ORIGINAL PAPER

Molecular geochemical evaluation of Late Cretaceoussediments from Chad (Bornu) Basin, NE Nigeria:implications for paleodepositional conditions,source input and thermal maturation

Adebanji Kayode Adegoke & Babangida M. Sarki Yandoka &

Wan Hasiah Abdullah & Izuchukwu Mike Akaegbobi

Received: 20 November 2013 /Accepted: 18 February 2014 /Published online: 5 March 2014# Saudi Society for Geosciences 2014

Abstract The Late Cretaceous Gongila and Fika formationsediments in the Chad (Bornu) Basin, northeastern Nigeria,were analysed to evaluate their paleodepositional conditionsand source input as well as to determine their thermal maturity.These were interpreted based on their molecular composition.The composition and distribution of n-alkanes, isoprenoids,and biomarkers indicate a mixture of marine algal/bacterialand land-derived organic matter source input for the Gongilaand Fika formation sediments deposited in marine environ-ment. This is indicated by the abundance of short-chain n-alkanes, low CPI and high concentration of tricyclic terpanes,low C24 tetracyclic/C26 tricyclic, low to moderate hopane/sterane ratios and the relationship between regular steranecompositions. These were probably deposited in environmentalconditions which are mainly dysoxic. From the waxiness indexand terrigenous/aquatic ratio (TAR), it can be deduced thatmore terrigenous organic materials were deposited towardsthe northeastern part of the basin, probably because of theirproximity to the Lake Chad. Biomarker maturity parameterssuch as Ts/(Ts + Tm), C32 22S/(22S + 22R) homohopane,moretane/hopane and 20S/(20S + 20R) and ββ/(ββ + αα)

C29 sterane ratios also suggest that the sediments have reachedthe early to peak stages of hydrocarbon generation.

Keywords Fika formation . Late Cretaceous . Biomarkers .

Organic matter . Paleodepositional conditions

Introduction

Hydrocarbon exploration started in the Chad (Bornu) Basin(Fig. 1), the Nigerian part of the much larger Chad Basin, inthe 1980s (Obaje et al. 2004). During this campaign, 23 explor-atory wells were drilled, but minor gas shows were encounteredin only two of the wells, unlike in the other parts of the basin inNiger (Termit–Agadem Basin) and Chad (Doba, Doseo andBongor fields), where commercial hydrocarbon deposits havebeen discovered (Obaje et al. 2004; Mohammed et al. 1999).Although this exploration campaign was suspended for sometime, it has since begun again with the commencement of 3Dseismic data acquisition. The poor knowledge of the evolutionof the subsurface rocks in this part of the basin may have beenresponsible for this unsuccessful exploration attempt. The un-derstanding of the petroleum systems operating in any sedimen-tary basin is very vital to the exploration of hydrocarbon(Seewald 2003). On the other hand, petroleum geochemistryhas become a critical tool for the identification of source rocksand classification of crude oils into families as highlighted byprevious workers, e.g. Akinlua et al. (2007); Peters et al. (2005);Doust and Omatsola (1990), and Ekweozor et al. (1979). Thisobservation was also emphasized by Peters and Fowler (2002),who predicted that petroleum geochemistry would continue toimprove forecasting efficiency as exploration and productionbecome increasingly difficult in the twenty-first century.Detailed geochemical analysis of source rocks can give insight

A. K. Adegoke :B. M. Sarki Yandoka :W. H. AbdullahDepartment of Geology, University of Malaya, 50603 KualaLumpur, Malaysia

A. K. Adegoke (*)Department of Geology, Faculty of Science, Ekiti State University,P.M.B. 5363, Ado Ekiti, Nigeriae-mail: kayodeadebanji@gmail.com

B. M. Sarki YandokaNational Centre for Petroleum Research and Development,A.T.B.U., Bauchi, Nigeria

I. M. AkaegbobiDepartment of Geology, Faculty of Science, University of Ibadan,Ibadan, Nigeria

Arab J Geosci (2015) 8:1591–1609DOI 10.1007/s12517-014-1341-y

into their composition, and this can provide detailed informationon the environmental conditions during the time of depositionand the level of thermal maturation as well as the characteristicsof the hydrocarbons they will generate (Hunt 1996; Tissot andWelte 1984).

Although some studies had been undertaken on the basinsource rock potential and organic matter (OM) maturity(Alalade and Tyson 2010; Obaje et al. 2004; Olugbemiroet al. 1997; Petters and Ekweozor 1982), detailed organicgeochemical investigations on the origin of organic matterand reconstruction of the evolution of the depositional envi-ronment during deposition are lacking. More so, most of theearlier interpretations had been based primarily on pyrolysismethods, without significant input from biomarker parameters.Studies have shown that pyrolysis methods have their con-straints against organically lean sediments, because they aremore prone to mineral matrix effects (Peters 1986; Espitaliéet al. 1980). Most of the previous studies have established thepredominantly gas-prone nature of the potential source rocksin the basin but have not adequately examined the source inputand the depositional conditions of the organic matter.

This present study focuses on the molecular geochemicalevaluation of the Gongila and Fika formation, so as to providean overview of the source input, depositional setting, as well asthe thermal maturation stage of the analysed sediments.Biomarker parameters have been widely used effectively in thecharacterization of the environmental conditions during the

deposition of organic matter, the source input and the assessmentof maturation level of potential source rocks (e.g. Peters et al.2005; Peters and Moldowan 1993). The outcome of this studycould provide relevant information needed to answer explorationquestions related to type of source input, conditions of deposi-tional environment and thermal maturity of the organic matter ofthe Gongila and Fika formations of Chad (Bornu) Basin.

Geological settings

The Chad Basin is an extensive structural depression, occu-pying an area of about 2,335,000 km2 in six countries, includ-ing Chad Republic, Niger Republic and extending intoCameroon, Central Africa Republic, Sudan and northeastNigeria (Fig. 2). The Nigeria sector of the basin (also knownas Bornu Basin), which represents about one tenth of the totalarea extent of the Chad Basin, is believed to be geneticallylinked with the Benue Trough, thus representing the northernborder of a NE–SW trending aulacogen basin (Olade 1975)(Fig. 2). The Chad Basin is genetically and physically relatedto the fault and rift systems termed the West and CentralAfrican Rift Systems (WCARS), whose origin is generallyattributed to the Cretaceous breakup of Gondwana and theopening of the South Atlantic Ocean and the Indian Ocean(Fairhead 1986). The Benue–Chad axial trough is believed tobe the third and failed arm of a triple junction rift system that

Fig. 1 Geological map of Nigeria, showing the Chad (Bornu) Basin including location of the studied exploratory wells Kanadi-1 and Kuchalli-1 (afterAlalade and Tyson 2010; Whiteman 1982)

1592 Arab J Geosci (2015) 8:1591–1609

preceded the opening of the South Atlantic during the EarlyCretaceous and the subsequent separation of the African andSouth American continents (Genik 1992; Avbovbo et al.1986; Olade 1975; Burke et al. 1972; Carter et al. 1963)(Fig. 2). The stratigraphic units represented in Bornu Basinrange in age from Albian to recent (Okosun 1995; Whiteman1982; Petters 1981). Deposition took place under varyingconditions with each deposit representing one complete cycleof transgression and regression. It has been divided into sixformations based on the nature of sedimentary deposits withinthe depression. The divisions are named Bima, Gongila, Fika,Gombe, Kerri-Kerri and Chad formations as shown in Fig. 3(after Okosun 1995; Avbovbo et al. 1986; Carter et al. 1963).The oldest sediments are Albian to Cenomanian and comprisecontinental, sparsely fossiliferous, poorly sorted, thickly bed-ded, cross-stratified fine to coarse-grained feldspathic sand-stones of the Bima formation, which rest unconformably onbasement (Okosun 1995).

The early Turonian Gongila formation, which consists ofthin to moderately thick bedded, grey to dark grey calcareousshales, silty sandstones and sandstones, conformably overliesthe Bima formation and represents a transitional sequencebetween the underlying continental Bima formation and theoverlying marine Fika formation. Volcanic intrusives whichoccur as diorite sills are present at several horizons within theGongila formation (Okosun 1995). The Fika formation com-prises blue-black, ammonite-rich, and open marine shalewhich are occasional gypsiferous and with intercalations ofthin limestone beds. There is also an occurrence of volcanicintrusives which occur as diorite sills at numerous horizons of

the formation. The formation is diachronous, and it has beendated Turonian–Santonian. The estuarine/deltaic Gombe for-mation rests unconformably on the Fika formation and occursonly in some parts of the basin. It comprises sandstone andsiltstone with minor interbeds of claystone, ironstone andshale with thin coal beds. The formation was deposited simul-taneously with the deformation episode of the Maastrichtian,which resulted in a widespread uplift, along the long axis ofthe basin. Overlying the Gombe ‘Sandstone’ formation is thecontinental (lacustrine and deltaic-type) Kerri-Kerri formationof Paleocene age. The youngest stratigraphic unit in the basinis the continental (lacustrine and fluviatile) Chad formation,which is made-up of Quaternary sedimentary sequence of fineto coarse-grained sand and clay. The sand is uncemented withangular and subangular quartz grains of variable colour fromyellow, brown, white to grey, while the clay is massive andlocally gritty in texture due to the presence of angular tosubangular quartz grains (Okosun 1995). This Chad formationhas been dated Pleistocene by Carter et al. (1963).

Samples and experimental procedure

For this study, 14 cutting samples from two different explor-atory wells drilled in the Chad (Bornu) Basin (Fig. 1) wereselected within the Gongila and Fika formation based on theircolour (mostly dark grey) and their availability. The wellswere located in the southwest–northeast transect sector ofthe basin. The samples were sieve washed with distilled waterto remove possible drilling mud contaminations and dried in

Fig. 2 Regional tectonic map ofWestern and Central Africanrifted basins showing therelationship of the Bornu (Chad)Basin to the Benue, Muglad,Doba and East Niger Basins.Locations of regional shear zones(marked with half arrow) andmajor zone extension (completearrow) are shown (adapted fromObaje et al. 2004; Schull 1988)

Arab J Geosci (2015) 8:1591–1609 1593

the oven overnight at a temperature of 40 °C. The sampleswere crushed to 100 mesh and were subsequently subjected toSoxhlet extraction of bitumen. This was followed by molec-ular organic geochemical analysis (gas chromatography (GC)and gas chromatography mass spectrometry (GC-MS)).Approximately 25–30 g of the crushed samples was extractedin a Soxhlet apparatus for 72 h, using an azeotropic mixture ofdichloromethane (DCM) and methanol (93:7). After evapora-tive removal of the extraction solvent, the bitumen extractswere fractionated using liquid column chromatography, intoaliphatic (saturated) hydrocarbon, aromatic hydrocarbon andNSO compound fractions. The saturated fractions were furtheranalysed using GC and GC-MS. An FID gas chromatographwith HP-5MS column, temperature programmed from 40 to300 °C at a rate of 4 °C/min and then held for 30 min at300 °C, was used for GC analysis. GC-MS experiments wereperformed on a V5975B inert MSD mass spectrometer with agas chromatograph attached directly to the ion source (70-eVionization voltage, 100-mA filament emission current, 230 °Cinterface temperature). The fingerprints (fragmentograms) ac-quired from these analyses were used for biomarker identifi-cation, which were quantified by measuring peak heights inthe m/z 191 and m/z 217 mass fragmentograms. Individual

compounds were identified on the basis of retention time andcomparison of the mass spectra from published data (e.g.Peters and Moldowan 1993; Philp 1985).

Results and discussion

Organic matter source input and paleodepositional conditions

Normal alkanes and acyclic isoprenoids

Biomarker concentration ratios were used to describe sourceinput and conditions of depositional environment of theanalysed samples (Peters et al. 2005; Peters and Moldowan1993). Gas chromatograms of saturate fractions from two rep-resentative samples are shown in Figs. 4a and 5a. The distribu-tion of n-alkanes may be an indicative of source input of originalorganism(s). For example, algal-sourced organicmatter is rich inshort-chain n-alkanes < C20, and land plant-derived organicmatter is dominated by long-chain n-alkanes > C27 with odd-over-even carbon number predominance (Cranwell et al. 1987;Eglinton and Hamilton 1967). In this study, the chromatogramsindicate that the saturated hydrocarbons are dominated by n-

Fig. 3 Stratigraphic successionin the Chad (Bornu) Basin,northeastern Nigeria (afterOkosun 1995; Avbovbo et al.1986; Carter et al. 1963)

1594 Arab J Geosci (2015) 8:1591–1609

C13–n-C32 and isoprenoid hydrocarbons (pristane and phytane).The n-alkane patterns in the analysed samples show mainlyunimodal distribution and a clear dominance of short-chain/low (n-C13–n-C20) to medium (n-C13–n-C20) molecular weightversus long-chain n-alkanes (n-C27–n-C31) in almost all of theanalysed samples (Figs. 4a and 5a).

Acyclic isoprenoids occur in significant amounts in all thestudied samples (Figs. 4a and 5a, Table 1) as indicated bypristane/n-C17 versus phytane/n-C18 ratios generally less than

1. Pristane (Pr) and phytane (Ph) are usually the most impor-tant acyclic isoprenoid hydrocarbons in terms of concentrationand reflect the paleoenvironmental conditions of source rocks(Powell and McKirdy 1973). Their ratios have been widelyused to assess the redox conditions during sedimentation anddiagenesis (Chandra et al. 1994; Didyk et al. 1978). The mostcommon origin of pristane (C19) and phytane (C20) is thephytyl side chain of chlorophyll-a in phototrophic organisms.Other sources include chlorophyll-b and bacteriochlorophyll-

Fig. 4 a A representative gaschromatogram of a saturatedhydrocarbon fraction (total ioncurrent, TIC), b a typical massfragmentogram (m/z 191)measured in the single ionmonitoring (SIM) mode ofterpenoids, and c a typical massfragmentogram (m/z 217, SIMmode) of steranes, obtained fromone of the analysed samples fromFika formation

Arab J Geosci (2015) 8:1591–1609 1595

a in purple sulphur bacteria (Powell and McKirdy 1973;Brooks et al. 1969). It has been stated that the formation ofpristane from phytol occurs in oxidizing environments, suchas peat swamp or bogs, while phytane occurs in reducing typeenvironments (Farhaduzzaman et al. 2012; Peters andMoldowan 1993). For oils and rocks within the oil-generative window, pristane/phytane (Pr/Ph) ratios correlateweakly with the depositional redox conditions (Large andGize 1996; Didyk et al. 1978; Powell and McKirdy 1973).

Low Pr/Ph ratio values (<0.8) indicate anoxic conditions,commonly carbonate or hypersaline environments, and highvalues (>3.0) usually typify oxic conditions often associatedwith terrigenous organic matter input (Peters et al. 2005;Peters and Moldowan 1993). However, Pr/Ph ratio can beinfluenced by other source materials than phytol from chloro-phylls (Ten Haven et al. 1987) and by thermal maturity (Vuet al. 2009). Pristane generally occurs in high relative concen-trations in the analysed samples, possessing pristane/phytane

Fig. 5 a A representative gaschromatogram of a saturatedhydrocarbon fraction (total ioncurrent, TIC), b a typical massfragmentogram (m/z 191)measured in the single ionmonitoring (SIM) mode ofterpenoids, and c a typical massfragmentogram (m/z 217, SIMmode) of steranes, obtained fromone of the analysed samples fromGongila formation

1596 Arab J Geosci (2015) 8:1591–1609

(Pr/Ph) ratios in the range of 0.72–1.23 (Table 1), whichsuggests that the Gongila and Fika formation sedimentspenetrated by these wells were deposited under marinedysoxic to relatively anoxic conditions (Hakimi et al.2011; Peters and Moldowan 1993). The cross-plot ofpristane/n-C17 versus phytane/n-C18 is sometimes usedto interpret source rock depositional environment condi-tion and organic matter type (van Koeverden et al. 2011;Peters et al. 2005). This plot suggests that the organicmatter in the investigated samples was derived mainlyfrom mixed marine and terrigenous materials depositedunder dysoxic conditions and that the samples wererelatively matured (Fig. 6).

Carbon preference index (CPI) of n-alkanes between n-C22

and n-C30 was also calculated to provide some insights intothe source input and depositional conditions of the organicmatter. CPI value that is less than 1.0 would indicate reducingdepositional conditions (Meyers and Snowdon 1993). Thevalue, which is greatly influenced by organic matter input,was calculated based on the formula proposed by Peters andMoldowan (1993), which is 2(C23+C25+C27+C29)/[C22+2(C24+C26+C28)+C30]. The CPI values for all the analysedsamples are slightly greater than or less than 1.0 (Table 1).This indicates a mixed input of marine and terrigenous organicmatter deposited under relatively reducing (dysoxic) condi-tions. The cross-plot of CPI against Pr/Ph (Fig. 7) furthersupports this interpretation (Akinlua et al. 2007; Meyers andSnowdon 1993).

Waxiness index may be used to determine the amount ofland-derived organic materials in the sediments. Thismethod is based on the assumption that terrigenous materialcontributes high molecular weight normal alkane compo-nents to the extracts (Peters et al. 2005). The degree ofwaxiness in this study is expressed by the ∑(n-C21–n-C31)/∑(n-C15–n-C20) ratios. Samples from Kuchalli-1 wells gen-erally contain higher waxy ratios (Table 1), reflectingrelatively higher amounts of land plant-derived biomarkersthan samples from Kanadi-1 well. This is supported by thecalculated terrigenous/aquatic ratios (TAR), in which theanalysed samples from Kanadi-1 well generally show lowerTAR values. The cross-plot of waxiness and pristane/phytane ratio further corroborates this interpretation(Fig. 8).

Terpanes and steranes

Hopane and sterane distributions (Appendix Table 4) of theanalysed samples were determined based onm/z 191 (Figs. 4band 5b) andm/z 217 (Figs. 4c and 5c) traces respectively. Peakidentifications of the m/z 191 and m/z 217 fragmentogramswere made on the basis of retention times and publishedliteratures (Farhaduzzaman et al. 2012; Hakimi et al. 2011;Sachse et al. 2011; Wan Hasiah 1999; Waples and Machihara1991; Philp 1985). The identified peaks are listed in AppendixTable 5, and the calculated ratios are shown in Tables 2 and 3.The m/z 191 mass chromatograms of the saturated

Table 1 n-Alkanes and isoprenoid ratios of the studied samples

Sample ID Depth (m) Formation Pr/Ph Pr/n-C17 Ph/n-C18 CPI OEP Waxinessindex

TAR n-alkanemaximum

Kanadi-1

KAN 900 900 Fika 1.08 1.6 1.06 1.08 1.1 0.53 0.14 C16

KAN 1205 1,205 Fika 1.21 1.45 0.78 1.01 0.97 0.45 0.13 C16

KAN 1370 1,370 Fika 1.23 1.31 0.78 1.06 1.06 1.18 0.27 C20

KAN 2450 2,450 Gongila 1.05 0.93 0.75 1 1.05 0.49 0.03 C18

KAN 2545 2,545 Gongila 1.04 1.2 1 0.98 0.99 0.58 0.03 C19

KAN 2575 2,575 Gongila 1.01 0.96 0.89 1 1 0.55 0.03 C19

Kuchalli-1

KUC 1620 1,620 Fika 0.72 0.99 1.29 0.97 0.95 0.68 0.19 C20

KUC 1680 1,680 Fika 0.87 1.52 1.35 1.09 1.07 0.41 0.15 C18

KUC 1950 1,950 Fika 0.95 1.49 1.2 1.1 0.96 0.47 0.06 C18

KUC 2070 2,070 Fika 1.16 1.4 1.14 0.99 0.97 1.3 0.51 C19

KUC 2190 2,190 Fika 0.91 1.22 1.04 0.98 0.96 0.42 0.06 C19

KUC 2310 2,310 Gongila 1.08 1.11 1.12 1.03 1.05 0.53 0.16 C16

KUC 2460 2,460 Gongila 1.04 1.47 1.29 1.04 1 1.19 0.34 C19

KUC 2850 2,850 Gongila 0.86 1.13 1.08 1.03 1 1.14 0.22 C19

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)], OEP improved odd-even predominance C21+6C23+C25/4C22+4C24, TAR terrigenous/aquatic ratio (C27+C29+C31)/(C15+C17+C19), waxiness index ∑ (n-C21-n-C31)/∑ (n-C15-n-C20)

Arab J Geosci (2015) 8:1591–1609 1597

hydrocarbon fractions of all the analysed samples show mod-erate abundances of pentacyclic and tricyclic terpanes withlower amounts of tetracyclic terpanes. Hopanoids are impor-tant biomarkers for indicating organic matter that was derivedfrom bacteria (Ourisson et al. 1979). Their composition anddistribution are similar in most of the samples and mainlyconsist of C27 to C34 17α,21β(H)-hopanes with C29αβ andC30αβ hopanes as major compounds (Figs. 4b and 5b).However, the relative abundance of C29 hopane is generallyless than that of C30 hopane in most of the analysed samples,with C29/C30 17α(H)-hopane ratios ranging from 0.4 to 1.22(Table 2). The predominance of C30 hopane is often associatedwith clay-rich source rocks (Gürgey 1999). This statement is

in agreement with the lithofacies of the studied samples. The17β,21α(H)-moretane was also detected in all the samples,though in low concentrations in many of the analysed sam-ples. The 18α(H)-oleanane, which is an important land plant-derived biomarker, was identified in low proportion in almostall the analysed samples, supporting the presence of terrige-nous organic matter input in the sediments (Peters et al. 2005;Peters and Moldowan 1993). The ol/C30-hopane ratio(oleanane index) ranges from 0.01 to 0.18. Murray et al.(1997) and other workers, e.g. Alias et al. (2012), have alsoreported that the presence of oleanane suggests probable ma-rine influence. The presence or absence of oleanane may alsobe an indicative of age. According to Peters and Moldowan

Fig. 6 Graph of pristane/n-C17

versus phytane/n-C18 for theinvestigated samples (cf vanKoeverden et al. 2011; Peterset al. 2005)

Fig. 7 Pristane/phytane versusCPI, indicating the depositionalenvironment conditions of thestudied samples (modified afterAkinlua et al. 2007; Meyers andSnowdon 1993)

1598 Arab J Geosci (2015) 8:1591–1609

(1993), oleanane is present in Middle/Late Cretaceous toyounger sediments and is absent in Jurassic or older sedi-ments. This confirms the age of the Gongila and Fika forma-tion sediments as not older than that of the Cretaceous.

Gammacerane is present in almost all the samplesanalysed, although in very low concentration in some of thesamples. Gammacerane is widely believed to have been de-rived from tetrahymanol in bacterivorous ciliates living at theboundary of a high-salinity water layer with an upper layer ofless saline water (Sinninghe Damsté et al. 1995; Ten Havenet al. 1988). Therefore, it is generally regarded as an indicatorof salinity-stratified water column and possible marker forphotic zone anoxia. Gammacerane index for the samplesanalysed in this study ranges between 0.06 and 0.43, and thisis an evidence for the existence of salinity-stratified watercolumn during the deposition of the Gongila and Fika forma-tion sediments penetrated within these wells. This also sug-gests that redox conditions during the deposition of the sedi-ments were moderately reducing (dysoxic), which supportsthe interpretation based on Pr/Ph values.

Tricyclic and tetracyclic terpanes are usually common inmarine environment and are believed to have their origin fromalgae, especially Tasmanites and bacteria (Marynowsky et al.2000; Aquino Neto et al. 1989; 1983; Volkman et al. 1989).However, Wan Hasiah (1999) and Philp (1985) have suggestedthat tricyclics may also be formed by partial aerobic oxidationof bacterial membrane and that they may not necessarily beoriginated from Tasmanites. This means that their abundancesin rocks and oils may be more related to diagenetic factors thanto direct biosynthetic production by specific organisms. C24

tetracyclics have also been reported in high concentration inAustralian oil with a probable terrestrial source (Philp andGilbert 1986). Thus, it seems likely that there may be morethan one origin for these compounds. Generally, the concentra-tion of tricyclic terpanes is much higher than that of tetracyclics

in most of the samples analysed in this study (represented byC24 tetracyclic/C26 tricyclic; Table 2). The relatively high abun-dance of the C23 tricyclic terpane compared with that of the C24

tetracyclic in all the analysed samples may also be related todiagenetic factors as well as an algal origin for the organicmatter or a deposition in marine environment. It may bereflecting an enhanced microbial activity within the sediments(as supported by moderate hopane/sterane ratio) and an envi-ronmental condition that is dysoxic. In addition, the ratios ofvarious tricyclic terpanes by carbon number have been ac-knowledged to give some insight into the source of organicmaterial (Adekola et al. 2012; Peters et al. 2005). In the currentstudy, the low to moderate C24/C23 and low C22/C21 tricyclicterpane values (Table 2, Figs. 4b and 5b) in the Gongila andFika formation samples are interpreted as indicative of amixture of marine and terrigenous organic matter. Accordingto Qiuhua et al. (2011), C21/C23 tricyclic terpane ratio less than0.5 is in accordance with marine source rock. The ratios in thesamples range from 0.33 to 1.27, which indicate a mixture ofmarine and terrigenous organic matter. The cross-plots of tri-cyclic terpane C26/C25 versus C24 tetracyclic terpane/C26 tricy-clic terpane (Fig. 9) and tricyclic terpane C23/C24 versus C24

tetracyclic terpane/C26 tricyclic terpane (Fig. 10) further sup-port the increasing terrigenous organic matter input towards thenortheastern part of the basin, where Kutchalli-1 well is located(Fig. 1). The relative amounts of C26 tricyclic terpane to C25

tricyclic terpane can also be used to differentiate marine fromlacustrine source rock (Peters et al. 2005; Volk et al. 2005). Inall of the studied samples, the C26 tricyclic terpane is lessabundant than the C25 tricyclic terpane (Table 2, Figs. 4b and5b). This attribute is consistent with a source from mixedmarine and terrigenous organic materials (Adekola et al.2012). Also, the cross-plot of two tricyclic terpane ratios (C24/C23 versus C22/C21) indicates that most of the analysed samplesare deposited under essentially the same conditions (Fig. 11).

Fig. 8 Cross-plot of waxinessindex versus pristane/phytaneratios (cf El Diasty andMoldowan 2012)

Arab J Geosci (2015) 8:1591–1609 1599

Tab

le2

Hopanebiom

arkerparameterscalculated

from

m/z191masschromatogramsof

theanalysed

samples

(see

AppendixTable4and5)

Sam

pleID

Depth

(m)

Form

ation

Therm

almaturity

Sourceinputand

paleodepositionalconditions

Tm/Ts

Ts/(Ts

+Tm)

C30Mor/C30H

C32

22S/

(22S

+22R)

C29H/

C30H

Ga/C30H

ol/C30H

C19T/

C23T

C21T/

C23T

C22T/

C21T

C24T/

C23T

C26T/

C25T

C24Tet/

C26T

C23T/

C24T

C23T/

C24Tet

Kanadi-1

KAN900

900

Fika

0.86

0.54

0.24

0.37

0.45

0.24

0.01

0.2

1.16

0.18

0.62

0.79

0.87

1.6

5.31

KAN1205

1,205

Fika

0.76

0.58

0.17

0.57

0.81

0.21

0.03

0.25

0.73

0.23

0.45

0.81

0.14

2.21

33.2

KAN1370

1,370

Fika

0.77

0.57

0.18

0.64

0.67

0.15

0.03

0.4

0.71

0.28

0.44

0.78

0.29

2.25

22.5

KAN2450

2,450

Gongila

0.82

0.55

0.17

0.57

1.22

0.1

0.04

1.04

10.22

0.55

0.67

0.29

1.82

25.5

KAN2545

2,545

Gongila

0.71

0.58

0.22

0.55

0.97

0.06

0.07

0.81

10.28

0.54

0.73

0.15

1.86

40.5

KAN2575

2,575

Gongila

0.77

0.57

0.19

0.56

0.95

nd0.06

0.54

0.33

0.67

0.61

0.35

0.5

1.64

25

Kuchalli-1

KUC1620

1,620

Fika

0.67

0.6

0.19

0.39

0.4

0.36

0.01

0.37

1.09

0.24

0.63

0.94

0.76

1.59

3.89

KUC1680

1,680

Fika

0.72

0.59

0.2

0.38

0.5

0.29

0.02

0.65

1.27

0.19

0.58

0.85

1.06

1.71

5.33

KUC1950

1,950

Fika

0.23

0.81

0.5

0.53

1.17

0.13

0.02

0.88

1.4

0.2

0.59

0.8

0.25

1.68

34.5

KUC2070

2,070

Fika

0.24

0.8

0.33

0.53

0.82

0.16

0.02

0.66

1.07

0.26

0.38

0.64

0.29

2.61

43

KUC2190

2,190

Fika

0.26

0.79

0.25

0.56

0.73

0.14

0.04

0.45

1.06

0.24

0.44

0.62

0.25

2.3

50.5

KUC2310

2,310

Gongila

0.91

0.5

0.23

0.6

10.43

0.08

0.27

0.8

0.33

0.48

0.6

0.21

2.06

46.3

KUC2460

2,460

Gongila

0.89

0.53

0.23

0.6

1.1

0.15

0.18

0.15

0.36

0.74

0.55

0.62

nd1.83

nd

KUC2850

2,850

Gongila

0.77

0.57

0.17

0.56

0.71

0.12

0.07

0.21

0.53

0.39

0.62

0.6

0.17

1.61

37

1600 Arab J Geosci (2015) 8:1591–1609

The steranes are another group of important biomarkers.Steranes are derived from sterols that are found in higherplants and algae but rare or absent in prokaryotic organisms(Volkman 1986). The relative proportions of each of the‘regular’ steranes (C27, C28 and C29) can vary greatly fromsample to sample, depending upon the type of organic matterinput to the sediment. Huang and Meinschein (1979) pro-posed that the relative proportions of the C27, C28 and C29

regular steranes in sediments might provide valuablepaleoenvironmental information. They suggested that a dom-inance of C27 steranes would indicate a preponderance of

marine phytoplankton, whereas a dominance of C29 wouldindicate a strong terrestrial contribution, and C28 steranesmight indicate a heavy contribution by lacustrine algae.However, Volkman (1988) subsequently noted that certainmarine organisms contribute to C29 regular steranes. Also,Nichols et al. (1990) observed that large amounts of C29

sterols are produced by marine diatoms during the springbloom in freezing Antarctic waters. Therefore, these biomark-er interpretations should be used with caution. Both C27 andC29 steranes are the most abundant tetracyclic components inthe Gongila and Fika formation extracts. However, these

Table 3 Sterane biomarker parameters (calculated from m/z 217 fragmentograms) of the analysed samples (see Appendix Table 4 and 5)

Sample ID Depth (m) Formation Thermal maturity Source input and paleodepositional conditions

Ster-C29 20S/(20S+20R)

Ster-C29 ββ/(ββ+αα)

C27-ster(%)

C28-ster(%)

C29-ster(%)

Ster-C27/Ster-(C27+C29)

Ster-C29/Ster-C27

Hop/Ster

Diaste/Ster

Kanadi-1

KAN 900 900 Fika 0.13 0.81 34 19 47 0.42 1.4 1.3 0.72

KAN 1205 1,205 Fika 0.32 0.72 40 19 41 0.5 1 3.6 0.88

KAN 1370 1,370 Fika 0.58 0.67 35 21 44 0.44 1.29 4.4 1.3

KAN 2450 2,450 Gongila 0.6 0.67 42 24 34 0.55 0.81 3.7 2.96

KAN 2545 2,545 Gongila 0.55 0.65 43 25 32 0.56 0.78 3.2 3.04

KAN 2575 2,575 Gongila 0.58 0.64 37 23 40 0.48 1.1 3.9 2.06

Kuchalli-1

KUC 1620 1,620 Fika 0.12 0.75 30 19 51 0.37 1.69 0.6 0.12

KUC 1680 1,680 Fika 0.25 0.72 42 21 37 0.53 0.88 0.8 0.1

KUC 1950 1,950 Fika 0.43 0.65 43 21 36 0.55 0.84 3.7 0.73

KUC 2070 2,070 Fika 0.53 0.8 39 21 40 0.49 1.02 4.1 0.72

KUC 2190 2,190 Fika 0.56 0.72 40 20 40 0.5 1 0.2 0.59

KUC 2310 2,310 Gongila 0.53 0.59 32 17 51 0.34 1.9 4.8 3.2

KUC 2460 2,460 Gongila 0.64 0.6 49 17 35 0.58 0.71 3 1.9

KUC 2850 2,850 Gongila 0.6 0.67 52 18 30 0.6 0.58 0.2 1.42

Fig. 9 Cross-plot of C26T/C25Tversus C24Tet/C26T showingincreasing terrigenous input in theKutchalli-1 samples (cf El Diastyand Moldowan 2012)

Arab J Geosci (2015) 8:1591–1609 1601

samples show a higher proportion of C27 (32–52%) comparedwith C29 (20–51 %) and C28 (17–24 %) steranes (Table 3),reflecting a high contribution of aquatic algae—bacterial or-ganic matter (Peters and Moldowan 1993) as indicated by theregular sterane ratio ternary diagram (Fig. 12; Huang andMeinschein 1979). This is corroborated by the moderatehopane/sterane ratios and the low to moderate C29/C27 steraneratios in the analysed Gongila and Fika formation samples(Table 3). The cross-plots of hopane/sterane versus pristane/phytane ratios (Fig. 13) and sterane C27/(C27+C29) ratiosversus pristane/phytane ratios (Fig. 14) further support thisinterpretation. The organic matter in these sediments wasdeposited mainly in open marine environment, as indicatedby the regular sterane ratio ternary diagram (Fig. 15).According to Moldowan et al. (1985), marine source rockextracts or oils tend to be richer in steranes, evidenced by

lower values of hopane/sterane ratio(s), compared with oilsfrom non-marine sources. The hopane/sterane ratios in thesamples, which range from 0.2 to 4.8, further support theinterpreted marine depositional environment. Low to moder-ate diasterane/sterane ratio (0.1–3.2) in the analysed samplesis also an indication of dysoxic depositional conditions.

Organic maturation

In this study, n-alkane data and biomarker maturity parametershave been used to evaluate the level of thermal maturity of theGongila and Fika formations. The proportion of odd versuseven carbon numbered n-alkanes may be used to obtain arough estimate of organic maturation level of the sediments(Peters et al. 2005; Peters and Moldowan 1993; Bray andEvans 1961). These measurements include the CPI, based on

Fig. 10 Cross-plot of C23T/C24Tversus C24Tet/C26T, also showingincreasing terrigenous input in theKutchalli-1 samples (cf El Diastyand Moldowan 2012)

Fig. 11 Tricyclic terpane C24/C23

versus tricyclic terpane C22/C21

derived from the distribution oftricyclic terpanes in sampleextracts from the Chad (Bornu)Basin, indicating organic mattersource and deposition underessentially the same conditions(after Peters et al. 2005)

1602 Arab J Geosci (2015) 8:1591–1609

the formula proposed by Peters and Moldowan (1993), whichis 2(C23+C25+C27+C29)/[C22+2(C24+C26+C28)+C30] andthe improved odd-even preference (OEP) by Scalan andSmith (1970). According to Peters and Moldowan (1993),CPI or OEP values significantly above 1.0 (odd preference)or below 1.0 (even preference) indicate thermal immaturity,while values of 1.0 suggest that the organic matter is thermallymature. They, however, noted that CPI and OEP values aregreatly influenced by organic matter input, and as a result, thematurity interpretation obtained from these values should becomplemented with other maturity data. The CPI values for all

the analysed samples are equal to 1.0 or almost equal to 1.0,and there is a slight odd carbon number preference between n-C22 and n-C30 (Table 1). This indicates that most of theinvestigated samples are thermally mature in terms of hydro-carbon generation (Bray and Evans 1961). The relationshipbetween isoprenoids Pr/n-C17 and Ph/n-C18 ratios also showsthat all the analysed samples are thermally mature for hydro-carbon generation (Fig. 6).

The biomarker parameters indicative of thermal matu-rity are listed in Tables 2 and 3 and are further discussedbelow. The biomarker maturation parameters, such as Ts/

Fig. 12 Ternary plot showing therelationship between steranecomposition and source input(after Huang and Meinschein1979)

Fig. 13 Cross-plot of hopane/sterane ratios versus pristane/phytane ratios (cf El Diasty andMoldowan 2012)

Arab J Geosci (2015) 8:1591–1609 1603

(Ts + Tm), C32 22S/(22S + 22R) homohopane, moretane/hopane and 20S/(20S + 20R) and ββ/(ββ + αα) C29sterane ratios, were widely used as maturity indicators(Waples and Machihara 1991; Mackenzie et al. 1980).However, some of these ratios may be influenced by bothsource input and thermal maturation. For example, the Ts/(Ts + Tm) ratio (sometimes referred to as Ts/Tm) isdependent on both maturity and source input of organicmatter (Peters and Moldowan 1993). According to Seifertand Moldowan (1978), 17α-22,29,30-trisnorhopane (Tm)has been observed to be less stable than 18α-22,29,30-

trisnorneohopane (Ts) during the catagenesis phase ofhydrocarbon generation, so the ratio of Ts to Tm shouldincrease with maturity. In the studied samples, the Ts/(Ts+Tm) ratio ranges from 0.5 to 0.81 (Table 2). This indicatesearly mature to mature level of thermal maturity for all ofthe analysed samples. Homohopanes are dominated byC31-hopane in all the samples analysed. Among thehomohopanes (C31–C34), S-isomers are dominant over R-isomers in most of the samples, indicating the samples’maturity. Similarly, Seifert and Moldowan (1980) reportedthat the 22S/(22S + 22R) homohopane ratio increases

Fig. 14 Cross-plot of C27/(C27+C29) regular steranes versuspristane/phytane ratios showingdepositional environmentconditions and source input (cfHossain et al. 2009)

Fig. 15 Ternary plot showing therelationship between steranecomposition and depositionalenvironment (after Huang andMeinschein 1979)

1604 Arab J Geosci (2015) 8:1591–1609

from 0 to ~0.6 but normally ranges from 0.57 to 0.62during thermal maturation. C31- or C32-homohopane resultsare normally used to calculate 22S/(22S + 22R) isomeriza-tion values. The C32 22S/(22S + 22R) ratios for theanalysed samples range from 0.37 to 0.64. This indicatesthat most of the samples are thermally mature for hydro-carbon generation.

The 17β,21α(H)-moretanes are thermally less stablethan the 17α,21β(H)-hopanes, so the concentrations ofthe C29-hopanes and C30-moretanes decrease relative tothe corresponding hopanes with thermal maturity (Peterset al. 2005). The C30-moretane/C30-hopane ratio decreaseswith increasing maturity of organic matter from about 0.8in immature bitumen to values of less than 0.15 in maturesource rocks and in oils to a minimum of 0.05 (Peters et al.2005; Mackenzie et al. 1980; Seifert and Moldowan 1980).The values obtained for the analysed Gongila and Fikaformation samples range between 0.17 and 0.33 and sug-gest that they are within the oil window. This is consistentwith the previous maturity data described above.Diasteranes (also referred to as rearranged steranes) arepresent in significant amounts in most samples analysed(Figs. 4c and 5c). Diasteranes are believed to result fromthe conversion of sterols during diagenesis and earlycatagenesis, and they appear to be more stable thansteranes at high thermal maturity (Peters et al. 2005;Peters and Moldowan 1993). As a result, diasterane/regular sterane ratios were also used as indicators of ma-turity. In the analysed samples, the values range from 0.1 to3.2, which suggest that most of the samples are matured forhydrocarbon generation. Other maturity parameters calcu-lated from the m/z 217 ion mass fragmentograms are theC29-5α,14α,17α(H)-20S/(20S + 20R) and the ββ/(ββ +αα) for C29 steranes. These ratios increase with increasingthermal maturity (Peters et al. 2005). Although it has beenstated that the 20S/(20S + 20R) ratio equilibrates at 0.52 to0.55, Gallegos and Moldowan (1992), however, showedthat these equilibrium values are probably too high and thatthe maximum values in oils and source rocks are 0.5 orless. They reported that the chromatographic peak corre-sponding to the C29ααα20S isomer is normally contami-nated by other C29 sterane isomers. The ββ/(ββ + αα), onthe other hand, may be affected by lithofacies types (Petersand Moldowan 1993; Rullkötter and Marzi 1988).Korkmaz and Kara Gülbay (2007) also reported that thevalues of 20S/(20S + 20R) and ββ/(ββ + αα) C29 may notbe completely equilibrated and that some of the variance inthe extent of sterane isomerization may be attributed toother factors such as organic facies, environment and li-thology. The values of 20S/(20S + 20R) and ββ/(ββ +αα) for the Gongila and Fika formation samples rangefrom 0.13 to 0.64 and 0.59 to 0.81, respectively. Thesevalues suggest that the analysed samples are thermally

mature for hydrocarbon generation. Generally, most ofthese biomarker maturity parameters indicate that theanalysed samples from Gongila formation are relativelymore mature than those from Fika formation (Tables 2and 3).

Conclusions

The molecular geochemical analyses of selected samples fromthe Gongila and Fika formation penetrated by two exploratorywells (Kanadi-1 and Kuchalli-1) in Chad (Bornu) Basin sug-gest the following:

(1) There is no distinct molecular parameter that could dis-tinguish between the Gongila formation and the Fikaformation except for the biomarker maturation parame-ters which include Ts/(Ts + Tm), C32 22S/(22S + 22R)homohopane, moretane/hopane and 20S/(20S + 20R)and ββ/(ββ + αα) C29 sterane ratios. These parametersindicate that the analysed samples from Gongila forma-tion are relatively more mature than those from Fikaformation.

(2) The composition and distribution of n-alkanes,isoprenoids, and biomarkers indicate a mixture ofmarine algal/bacterial and land-derived organicmatter source input for the Gongila and Fika for-mation sediments. This is indicated by the abun-dance of short-chain n-alkanes, low CPI and highconcentration of tricyclic terpanes, low C24 tetra/C26 tri and hopane/sterane ratios. The cross-plots ofpristane/n-C17 versus phytane/n-C18 and hopane/sterane versus pristane/phytane and the ternary di-agram showing the relationship between regularsterane compositions further suggest a mixture ofmarine and terrigenous biomass in the analysedsediments.

(3) This organic matter was probably deposited in environ-mental conditions which are mainly dysoxic.

(4) From the waxiness index and TAR, it can be de-duced that more terrigenous organic materials weredeposited in the sediments towards the northeasternpart of the basin, probably because of their prox-imity to the Lake Chad.

(5) Various biomarker distribution maturity data suggestedthat the sediments have reached the early to peak stagesof hydrocarbon generation.

Acknowledgments This work was supported by the University ofMalaya IPPP Research Grant Nos.

PV016-2012A and PG140-2012B. The authors are grateful to theNigerian Geological Survey Agency (NGSA) and the Frontier Explora-tion Services of the Nigerian National Petroleum Corporation (NNPC),for supplying the samples and data for this research.

Arab J Geosci (2015) 8:1591–1609 1605

Appendix

Table 4 Definitions and mea-surement procedures used in theliterature

22S/(22R + 22S) C3217α(H),21β(H)22S/[C3217α(H),21β(H)22(R + S)]

Ts/(Ts + Tm) 18α(H)-22,29,30-trisnorneohopane/[18α(H),22,29,30-trisnorneohopane +

17α(H),22,29,30-trisnorhopane]

C29-hop/C30-hop C2917α(H),21β(H)-hopane/C3017α(H),21β(H)-hopane

C22T/C21T C22 tricyclic terpane/C21 tricyclic terpane

C24T/C23T C24 tricyclic terpane/C23 tricyclic terpane

C26T/C25T C26 tricyclic terpane/C25 tricyclic terpane

C24Tet/C26T C24 tetracyclic terpane/C26tricyclic terpane

ol/C30-hop 18α(H)-oleanane/C3017α(H),21β(H)-hopane

C30-mor/C30-hop C3017β(H),21α(H)-hopane/C3017α(H),21β(H)-hopane

Diasteranes/steranes C2913β(H),17α(H)20(R + S) diasteranes/C29ααα + αββ20(R + S) steranes

20S/(20S + 20R) C295α(H),14α(H),17α(H),20S/[C295α(H),14α(H),17α(H),20(S + R)]

ββ/(ββ + αα) [5α(H),14β(H),17β(H)(20R + 20S)C29sterane]/[5α(H),14β(H),17β(H)(20R + 20S) +5α(H),14α(H),17α(H)(20R + 20S)]C29 steranes

Hopanes/steranes Relative abundance of hopanes in m/z 191/relative abundance of steranes in m/z 217

(C29–33 αβ hopanes [22S + 22R])/(C27–29 ααα [20S + 20R] and αββ [20S + 20R]steranes)

Peak identity Compound Carbon no.

(I)

Fragmentogram

m/z 191

C19 C19 Tricyclic (Cheilanthane) 19

C20 C20 Tricyclic (Cheilanthane) 20

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) 31

(22S and 22R)

1606 Arab J Geosci (2015) 8:1591–1609

Table 5 Peak assignments for al-kane hydrocarbons in the gaschromatograms of the saturatefractions (I) in the m/z 191 massfragmentogram and (II) m/z 217mass fragmentogram

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