterns of
these compounds in the crude oils and source rocks from Niger
Delta Basin, Nigeria. This study also explores the applications of
DBTs, BNTs, and PhDBTs as source facies and thermal matu-
rity indicators.
Geological and stratigraphic settings of Niger
Delta Basin
The Niger Delta is a sedimentary basin in the Gulf of Guinea,
the re-entrance of West Africa (Ogbesejana et al. 2020). The
Niger Delta Basin sub-aerial portion comprises about 75,000
km
2
and extends about 200 km from apex to mouth
(Ogbesejana et al. 2020). The Niger Delta covers an area of
nearly 140,000 km
2
with a cumulative sedimentary sequence
of about 12,000 m (Knox and Omatsola 1989; Bankole et al.
2014). These sequences have been categorized into three main
sedimentary groups, namely the Akata, Agbada, and Benin
formations (Bankole et al. 2014). The Eocene to Recent
Akata Formation is the oldest of these three groups (Short
and Stauble 1967; Reijers et al. 1997). The Akata Formation
was formed in a marine environment characterized by contin-
uous, uniform shale deposition. The formation is characteris-
tically made up of dark gray shale, mostly sandy or silty of
prodelta origin (Short and Stauble 1967; Akpoyovbike 1978;
Bankole et al. 2014). The shales of this formation are often
under compacted (Akpoyovbike 1978). On top of the marine
sequence, the Eocene to Recent Agbada formation is found
(Bankole et al. 2014). The Agbada Formation makes up the
Fig. 1 Niger Delta map showing
sample depobelts, formation, and
locations (after Stacher 1995)
Arab J Geosci (2021) 14:592 Page 3 of 22 592
Authors personal copy
real deltaic portion of the sequence (Bankole et al. 2014). This
is known to have accumulated in deltaic front, delta-topset,
and deltaic fluvial environments (Corredor et al. 2005;
Bankole et al. 2014) and is primarily composed of alternating
sandstone/sand bodies of mudstone. The Agbada Formation is
considered to consist, according to Weber (1971), of cyclic
marine and fluvial deposit sequences. Capping the sequence is
the mainly continental Benin Formation which was deposited
during Oligocene to Recent (Reijers et al. 1997; Bankole et al.
2014). The Benin Formation represents an almost continuous
series of continental deposits, ranging from medium to coarse-
grained sand/sandstones (with only occasional short-lived ma-
rine incursions) (Bankole et al. 2014). Short and Stauble
(1967) described the sands/sandstones of Benin Formation
as generally coarse-grained, very granulated, and pebbled,
but also varying from grain to fine grain (Bankole et al. 2014).
Materials and methods
Sampling
A total of 39 crude oil samples were collected at depths rang-
ing from 1610 to 3605 m in Agbada Formation, northern and
offshore Niger Delta Basin, from five wells in five oilfields
represented as ADL, OKN, MJI, MJO, and WZB. Twenty-
one rock samples were also obtained for study from three
wells in three of the fields. The map showing depobelts from
Niger Delta and the locations of the samples is shown in Fig. 1
(Stacher 1995).
Sample preparation, extraction, and fractionation
The samples were crushed with agate mortar and pow-
dered to less than 100 mesh size, prior to extraction.
Around 50 g of each powdered samples was subjected
to Soxhlet extraction for 72 h using azeotropic mixture
of dichloromethane:methanol (93:7, v/v). Activated cop-
per powder was added in order to remove elemental
sulfur from the extracts. Excess solvent was extracted
using rotary evaporator to an aliquot quantity of around
3 mL. The aliquot was then transferred to a weighed
smooth vial with a micropipette, and the remaining sol-
vent was removed at temperatures below 50°C under a
stream of nitrogen gas. The rock extracts and oils were
fractionated by column chromatography using silica gel/
alumina as the stationary phase. The saturated hydrocar-
bon fractions were eluted with 35 ml of hexane, while
Table 1 Geological information and relative abundance of phenyldibenzothiophene isomers in Niger Delta source rocks
Field Depth(m) Reservoir formation Geological age Depobelt Relative abundance (%)
1-PhDBT 4-PhDBT 2-PhDBT 3-PhDBT
MJI 2079–2098 Agbada Eocene to Recent Offshore 12.8 59.0 12.4 15.6
MJI 2299–2308 Agbada Eocene to Recent Offshore 22.3 24.3 13.8 39.3
MJI 2637–2655 Agbada Eocene to Recent Offshore 29.9 26.5 17.6 25.9
MJI 2857–2875 Agbada Eocene to Recent Offshore 26.6 41.8 15.0 16.4
MJI 2994–3012 Agbada Eocene to Recent Offshore 19.1 46.4 29.4 4.9
MJI 3085–3104 Agbada Eocene to Recent Offshore 27.6 42.0 20.1 10.1
MJI 3232–3250 Agbada Eocene to Recent Offshore 23.0 51.5 15.0 10.3
MJI 3323–3332 Agbada Eocene to Recent Offshore 26.3 43.2 15.6 14.9
MJI 3405–3424 Agbada Eocene to Recent Offshore 23.0 63.6 5.2 8.2
MJO 1616–1707 Agbada Eocene to Recent Offshore 16.7 68.6 8.7 6.0
MJO 1771–1872 Agbada Eocene to Recent Offshore 19.3 65.1 10.9 4.8
MJO 2091–2101 Agbada Eocene to Recent Offshore 19.64 67.8 5.6 7.0
MJO 2293–2366 Agbada Eocene to Recent Offshore 24.9 26.1 7.7 41.2
MJO 2570–2588 Agbada Eocene to Recent Offshore 24.9 26.1 7.7 41.2
MJO 2808–2817 Agbada Eocene to Recent Offshore 18.3 32.2 10.7 38.8
OKN 1537–1555 Agbada Eocene to Recent Offshore 15.5 59.0 8.7 16.7
OKN 1729–1747 Agbada Eocene to Recent Offshore 10.1 75.0 7.3 7.5
OKN 2625–2643 Agbada Eocene to Recent Offshore 7.3 26.3 35.0 31.4
OKN 2780–2799 Agbada Eocene to Recent Offshore 12.0 61.4 17.3 9.2
OKN 2863–2881 Agbada Eocene to Recent Offshore 22.7 38.0 20.8 18.6
OKN 2909–2927 Agbada Eocene to Recent Offshore 27.5 29.1 20.8 22.5
Note: PhDBT, phenyldibenzothiophene
592 Page 4 of 22 Arab J Geosci (2021) 14:592
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Table 2 The absolute concentrations of dibenzothiophenes, benzo[b]naphthothiophenes, and phenyldibenzothiophenes in Niger Delta crude oils
Sample Depth (m) Absolute concentrations (μg/g oil) Relative abundance (%) Absolute concentrations (μg/g oil)
DBT MDBT DMDBT TMDBT BNTs DBTs [2,1]BNT [1,2]BNT [2,3]BNT 1-PhDBT 4-PhDBT 2-PhDBT 3-PhDBT PhDBTs
ADL1 2602–2607 6.2 7.2 20.5 9.8 1.3 45.2 57.8 15.3 26.8 1.1 1.7 0.4 0.9 4.0
ADL2 2602–2607 6.6 10.0 23.1 11.0 1.6 52.0 54.1 17.2 28.7 1.1 1.8 0.4 0.9 4.3
ADL3 2759–2763 4.7 8.1 17.6 8.3 1.3 40.1 48.0 14.3 37.6 1.5 1.7 0.4 0.7 4.3
ADL4 2766–2770 8.8 15.5 28.6 15.1 1.9 69.9 57.1 14.5 28.4 1.6 2.1 0.7 0.9 5.3
ADL5 2905–2908 3.6 7.3 16.5 8.0 0.9 36.3 56.3 13.6 30.1 0.9 1.1 0.4 0.6 3.0
ADL6 2964–2967 8.5 17.5 35.1 18.9 2.6 82.6 60.0 23.9 16.1 4.0 4.7 1.4 2.7 12.8
ADL7 3052–3064 11.9 25.1 46.3 20.0 3.1 106.3 60.2 16.2 23.6 1.8 3.7 0.8 1.8 8.1
OKN-1 1749–1750 24.6 94.0 193.6 104.8 8.9 425.9 66.4 9.6 24.0 6.2 6.2 1.8 1.2 15.4
OKN-2 1892–1895 18.1 51.7 94.8 54.0 7.4 226.0 62.7 13.0 24.3 3.7 6.2 1.8 0.7 12.4
OKN-3 1905–1907 17.7 47.5 88.4 50.5 8.5 212.6 69.0 12.9 18.1 3.4 5.3 1.7 0.7 11.0
OKN-4 1952–1955 27.7 101.0 200.6 107.9 11.0 448.2 60.3 17.8 21.9 6.0 7.1 2.1 1.5 16.7
OKN-5 2050–2059 15.0 43.2 79.1 45.5 7.3 190.0 68.4 9.1 22.5 3.3 5.3 1.6 0.8 11.0
OKN-6 2369–2555 3.5 9.9 18.6 10.2 1.8 43.9 64.0 14.6 21.5 1.0 1.6 0.7 0.5 3.7
OKN-7 2377–2672 17.3 50.1 87.9 50.8 10.0 216.1 62.3 14.3 23.1 4.3 6.3 1.9 1.0 13.4
OKN-8 2469–2782 16.8 48.0 88.0 51.5 8.7 213.0 59.6 13.5 26.9 3.3 6.1 1.8 0.9 12.1
OKN-9 2485–2793 17.5 50.5 93.8 52.9 8.3 222.9 57.7 14.9 27.4 4.5 6.1 2.2 0.9 13.8
OKN-10 2489–2491 11.8 27.1 43.7 23.1 3.5 109.3 58.6 18.1 23.3 1.5 1.9 0.7 0.6 4.7
OKN-11 2521–2523 10.9 26.3 45.3 25.1 2.6 110.2 58.9 12.7 28.4 2.1 2.4 1.0 0.8 6.3
OKN-12 2530–2537 18.3 47.6 78.8 39.7 5.5 189.8 57.0 14.4 28.6 3.6 4.2 1.7 1.1 10.6
OKN-13 2566–2568 21.0 81.3 166.8 88.1 8.3 365.5 61.4 14.8 23.8 5.5 5.7 1.4 1.8 14.4
OKN-14 2677–2683 12.9 34.0 61.2 34.5 4.5 147.1 52.9 16.7 30.5 2.3 3.3 1.0 0.8 7.5
OKN-15 3148–3154 13.6 35.5 62.5 34.1 3.9 149.6 53.0 15.3 31.6 1.9 2.7 0.9 0.7 6.1
OKN16 3593–3605 14.1 47.2 81.5 42.5 5.2 190.5 68.7 13.7 17.6 3.4 4.3 0.5 1.1 9.2
MJO-1 2207–2216 48.3 125.9 186.5 91.3 17.5 469.6 72.2 12.1 15.7 6.3 9.1 3.8 1.6 20.8
MJO-2 2070–2081 65.8 167.9 248.1 123.7 24.0 629.5 70.9 15.8 13.3 7.2 10.1 3.6 2.4 23.2
MJO-3 2091–2104 51.6 136.8 203.7 100.4 21.3 513.72 71.0 14.1 14.9 4.6 8.6 3.0 2.0 18.2
MJO-4 2096–2101 56.5 142.7 212.5 105.5 19.3 536.6 75.0 11.9 13.1 5.2 8.7 3.1 0.7 17.7
MJI-1 1607–1611 20.6 57.8 111.2 59.5 8.7 257.8 51.3 13.8 35.0 5.5 6.3 2.2 2.0 16.1
MJI-2 1777–1779 42.2 111.1 180.4 95.6 21.4 450.7 62.5 13.7 23.8 9.1 10.1 2.5 2.8 24.5
MJI-3 1795–1797 34.4 83.4 133.8 72.3 14.1 338.1 66.6 16.1 17.3 5.5 8.3 1.7 1.4 16.8
MJI-4 1920–1921 31.1 90.7 148.9 76.7 12.5 359.8 67.9 11.1 21.0 5.6 7.4 2.6 1.4 16.9
MJI-5 1936–2342 21.2 55.1 105.1 56.9 6.6 244.9 53.2 13.6 33.1 4.7 6.2 2.2 2.1 15.1
MJI-6 1944–1947 46.6 121.3 203.4 107.2 20.5 499.0 57.4 13.6 29.1 8.0 10.8 3.2 2.2 24.1
MJI-7 1948–1950 35.8 92.3 148.8 75.4 13.2 365.4 71.0 11.3 17.7 5.6 7.0 1.5 2.1 16.3
Arab J Geosci (2021) 14:592 Page 5 of 22 592
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the aromatic hydrocarbon fractions were eluted with
35 ml of dichloromethane:hexane (2:1, v: v).
Gas chromatography-mass spectrometry (GC-MS)
analyses
The GC-MS analyses of the saturated and aromatic hydrocar-
bon fractions were performed on an Agilent 5975i gas chro-
matography (GC) equipped with an HP-5MS (5%
phenylmethylpolysiloxane) fused silica capillary column
(60m × 0.25mm i.d., × 0.25μm film thickness) coupled to
an Agilent 5975i mass spectrometry (MS). The GC operating
conditions are as follows: the oven temperature was held iso-
thermally at 80°C for 1 min, ramped to 310°C at 3°C/min and
held isothermal for 16 min (Li et al. 2012). Helium was used
as the carrier gas with constant flow rate of 1.2 mL/min. The
MS was operated in the electron impact (EI) mode at 70eV, an
ion source temperature of 250°C, and injector temperature of
285°C. The identification and elution order of
dibenzothiophenes, phenyldibenzothiophenes, and
benzo[b]naphthothiophenes were determined by comparison
of their mass spectra and relative retention times in the corre-
sponding mass chromatograms with those reported in litera-
ture (Santamaría-Orozco et al. 1998; Meyer zu Reckendorf
2000;Marynowskietal.2002;Lietal.2012,2013a,2013b,
2014;Ogbesejanaetal.2019a; Zhu et al. 2019). The quanti-
fication of DBTs, PhDBTs, and BNTs was done using
dibenzothiophene-d8 (DBT-d8; molecular formula, C
12
D8S;
molecular mass, 192.31; purity = 99.5%, laboratory of Dr.
Ehrenstorfer, Augsburg, Germany). The relative abundance
was calculated from integrated peak areas in the relevant ion
chromatograms.
Results
Identifications and absolute concentrations of DBTs,
PhDBTs, and BNTs
The mass chromatograms m/z 184, 198, 212, 226, 260, and 234
showing the distribution of dibenzothiophene (DBT),
methyldibenzothiophenes (MDBT), trimethyldibenzothiophenes
(TMDBTs), phenyldibenzothiophenes (PhDBTs), and
benzo[b]naphthothiophenes (BNTs) in the Niger Delta source
rock extracts and crude oils are shown in Figs. 2and 3.Their
relative abundances, absolute concentrations, and calculated geo-
chemical parameters are given in Tables 1,2,3,and4.The
distribution and relative abundance of DBTs and BNTs in the
present source rocks have recently been reported (Ogbesejana
et al. 2019a). In the present study, four isomers of
phenyldibenzothiophenes were detected in the m/z 260 mass
chromatograms of the aromatic fractions of the rock extracts,
and their relative abundances, i.e., 1-phenyldibenzothiophene
Table 2 (continued)
Sample Depth (m) Absolute concentrations (μg/g oil) Relative abundance (%) Absolute concentrations (μg/g oil)
DBT MDBT DMDBT TMDBT BNTs DBTs [2,1]BNT [1,2]BNT [2,3]BNT 1-PhDBT 4-PhDBT 2-PhDBT 3-PhDBT PhDBTs
MJI-8 1979–2398 28.9 74.5 132.8 71.1 11.6 319.0 76.3 12.4 11.3 4.8 7.0 2.1 2.3 16.3
MJI-9 2442–2444 49.3 118.9 192.2 100.6 18.5 479.6 65.9 13.0 21.0 10.5 12.4 3.3 5.0 31.1
MJI-10 3030–3036 37.4 74.3 102.1 51.9 8.9 274.6 70.8 13.8 15.4 3.4 6.6 0.9 1.0 11.9
WZB1 1610–2647 51.3 113.8 175.6 106.4 13.1 460.1 55.9 23.9 20.2 14.1 15.0 5.8 3.3 38.2
WZB2 1811–1957 42.2 101.3 152.4 91.3 14.0 401.2 58.8 28.5 12.7 13.9 13.2 3.9 3.2 34.1
Note: DBT, dibenzothiophene; MDBT, methyldibenzothiophene; DMDBT, dimethyldibenzothiophene; TMDBT, trimethyldibenzothiophene; BNT, benzo[b]naphthothiophene; PhDBT,
phenyldibenzothiophene
592 Page 6 of 22 Arab J Geosci (2021) 14:592
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(1-PhDBT), 4-phenyldibenzothiophene (4-PhDBT), 2-
phenyldibenzothiophene (2-PhDBT), and 3-
phenyldibenzothiophene (3-PhDBT), are in the range of 7.3 to
29.9%, 24.4 to 75.0%, 5.6 to 35.0%, and 4.8 to 41.2%, respec-
tively (Table 1).
The absolute concentrations of DBT, MDBTs, DMDBTs,
and TMDBTs in the crude oil samples range from 3.5 to 65.8
μg/g oil, 7.3 to 167.9 μg/g oil, 16.5 to 248.1 μg/g oil, and 8.0 to
123.7μg/g oil, respectively (Table 2). Three isomers of BNTs,
i.e., benzo[b]naphtho[2,1-d]thiophene ([2,1]BNT),
benzo[b]naphtho[1,2-d]thiophene ([1,2]BNT), and
benzo[b]naphtho[2,3-d]thiophene ([2,3]BNT), were identified
in the aromatic fractions of the crude oils. The relative abun-
dances of [2,1]BNT, [1,2]BNT, and [2,3]BNT in the oils have
ranges of 48.0 to 76.3%, 9.1 to 28.5%, and 11.3 to 37.6%,
respectively (Table 2). The absolute concentrations of 1-
PhDBT, 4-PhDBT, 2-PhDBT, and 3-PhDBT in the oils range
from 0.9 to 14.1 μg/g oil, 1.1 to 15.0 μg/goil,0.38to5.8μg/g
oil, and 0.5 to 5.0 μg/g oil, respectively (Table 2). The total
absolute concentrations of DBTs,PhDBTs,andBNTsintheoils
are in the ranges of 36.3 to 460.1 μg/g oil, 3.0 to 38.2 μg/g oil,
and 0.9 to 24.0 μg/g oil, respectively (Table 2).
Parameters related to source, lithology, and
depositional environments
In rock samples, the Pr/Ph values range from 1.51 to 4.53
(Table 3) (Ogbesejana et al. 2018b,2019b;Ogbesejanaand
Bello 2020). These values suggest source rocks formed
from mixed marine and terrigenous organic matter and de-
posited under oxic to sub-oxic conditions (Didyk et al.
1978; Chandra et al. 1994;Escobaretal.2011). The ratio
of dibenzothiophene/phenanthrene (DBT/P) alone is an in-
dicator of source rock lithology, with the ratios of carbon-
ates > 1 and ratios of shales < 1 (Hughes et al. 1995). The
DBT/P values in the present study are within the range of 0.03
to 0.13 (Table 3), suggesting a shale lithology for the rock
samples investigated. The (1+4)-/(2+3)-MDBT values in the
rock samples vary between 1.31 and 2.54 (Table 3), suggest-
ing a shale lithology. In the crude oils, the Pr/Ph and DBT/P
values range from 1.48 to 4.71 (Ogbesejana et al. 2017,
2019b) and 0.10 to 0.22, respectively (Table 4). These values
suggest crude oils with shale lithology and mixed input of
terrestrial and marine organic matter and deposited under oxic
to sub-oxic conditions (Didyk et al. 1978; Chandra et al. 1994;
Table 3 Geological information and geochemical parameters calculated from dibenzothiophenes and related parameters
Field Depth(m) Reservoir
formation
Geological age Depobelt Pr/Ph % Ro DBT/P (1+4)-/
(2+3)-
MDBT
PhDR-1 PhDR-2 4-PhDBT/1-
PhDBT
MJI 2079–2098 Agbada Eocene to Recent Offshore 3.2 0.29 0.04 2.29 0.21 0.48 4.60
MJI 2299–2308 Agbada Eocene to Recent Offshore 2.31 0.03 2.28 0.08 0.21 1.09
MJI 2637–2655 Agbada Eocene to Recent Offshore 3.91 0.41 0.04 2.43 0.57 0.74 0.89
MJI 2857–2875 Agbada Eocene to Recent Offshore 3.98 0.06 2.48 0.36 0.75 1.57
MJI 2994–3012 Agbada Eocene to Recent Offshore 4.26 0.43 0.04 2.48 0.63 1.64 2.43
MJI 3085–3104 Agbada Eocene to Recent Offshore 4.53 0.05 2.50 0.48 0.72 1.52
MJI 3232–3250 Agbada Eocene to Recent Offshore 2.87 0.37 0.05 2.30 0.29 0.49 2.24
MJI 3323–3332 Agbada Eocene to Recent Offshore 4.46 0.13 2.11 0.36 0.71 1.64
MJI 3405–3424 Agbada Eocene to Recent Offshore 2.92 0.58 0.05 2.47 0.67 2.19 2.77
MJO 1616–1707 Agbada Eocene to Recent Offshore 4.31 0.3 0.06 1.75 0.13 0.21 4.11
MJO 1771–1872 Agbada Eocene to Recent Offshore 2.16 0.04 2.54 0.08 0.24 3.37
MJO 2091–2101 Agbada Eocene to Recent Offshore 2.08 0.38 0.04 2.11 0.17 0.19 3.45
MJO 2293–2366 Agbada Eocene to Recent Offshore 2.96 0.43 0.07 2.13 0.59 1.87 1.05
MJO 2570–2588 Agbada Eocene to Recent Offshore 3.5 0.4 0.07 2.49 0.30 1.87 1.05
MJO 2808–2817 Agbada Eocene to Recent Offshore 3.6 0.35 0.07 2.31 0.33 1.54 1.76
OKN 1537–1555 Agbada Eocene to Recent Offshore 1.85 0.23 0.09 1.31 0.15 0.43 3.80
OKN 1729–1747 Agbada Eocene to Recent Offshore 2.61 0.23 0.08 1.53 0.10 0.20 7.42
OKN 2625–2643 Agbada Eocene to Recent Offshore 1.51 0.07 1.83 0.55 1.04 3.61
OKN 2780–2799 Agbada Eocene to Recent Offshore 1.73 0.29 0.06 1.53 0.28 0.43 5.10
OKN 2863–2881 Agbada Eocene to Recent Offshore 2.06 0.31 0.06 1.74 1.33 1.49 1.67
OKN 2909–2927 Agbada Eocene to Recent Offshore 2.27 0.27 0.05 1.57 0.72 2.53 1.06
Note: Pr/Ph, pristane/phytane; %Ro, vitrinite reflectance; DBT/P, dibenzothiophene/phenanthrene; (1+4)-/(2+3)-MDBT, (1-methyldibenzothiophene+
4-methyldibenzothiophene)/(2-methyldibenzothiophene+3-methyldibenzothiophene); PhDR-1, phenyldibenzothiophene ratio-1; PhDR-2,
phenyldibenzothiophene ratio-2; 4-PhDBT/1-PhDBT, 4-phenyldibenzothiophene/1-phenyldibenzothiophene
Arab J Geosci (2021) 14:592 Page 7 of 22 592
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Escobar et al. 2011). The benzo[b]naphtho[2,1-d]thiophene/
benzo[b]naphtho[2,1-d]thiophene ([2,1]BNT/[1,2]BNT)
values in the crude oils are in the range of 2.03 to 7.51
(Table 4), suggesting microbial contribution to the source
rocks that expelled the oils (Kropp et al. 1994;Ogbesejana
et al. 2019a).
Parameters related to maturation
The values of vitrinite reflectance values, PhDR-1, PhDR-2,
and 4-PhDBT/1-PhDBT, in the rock samples range from 0.27
to 0.58% (Ogbesejana et al. 2018a,2019a,2019c), 0.08 to
0.67, 0.20 to 2.53, and 0.89 to 7.42, respectively ( Table 3).
Also, the 4-/1-MDBT, 4,6-/(1,4 + 1,6)-DMDBT, 2,4,6-/(2,4,7
+ 2,4,8)-TMDBT, and calculated vitrinite reflectance (VRc)
values in the crude oils range from 1.52 to 5.82, 0.76 to 1.98,
0.77 to 1.22, and 0.77 to 0.97 % (Table 4), respectively,
whereas the PhDR-1, PhDR-2, and 4-PhDBT/1-PhDBT
values in the oils vary from 0.11 to 0.42, 0.30 to 0.89, and
0.95 to 2.07, respectively (Table 4).
Table 4 Geochemical parameters calculated from dibenzothiophenes and related parameters
Sample Depth (m) Pr/Ph VRc DBT/P (1+4)-/
(2+3)-
MDBT
[2,1]-/
[1,2]BNT
C
27
Dia/
C
27
Steranes
4-/1-MDBT 4,6-/(1,4
+ 1,6)-
DMDBT
2,4,6-/
(2,4,7 + 2,4,8)-
TMDBT
PhDR-1 PhDR-2 4-
PhDBT/
1-PhDBT
ADL1 2602–2607 3.61 0.84 0.22 1.86 3.77 0.38 3.25 1.98 0.79 0.21 0.75 1.60
ADL2 2602–2607 3.95 0.83 0.21 1.74 3.15 0.37 3.17 1.92 0.79 0.22 0.72 1.62
ADL3 2759–2763 4.13 0.82 0.20 1.70 3.35 0.46 2.43 1.89 0.77 0.24 0.66 1.13
ADL4 2766–2770 4.30 0.87 0.19 1.43 3.94 0.39 3.17 0.76 0.80 0.30 0.74 1.39
ADL5 2905–2908 4.08 0.87 0.15 1.71 4.15 0.40 1.52 1.93 0.81 0.28 0.89 1.23
ADL6 2964–2967 3.61 0.89 0.18 1.91 2.51 0.50 2.59 0.99 0.85 0.30 0.86 1.19
ADL7 3052–3064 3.61 0.90 0.15 1.50 3.72 0.47 3.05 1.51 0.83 0.32 0.69 2.07
OKN-1 1749–1750 2.39 0.97 0.13 1.74 6.94 0.22 3.61 1.64 1.09 0.11 0.48 1.01
OKN-2 1892–1895 2.19 0.95 0.12 1.80 4.83 0.39 3.80 1.39 1.00 0.29 0.41 1.68
OKN-3 1905–1907 2.11 0.95 0.13 1.79 5.34 0.36 3.66 1.38 0.99 0.32 0.45 1.56
OKN-4 1952–1955 2.39 0.96 0.14 1.72 3.39 0.28 3.52 1.59 1.08 0.29 0.51 1.18
OKN-5 2050–2059 2.18 0.94 0.11 1.76 7.51 0.44 3.92 1.41 1.01 0.30 0.46 1.62
OKN-6 2369–2555 2.29 0.97 0.10 2.07 4.39 0.29 3.64 1.63 1.22 0.42 0.72 1.57
OKN-7 2377–2672 2.10 0.94 0.10 2.03 4.37 0.28 4.03 1.54 1.10 0.30 0.45 1.47
OKN-8 2469–2782 2.25 0.93 0.10 1.93 4.40 0.24 4.29 1.53 1.08 0.29 0.44 1.84
OKN-9 2485–2793 2.18 0.94 0.12 1.78 3.86 0.29 3.87 1.40 1.03 0.37 0.52 1.34
OKN-10 2489–2491 2.72 0.85 0.14 1.84 3.24 0.23 4.07 1.78 1.11 0.37 0.70 1.23
OKN-11 2521–2523 2.62 0.82 0.14 1.75 4.63 0.27 3.30 1.60 0.97 0.42 0.75 1.15
OKN-12 2530–2537 2.83 0.85 0.13 1.68 3.96 0.29 3.88 1.67 1.05 0.40 0.66 1.18
OKN-13 2566–2568 2.49 0.96 0.12 1.95 4.15 0.23 3.82 1.77 1.20 0.37 0.56 1.02
OKN-14 2677–2683 2.10 0.77 0.13 1.83 3.17 0.30 3.76 1.49 1.07 0.31 0.55 1.44
OKN-15 3148–3154 2.28 0.79 0.13 1.90 3.46 0.20 3.85 1.54 1.11 0.40 0.57 1.45
OKN16 3593–3605 2.17 0.82 0.11 1.92 5.04 0.21 4.51 1.45 1.19 0.42 0.35 1.27
MJO-1 2207–2216 1.76 0.86 0.10 1.86 5.99 0.24 4.13 1.29 1.04 0.42 0.60 1.44
MJO-2 2070–2081 1.76 0.86 0.11 1.80 4.49 0.20 3.99 1.25 1.01 0.36 0.59 1.41
MJO-3 2091–2104 1.68 0.87 0.11 1.80 5.04 0.30 4.14 1.25 1.02 0.35 0.58 1.88
MJO-4 2096–2101 1.70 0.87 0.11 1.84 6.28 0.30 4.30 1.27 1.03 0.36 0.45 1.67
MJI-1 1607–1611 2.93 0.86 0.11 1.70 3.72 0.29 4.16 1.73 1.16 0.35 0.67 1.15
MJI-2 1777–1779 3.33 0.90 0.11 1.36 4.57 0.23 5.29 1.71 0.98 0.25 0.52 1.10
MJI-3 1795–1797 3.33 0.90 0.11 1.46 4.14 0.28 5.40 1.82 1.01 0.20 0.36 1.51
MJI-4 1920–1921 4.37 0.97 0.11 1.47 6.12 0.37 5.82 1.93 1.06 0.36 0.54 1.32
MJI-5 1936–2342 2.74 0.87 0.12 1.59 3.90 0.22 3.71 1.68 0.92 0.35 0.69 1.30
MJI-6 1944–1947 3.24 0.89 0.12 1.46 4.22 0.29 4.51 1.73 1.17 0.29 0.49 1.34
MJI-7 1948–1950 4.71 0.92 0.11 1.41 6.29 0.26 5.17 1.70 1.00 0.22 0.52 1.25
MJI-8 1979–2398 3.08 0.89 0.12 1.52 6.14 0.30 4.30 1.67 1.16 0.30 0.63 1.45
MJI-9 2442–2444 3.26 0.90 0.12 1.38 5.06 0.24 4.84 1.69 0.98 0.26 0.67 1.18
MJI-10 3030–3036 4.43 0.90 0.13 1.41 5.14 0.24 4.72 1.68 1.03 0.14 0.30 1.91
WZB1 1610–2647 2.35 0.91 0.18 1.43 2.34 0.26 2.31 0.82 0.97 0.38 0.61 1.06
WZB2 1811–1957 1.48 0.92 0.16 1.38 2.06 0.25 2.74 0.97 0.99 0.29 0.54 0.95
Note: Pr/Ph, pristane/phytane; VRc, calculated vitrinite reflectance; DBT/P, dibenzothiophene/phenanthrene; (1+4)-/(2+3)-MDBT, (1-
methyldibenzothiophene+4-methyldibenzothiophene)/(2-methyldibenzothiophene+3-methyldibenzothiophene); [2,1]-/[1,2]BNT,
benzo[b]naphtho[2,1-d]thiophene/benzo[b]naphtho[2,1-d]thiophene; C
27
Dia/C
27
steranes, C
27
diasteranes/C
27
steranes; 4-/1-MDBT, 4-
methyldibenzothiophene/1-methyldibenzothiophene; 4,6-/(1,4+1,6)-DMDBT, 4,6-dimethyldibenzothiophene/(1,4+1,6)-dimethyldibenzothiophene;
2,4,6-/(2,4,7+2,4,8)-TMDBT, 2,4,6-trimethyldibenzothiophene/(2,4,7+2,4,8)-trimethyldibenzothiophene; PhDR-1, phenyldibenzothiophene ratio-1;
PhDR-2, phenyldibenzothiophene ratio-2; 4-PhDBT/1-PhDBT, 4-phenyldibenzothiophene/1-phenyldibenzothiophene
592 Page 8 of 22 Arab J Geosci (2021) 14:592
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Data related to migration
The total absolute concentrations of DBTs in the crude
oilsfromADL,OKN,MJO,MJI,andWZBoilfields
range from 36.34 to 106.30 μg/g oil, 43.94 to 448.24
μg/g oil, 469.55 to 629.45 μg/g oil, 244.87 to 499.04
μg/goil,and401.17to460.08μg/g oil, respectively
(Table 2).
Discussion
Variations in the distribution of DBTs, PhDBTs, and
BNTs
The results showed that dimethyldibenzothiophenes predom-
inated over DBT and its other alkylated homologues in the
rock samples (Ogbesejana et al. 2019a), which was consistent
Fig. 2 m/z 184+198, 212, 226, 234, and 260 mass chromatograms showing the distribution of adibenzothiophene and methyldibenzothiophenes, b
dimethyldibenzothiophenes, ctrimethyldibenzothiophenes, dbenzo[b]naphthothiophene, and ephenyldibenzothiophenes in Niger Delta source rocks
Arab J Geosci (2021) 14:592 Page 9 of 22 592
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with previous reports on sediment extracts from the Liaohe
Basin, East China (Li et al. 2013b). Among the
benzo[b]naphthothiophenes reported by Ogbesejana et al.
(2019a), the abundance of benzo[b]naphtho[2,1-d]thiophene
was higher than other isomers (Fig. 2d). Moreover, the source
rocks are characterized by the predominance of 4-PhDBT and
1-PhDBT over other isomers (Fig. 2e;Table1). The
histograms of the relative abundance of the PhDBT isomers
also show that the 4-PhDBT dominated other isomers (Fig.
4a). Differences in lithology, source facies, and low thermal
maturities of source rocks can cause these distribution anom-
alies (Zhu et al. 2019). For instance, the coals with a thermal
maturity of about 0.96 %Ro contained 4-PhDBT as their dom-
inant isomer in the previous studies, while 1-, 2-, and 3-
Fig. 3 m/z 184+198+212+226, 234, and 260 mass chromatograms showing the distribution of adibenzothiophene, methyldibenzothiophenes,
dimethyldibenzothiophenes, and trimethyldibenzothiophenes; bbenzo[b]naphthothiophenes; and cphenyldibenzothiophenes in Niger Delta crude oils
592 Page 10 of 22 Arab J Geosci (2021) 14:592
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PhDBT were present only in trace amounts (Zhu et al. 2019).
Similarly, the less mature shales and carbonates (≤0.90 %Ro)
contained either 4- or 1-PhDBT as their dominant isomer,
while 2- and 3-PhDBT occurred in relatively small abun-
dances in previous reports (Marynowski et al. 2002;
Rospondek et al. 2008). Recently, the source rocks being
studied have been reported to have immature to low maturity
status (0.23 to 0.58 %Ro) (Ogbesejana et al. 2018a,2018b,
2019a; Ogbesejana and Bello 2020). Thus, the results of the
present study agreed with the previously published data
(Marynowski et al. 2002; Rospondek et al. 2008; Zhu et al.
2019). Additionally, the absolute concentrations of DBT,
MDBTs, DMDBTs, and TMDBTs calculated for the crude
oils showed that DMDBTs have the highest concentration
while DBT has the least. This claim is further confirmed by
the histogram of DBT and its isomers in Fig. 4b.
Among the isomers of BNT identified, [2,1]BNT has the
highest abundance in all the crude oils analyzed (Fig. 3b;
Table 2), and these findings have been confirmed with previous
data (Fang et al. 2016; Ogbesejana et al. 2019a). Furthermore,
the triangular plot of BNTs (Fig. 5a; Ogbesejana et al. 2019a)
showed that [2,1]BNT dominated over other isomers of BNT in
Fig. 4 The histograms of athe relative abundances of phenyldibenzothiophene isomers in Niger Delta source rocks and babsolute concentrations of
dibenzothiophene, methyldibenzothiophenes, dimethyldibenzothiophenes, and trimethyldibenzothiophenes in Niger Delta crude oils
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the oils. The absolute concentration values of DBTs, PhDBTs,
and BNTs show that DBTs have the highest concentrations
while BNTs have the least concentrations. This interpretation
is further supported by the histogram of DBTs, PhDBTs, and
BNTs (Fig. 5b). The BNTs in these oils could have been
formed microbially from benzo[b]thiophenes with
Pseudomonas (Kropp et al. 1994; Ogbesejana et al. 2019a).
Similarly, among the isomers of the PhDBTs in the oils, 4-
PhDBT has the highest absolute concentration which is typical
of crude oils within the oil window (Zhu et al. 2019).
Furthermore, the wide ranges of concentrations observed
among the compound classes (DBTs, BNTs, and PhDBTs)
may be due to the effects of source facies, depositional envi-
ronments, thermal maturity, or secondary migration processes
(Connan et al. 1986; Radke et al. 1986,1991; Hughes 1984;
Radke 1988; Budzinski et al. 1991; Radke and Willsch 1994;
Hughes et al. 1995;Wuetal.1995; Bao et al. 1996;
Chakhmakhchev et al. 1997; Santamaría-Orozco et al. 1998;
Huang and Pearson 1999; Mössner et al. 1999;Kruge2000;
Li 2000; Hwang et al. 2002;Wangetal.2004; Jinggui et al.
2005;Yangetal.2005;Lietal.2008,2013a,2013b,2013c,
2014;Fangetal.2016,2017;Chenetal.2017;Yangetal.
2016; Ogbesejana et al. 2019a), and these are discussed in the
sections below.
Applications of DBTs and BNTs as source facies and
depositional environment indicators
The values of Pr/Ph in the rock samples and crude oils suggest
source rocks formed from mixed marine and terrigenous organ-
ic matter and deposited under oxic to sub-oxic conditions
(Didyk et al. 1978; Chandra et al. 1994; Escobar et al. 2011),
10
b
a
20 30 40 50 60 70 80 90
10
20
30
40
50
60
70
80
9010
20
30
40
50
60
70
80
90
%[2,1]
%[1,2] %[2,3]
ADL Oils
OKN Oils
MJO Oils
MJI Oils
WZB Oils
Fig. 5 aTernary plot of benzo[b]naphthothiophene isomers (after Ogbesejana et al. 2019a) and bhistogram of the absolute concentrations of
dibenzothiophenes, benzo[b]naphthothiophenes, and phenyldibenzothiophenes in Niger Delta crude oils
592 Page 12 of 22 Arab J Geosci (2021) 14:592
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while the values of DBT/P indicate source rocks with shale
lithology (Hughes et al. 1995). The cross plot of DBT/P versus
Pr/Ph has been previously used to evaluate source rock
paleodepositional environments based on the premise that these
ratios reflect the different Eh-pH regimes resulting from the
significant microbiological and chemical processes occurring
during deposition and early diagenesis of sediments (Hughes
et al. 1995). The cross plot of DBT/P versus Pr/Ph in the present
study clearly showed that the rock samples and the crude oils
have received mixed input of marine and terrestrial organic
matter and deposited in a lacustrine-fluvial/deltaic environment
(Fig. 6a). In the present study, the (1+4)-/(2+3)-MDBT ratio is
proposed for the first time to indicate lithology with values >
2.70 and < 2.70 indicating carbonates and shale lithology, re-
spectively. The plot of (1+4)-/(2+3)-MDBT versus Pr/Ph clear-
ly supports that the rock samples were formed from mixed
origin of terrestrial and marine organic matter and deposited
in a lacustrine-fluvial/deltaic environment (Fig. 6b). This inter-
pretation agreed with the result of Hughes et al. (1995).
The cross plots of DBT/P and (1+4)-/(2+3)-MDBT versus
Pr/Ph further support mixed origin (terrestrial and marine) and
lacustrine-fluvial/deltaic environments for the crude oils (Fig.
7a and b). We proposed that (1+4)-/(2+3)-MDBT can be used
as potential lithology and depositional environment indicator
in crude oils and sediment extracts.
Although the origin of BNTs in geological materials is still
unknown, Kropp et al. (1994) reported that
benzonaphthothiophenes (BNTs) could be microbially pro-
duced from benzo[b]thiophenes with Pseudomonas.The
values of benzo[b]naphtho[2,1-d]thiophene/
benzo[b]naphtho[2,1-d]thiophene ([2,1]BNT/[1,2]BNT) in
the crude oils suggest microbial contribution to the source
rocks that expelled the oils (Kropp et al. 1994;Ogbesejana
et al. 2019a). Furthermore, the diasterane formation has been
shown to be affected by mineral catalysis; higher clay content
typically yields higher diasterane/sterane ratios (Rubinstein
et al. 1975; Sieskind et al. 1979; van Kaam-Peters et al.
1997; Cesar and Grice 2017). In the present study, the crude
oils showed little or no effect of clay mineral catalysis when
Fig. 6 The cross plots of pristane/
phytane versus a
dibenzothiophene/phenanthrene
(after Hughes et al. 1995)andb
(1+4)-/(2+3)-
methyldibenzothiophene in Niger
Delta source rocks
Arab J Geosci (2021) 14:592 Page 13 of 22 592
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[2,1]BNT/[1,2]BNT is plotted against C
27
diasteranes/C
27
steranes (Fig. 7c).
Applications of DBTs and PhDBTs as thermal maturity
indicators
DBT-based maturity indicators were widely used in petroleum
geochemistry (Radke et al. 1982;Radke 1988; Radke et al.
1991; Radke and Willsch 1994; Santamaría-Orozco et al.
1998;Lietal.2013a,2013b; Ogbesejana et al. 2019a).
Radke (1988) suggested that the 4-/1-MDBT (MDR) depends
on the same chemical basis as the methylphenanthrene ratio
(MPR), while Yang et al. (2018) indicated that the prevailing
degradation reactions and much higher thermodynamic stabil-
ity of 4-MDBT relative to 1-MDBT allow the MDR maturity
indicator to regularly increase to high maturity stages during
Fig. 7 The cross plots of pristane/phytane versus adibenzothiophene/phenanthrene (after Hughes et al. 1995)andb(1+4)-/(2+3)-
methyldibenzothiophene and c[2,1]-/[1,2]BNT versus C
27
diasteranes/C
27
steranes in Niger Delta crude oils
592 Page 14 of 22 Arab J Geosci (2021) 14:592
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the oil generation window. Similarly, Li et al. (2013a,2013b)
noted that the 4,6-/(1,4-+1,6)-DMDBT and 2,4,6-/(2,,4,7 +
2,4,8)-TMDBT ratios showed a linear increase with increased
thermal maturity in the Liaohe Basin, East China, sediments
and suggested that these ratios could be used as effective
maturity indicators for source rocks and crude oils within the
oil generation window. In addition, Zhuet al. (2019)proposed
two phenyldibenzothiophene maturity measures, i.e., PhDR-1
(2-PhDBT/4-PhDBT) and PhDR-2 ((2-PhDBT + 3-PhDBT)/
4-PhDBT), based on geochemical data and theoretical calcu-
lations and that these parameters are useful molecular markers
for higher thermal maturity assessment. In the present rock
samples, the application of dibenzothiophenes as maturity in-
dicators has already been reported (Ogbesejana et al. 2019a).
In this study, the values of vitrinite reflectance (%Ro), PhDR-
1, PhDR-2, and 4-PhDBT/1-PhDBT in the rock samples sug-
gest immature to early mature source rocks (Zhu et al. 2019).
The cross plots of values PhDR-1, PhDR-2, and 4-PhDBT/1-
PhDBT against burial depth (Fig. 8)and%Ro(Fig.9)clearly
indicate some degree of correlations, suggesting that these
parameters can be used as maturity indicators in Niger Delta
source rocks (Zhu et al. 2019). These interpretations are con-
sistent with the recent reports on the Niger Delta source rocks’
thermal maturity status (Ogbesejana et al. 2018a,2018b;
Ogbesejana and Bello 2020).
The 4-/1-MDBT, 4,6-/(1,4 + 1,6)-DMDBT, 2,4,6-/(2,4,7 +
2,4,8)-TMDBT, calculated vitrinite reflectance (VRc), PhDR-
1, PhDR-2, and 4-PhDBT/1-PhDBT values in the crude oils
Fig. 8 The cross plots of burial depths versus aPhDR-1, bPhDR-2, and c4-PhDBT/1-PhDBT in Niger Delta source rocks
Arab J Geosci (2021) 14:592 Page 15 of 22 592
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indicate that the crude oils are within the oil window (Zhu
et al. 2019;Ogbesejanaetal.2019b; Ogbesejana and Bello
2020). The cross plots of 4-/1-MDBT, 4,6-/(1,4 + 1,6)-
DMDBT, 2,4,6-/(2,4,7 + 2,4,8)-TMDBT, PhDR-1 and
PhDR-2 versus VRc, and 4-PhDBT/1-PhDBT versus 4-/1-
MDBT (Figs. 10 and 11) clearly illustrated that the crude oils
have similar maturity rank. These results agree with the pre-
vious results on Niger Delta crude oils (Ogbesejana et al.
2017; Ogbesejana 2018;Ogbesejanaetal.2019b,2019c).
Oil-source rock correlation
Dibenzothiophene and his alkylated homologues have been
widely applied to oil-source rock correlation studies based
on variations in source facies and depositional environments
in source rocks and crude oils (Fan et al. 1990,1991;Jinggui
et al. 2005). To determine the potential genetic relationship
between the source rocks and crude oils studied, the DBT/P
and (1+4)-/(2+3)-MDBT values were plotted against Pr/Ph;
ternary plot of the isomers of benzo[b]naphthothiophenes
Fig. 9 The cross plots of vitrinite
reflectance versus aPhDR-1, b
PhDR-2, and c4-PhDBT/1-
PhDBT in Niger Delta source
rocks
592 Page 16 of 22 Arab J Geosci (2021) 14:592
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was also plotted for the same purpose. The plots of DBT/P and
(1+4)-/(2+3)-MDBT values versus Pr/Ph (Fig. 11 and 12)and
ternary plots of benzo[b]naphthothiophene isomers (Fig. 13)
showed that the rock samples and crude oils have shale lithol-
ogies and have originated from terrestrial and marine organic
matter and deposited under oxic to sub-oxic conditions in
lacustrine-fluvial/deltaic environments (Hughes et al. 1995).
This shows that genetic relationship exists between the source
rocks and crude oils.
Dibenzothiophenes as migration indicators
Li et al. (2008) reported that the tetravalent sulfur atom of the
DBT molecule has two unsaturated lone pair electrons, which
can easily form an unstable hydrogen bond with a positively
charged bond in the carrier bed during the petroleum migra-
tion process (Chen et al. 2017). This scenario results in the
adsorption of DBT molecules by the carrier bed, thereby gen-
erating a geological chromatographic effect. Consequently,
total DBT concentrations decrease gradually with increasing
oil migration distance along migration pathways based on the
principle of the geological chromatography (Chen et al. 2017),
with the high absolute concentration of DBTs and the large
descending gradient in migration pathways indicating areas
with strong hydrocarbon filling capacity (Wang et al. 2004;
Li et al. 2012; Yang et al. 2016;Chenetal.2017). In the
present study, having tested the possible influence of source
facies and thermal maturity on the distribution and abundance
Fig. 10 The cross plots of
calculated vitrinite reflectance
versus a4-/1-MDBT, b4,6-/(1,4+
1,6)-DMDBT, and c2,4,6-/
(2,4,7+2,4,8)-TMDBT in Niger
Delta crude oils
Arab J Geosci (2021) 14:592 Page 17 of 22 592
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of dibenzothiophene and its derivatives in the crude oils under
study, and since oils from the same field belong to the same
family (also based on the applications of DBTs, PhDBTs, and
BNTs as source facies, lithology, depositional environment,
and maturity indicators in the oils under previous sections in
this study), it is safe to conclude that oils are affected by
migration fractionation effects and that the absolute concen-
trations of DBTs recorded in ADL-7, OKN-4, MJO-2, MJI-6,
and WZB-1, respectively, indicate areas with strong hydrocar-
bon filling capacities in the oilfields (Wang et al. 2004;Li
et al. 2012;Yangetal.2016;Chenetal.2017) and that the
source kitchens in these oilfields are located close to the ADL-
7, OKN-4, MJO-2, MJI-6, and WZB-1, respectively (Li et al.
2008,2014;Fangetal.2016;Yangetal.2016;Chenetal.
2017). However, further work is required on the migration
study because the contour maps of each of the oilfields are
needed in order to successfully evaluate the oil migration dis-
tances and trace the oil migration directions of the oilfields.
Fig. 11 The cross plots of
calculated vitrinite reflectance
versus aPhDR-1 and bPhDR-2,
and c4-PhDBT/1-PhDBT versus
4-/1-MDBT in Niger Delta crude
oils
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Fig. 12 The cross plots of
pristane/phytane versus a
dibenzothiophene/phenanthrene
(after Hughes et al. 1995)andb
(1+4)-/(2+3)-
methyldibenzothiophene in Niger
Delta source rocks and crude oils
10 20 30 40 50 60 70 80 90
10
20
30
40
50
60
70
80
9010
20
30
40
50
60
70
80
90
[2,1] %
[1,2] % [2,3] %
Crude oils
Source rocks
Fig. 13 Ternary plot of
benzo[b]naphthothiophene
isomers in Niger Delta source
rocks and crude oils (after
Ogbesejana et al. 2019a)
Arab J Geosci (2021) 14:592 Page 19 of 22 592
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Conclusion
The geochemical significance of dibenzothiophenes,
phenyldibenzothiophenes, and benzo[b]naphthothiophenes
in the Niger Delta source rocks and crude oils was investigated
by gas chromatography-mass spectrometry (GC-MS). The
dimethyldibenzothiophenes, 4-phenyldibenzothiophene, and
benzo[b]naphtho[2,1-d]thiophene were observed to be most
abundant among the dibenzothiophenes,
phenyldibenzothiophens, and benzo[b]napthothiophenes in
the crude oils and rock samples. The crude oils and source
rock samples were found to have shale lithologies and mixed
origin of terrestrial and marine organic matter and to have
deposited under oxic to sub-oxic conditions in lacustrine-flu-
vial/deltaic environments based on the distribution and abun-
dance of dibenzothiophenes and benzo[b]napthothiophenes.
The thermal maturity parameters based on dibenzothiophenes
and phenyldibenzothiophenes showed that the rock samples
had immature to early maturity status while the crude oils
were found to be within the oil window. A new lithology
and depositional environment indicator, i.e., (1+4)/(2+3)-
MDBT versus Pr/Ph which agreed with previously published
data (Hughes et al. 1995), was proposed in this study. This
study showed that dibenzothiophenes,
phenyldibenzothiophenes, and benzo[b]naphthothiophenes
were effective in determining the source facies, depositional
environments, and thermal maturity status of crude oils and
source rocks in the Niger Delta Basin, Nigeria.
Acknowledgements The authors thank the Department of Petroleum
Resources of Nigeria and Chevron Nigeria Limited for providing the
crude oils and rock samples for the present study. A. B. Ogbesejana
appreciates Cheng Quan and Zhao Jiang for their assistance in the labo-
ratory works. The authors gratefully acknowledge the State Key
Laboratory of Petroleum Resources and Prospecting, College of
Geosciences, China University of Petroleum, Beijing, China, for granting
A. B. Ogbesejana an international visiting research fellowship for carry-
ing out this research work. A. B. Ogbesejana appreciates Prof. Zhong
Ningning for his care during his visit to the State Key Laboratory of
Petroleum Resources and Prospecting, China University of Petroleum,
Beijing, China, for this research.
Declarations
Competing interests No conflicts of interest among the authors. The
manuscript has been seen and approved by all authors.
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