ORIGINAL PAPER

Organic geochemical characteristics of Cretaceous LamjaFormation from Yola Sub-basin, Northern Benue Trough, NENigeria: implication for hydrocarbon-generating potentialand paleodepositional setting

Babangida M. Sarki Yandoka & Wan Hasiah Abdullah & M. B. Abubakar &

Mohammed Hail Hakimi & Khairul Azlan Mustapha &

Adebanji Kayode Adegoke

Received: 18 August 2014 /Accepted: 10 November 2014# Saudi Society for Geosciences 2014

Abstract An integrated geochemical and molecular charac-terisation of the Cretaceous Lamja Formation shale and coalsediments from the Yola Sub-basin, Northern Benue Trough,northeastern Nigeria, has been undertaken to provide an over-view on the origin, richness, hydrocarbon generation potentialand paleodepositional conditions. This study is based ongeochemical analyses of whole rock (total organic carboncontent, pyrolysis, bitumen extraction and biomarker distribu-tions) and vitrinite measurements. The total organic carbon(TOC) contents of the Lamja Formation range from 0.8 to63 % and 0.8 to 1.16 % for coal and shale samples, respec-tively, with an average TOC value of 43.87 %. The hydrogenindex of these samples ranges from 93.1 to 228 mg hydrocar-bon (HC)/g TOC. The kerogen is predominantly type III witha significant mixture of type II kerogens, indicative of mainlygas with limited liquid hydrocarbon-generating potential. Theanalysed Lamja Formation samples have vitrinite reflectancein the range of 0.57–0.82 %Ro and pyrolysis temperature at

maximum (Tmax) in the range of 435–451 °C which indicatethat the samples are thermally mature and entered early matureto peak oil window stage. The molecular geochemical bio-markers are characterised by dominant odd carbon numberedn-alkanes in the range of n-C23 to n-C33, moderately highpristane/phytane (Pr/Ph) ratios (1.77–4.16), very high C27

17α(H)-22,29,30-trisnorhopane/C27 18α(H)-22,29,30-trisnorneohopane (Tm/Ts) ratios (>10) and high concentra-tions of regular sterane C29, indicating suboxic to oxic condi-tions, typical of delta plain/coastal marine environment ofdeposition with prevalent contribution of land plants andminor aquatic organic matter input. The occurrence ofoleanane in the analysed samples is also a strong indicator ofa terrestrial angiosperm plant source input and the presence ofmarine influence.

Keywords Cretaceous Lamja Formation . Hydrocarbongeneration potential . Total organic carbon

Introduction

The Yola Sub-basin in the Northern Benue Trough of Nigeriais one of the hydrocarbon exploration frontier basins where todate minimal data is available for adequate assessment of itshydrocarbon potential. It is part of the West and CentralAfrican Rift System (Fig. 1a) from which several petroleumexploration successes have been recorded in theMuglad Basinof Sudan and Doba and Termit basins of Chad and NigerRepublics. In view of the exploration successes in these basinswithin the same rift trend, the Northern Benue Trough ofNigeria has attracted the attention of many petroleum

B. M. Sarki Yandoka :W. H. Abdullah :K. A. Mustapha :A. K. AdegokeDepartment of Geology, University of Malaya,50603 Kuala Lumpur, Malaysia

B. M. Sarki Yandoka (*) :M. B. AbubakarNational Centre for Petroleum Research and Development,ATBU, Bauchi, Nigeriae-mail: bmsydgombe@yahoo.com

M. H. HakimiGeology Department, Faculty of Applied Science,Taiz University, 6803 Taiz, Yemen

A. K. AdegokeDepartment of Geology, Ekiti State University, P.M.B. 5363,Ado Ekiti, Nigeria

Arab J GeosciDOI 10.1007/s12517-014-1713-3

researchers and explorers. Three exploratory wells weredrilled in the Gongola Sub-basin of the Northern BenueTrough from 1999 to 2003 and an estimated reserve of 33billion cubic feet of gas was encountered in Kolmani River-1well (Obaje et al. 2004; Abubakar 2014). However, there is noreported drilled well or core in the Yola Sub-basin, and thus,there is poor knowledge on the organic facies variation anddistributions in the Yola Sub-basin (Fig. 1b).

Preliminary geochemical studies have previously been un-dertaken on some formations from Yola Sub-basin (e.g.Akande et al. 1998; Obaje et al. 2006), but detailed organicgeochemical investigation on the origin, type, richness,paleodepositional conditions as well as the assessment of thehydrocarbon potential and thermal maturation of the organicmatter is lacking. More so, the earlier interpretations had beenbased primarily on the pyrolysis method and have not exam-ined the source inputs, paleodepositional conditions and ther-mal maturation from biomarker parameters. Studies haveshown that pyrolysis methods have their constraints againstorganically lean sediments because they are more prone tomineral matrix effects (Peters 1986; Espitalié et al. 1980).

The present study focuses on the organic geochemicalcharacteristics of the Cretaceous Lamja Formation sedimentsfrom the Yola Sub-basin, so as to provide an overview on theorganic matter type, richness, source inputs, hydrocarbonpotential, paleodepositional conditions as well as the thermal

maturation. Since such studies of this kind have not beenpreviously conducted, selected outcrop samples were collect-ed from various stratigraphic intervals (Fig. 2) and detailedinvestigations were performed. This study is expected tocontribute to petroleum source rock prediction and assess-ment, which in turn will help in risk reduction for hydrocarbonexploration campaign in the Northern Benue Trough.

Geology and stratigraphy

The Benue Trough is one of the major rift basins formed fromthe tension generated by the separation of the African andSouth American plates (Abubakar 2014). It is a NE–SWtrending, intra-continental, Cretaceous sedimentary basin inNigeria that extends about 1000 km in length and 50 km inwidth (Fig. 1b). Several authors have presented tectonicmodels for the genesis of the Benue Trough (Abubakar2014). King (1950)proposed tensional movement resultingin a rift, while Stoneley (1966) proposed a graben-like struc-ture. The rift rift failed (RRF) triple junction model leading toplate dilation and opening of the Gulf of Guinea was proposedby Grant (1971). Olade (1975) considered the Benue Troughas the third failed arm or aulocogen of a three-armed riftsystem related to the development of hotspots. Benkhelil(1982, 1989) and Guiraud and Maurin (1992) considered

Fig. 1 a Regional tectonic map of western and central African rifted basins showing b the Nigerian Benue Trough and study area (adapted afterAbubakar 2006, 2014)

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wrench faulting as the dominant tectonic process during theBenue Trough evolution and defined it as a set of juxtaposedpull-apart basins.

The Benue Trough is geographically sub-divided intoSouthern, Central and Northern portions (Nwajide 2013).The Northern Benue Trough is made up of two major sub-

Fig. 2 Lithologic description of Yola Sub-basin successions and sedimentary log of the studied sediments of the Lamja Formation with location of thecollected shale and coals samples

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basins: the N–S trending Gongola Sub-basin and the E–Wtrending Yola Sub-basin (Fig. 1b). Carter et al. (1963),Offodile (1976), Benkhelil (1989), Zarboski et al. (1997)and Abubakar (2006) have described in detail the geologyand stratigraphy of the Northern Benue Trough. The strati-graphic succession in the Yola Sub-basin of the NorthernBenue Trough (Fig. 2) comprises the continental LowerCretaceous Bima Formation, the Cenomanian transitional ma-rine Yolde Formation, the marine late Cenomanian–NumanhaShales and Lamja formations (Carter et al. 1963; Abubakar2006; Sarki Yandoka et al. 2014).

The Lamja Formation was earlier described as “carbonaceousbeds” by Carter et al. (1963) and conformably overlies theNumanha Shales (Nwajide 2013; Abubakar 2006, 2014). Itconsists of a crystalline and shelly limestone, siltstone and yel-lowish to whitish fine-grained well-bedded sandstone, dark greyshale and dark coals (Fig. 2) deposited in a relatively shallowmarine environment (Carter et al. 1963; Nwajide 2013). Thisformation terminates the sedimentation of the Yola Sub-basin inthe Northern Benue Trough and was dated Santonian (Carteret al. 1963). Volcanic plugs of Tertiary age have been reported asintrusions in the Yola Sub-basin by Carter et al. (1963) andWright (1976) (Fig. 1b) although not significantly affecting thesediments of the Lamja Formation in the region.

Sampling and experimental methods

A total of 18 outcrop samples were collected from shale and coalintervals within the Lamja Formation from the Yola Sub-basinthat represent different sedimentary facies (Fig. 2). Sinceweathering is always a factor of concern for organic geochemicaland petrographic studies of outcrop sediments, the weatheredrock surfaces were removed by digging to approximately 0.5 min each sampling point. Prior to analysis, the samples werescrubbed and exhaustively cleanedwith distilledwater to removetraces of surficial dirt and plant growth and then dried at 35 °C for12 h. All of the samples were selected for organic geochemicaland petrographic analyses, which were carried out at theDepartment of Geology in the University of Malaya, Malaysia.

The samples were crushed into a fine powder (<150 μm)and screened using (SRA-Weatherford)-TOC/TPH instrument(equivalent to Rock-Eval instrument). Pyrolysis analysis wasperformed on 15 and 80 mg crushed coal and shale samples,respectively, which were heated to 600 °C in a helium atmo-sphere and measured several parameters such as S1, S2 andtemperature of maximum pyrolysis yield (Tmax) (Table 1).Total organic carbon (TOC) content was determined using aMulti EA2000 Analyser. Pyrolysis data are reported in thispaper as characterising, respectively, the organic richness,kerogen type, petroleum generation potential of the organicmatter and its thermal maturity (Espitalié et al. 1980; Espitaliéet al. 1985; Peters and Cassa 1994). Following the pyrolysis,

the samples were selected for further molecular geochemicalanalyses and vitrinite reflectance measurements.tgroup

For molecular geochemical analyses, about 30 g for shale and12 g for coal were subjected to bitumen extraction with Soxhletapparatus for 72 h using an azeotropic mixture of dichlorometh-ane (DCM) and methanol (CH3OH) (93:7). The extracts wereseparated into saturates, aromatics and nitrogen, sulfur and ox-ygen (NSO) compounds by liquid (column) chromatography.The saturated hydrocarbon fractions were dissolved in hexaneand analysed by a gas chromatography-mass spectrometry (GC-MS) on a HP 5975B MSD mass spectrometer with a gaschromatograph attached directly to the ion source (70 eVionisation voltage, 100 mA filament emission current, 230 °Cinterface temperature). Some selected saturated fractions weresubsequently analysed using gas chromatography-doublet massspectrometry (GC-MS/MS) onAgilent 7000BTriple quad, fittedwith a fused silica capillary column (60 m×0.25 mm I.D.,0.25 μm film thickness). Helium was the carrier gas at 30 psiconstant pressure, and the column was heated from 150 to300 °C at 2 °C/min, with a final holding temperature at 300 °Cfor 30 min. Vitrinite reflectance measurements were performedon polished blocks using Leica DM6000M microscope under amonochromatic light at 546 nm and using an optical sapphireglass standard having a reflectance of 0.589 % in oil immersion,following the procedures outlined by Taylor et al. (1998).

Results and discussion

Organic matter richness and generative potential

Organic matter richness and generative potential of organicmatter from Lamja Formation shales and coals were evaluatedusing pyrolysis S2 yield, TOC content and bitumen extractiondata (e.g. extractable organic matter (EOM) and hydrocarbonyields) (Table 1). The shales have relatively low to fair TOCcontent (0.88–1.16 wt%) and EOM yield (1167–896 ppm),whereas the coaly sediments as expected contain higher TOCcontent in the range 45.23–67.85 wt% and extractable bitu-men in the range 3876–17,987 ppm. The extractability and,hence, the proportion of organic carbon content of Lamjashale and coal sediments are high enough to classify them aspossessing good to excellent source rock generative potential(Tissot and Welte 1984; Peters and Moldowan 1993; Petersand Cassa 1994; Hunt 1995). In addition to the determinationof TOC and EOM content, the amount of hydrocarbon yield(S2) generated during pyrolysis is also a useful parameter toevaluate the generation potential of source rocks (Peters 1986;Bordenave et al. 1993). In the analysed samples, the hydro-carbon (S2) yield ranges from 0.82 to 125.53 mg hydrocarbon(HC)/g rock for all lithologies (Table 1). The pyrolysis S2 yieldfor shales ranges from 0.82 to 1.41 mg HC/g rock, while forcoals, it ranges from 45.49 to 125.53 mg HC/g rock (Table 1).

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The hydrocarbon yields (S2) are in agreement with TOCcontent, indicating that the Lamja coal and shale sedimentshave fair to excellent source rock generative potential basedon the classification given by Peters and Cassa (1994) (Fig. 3).The coal samples can act as very promising source rock forhydrocarbon generation as reflected by the high S2 and highTOC (wt%) content (Fig. 3).

Organic matter quality

To interpret the organic data in terms of paleoenvironmentalchanges and quality, information about the composition whichdiscriminates between marine and terrigenous sources is nec-essary (Stein 1991). Kerogen typing is also considered toproduce different types of hydrocarbons. Generally, type Iand II kerogens commonly derived from lacustrine andmarinesource rocks are capable of generating liquid hydrocarbons(Hakimi et al. 2011). Type III kerogen is mostly composed ofwoody materials and gas prone, and type IV is composedprimarily of inert materials and has no potential of generatinghydrocarbons (ibid). Based on the pyrolysis data, the kerogenclassification diagrams were constructed using hydrogen in-dex (HI) versus Tmax as carried out by previous workers (e.g.

Mukhopadhyay et al. 1995). The pyrolysis data (HI againstTmax) (Fig. 4) indicated that the analysed Lamja Formationsamples generally plot in the mature zone of mixed type II–IIIkerogens grading to type III kerogen (Fig. 4). This correspondsto their HI values in the range of 90–228 mg HC/g TOC(Table 1). Most samples are plotted in the type III field in thisdiagram, while some coal samples plotted within the mixedtype II–III kerogens (Fig. 4). These suggest that the LamjaFormation sediments can be expected to generate mainly gaswith limited capability to generate liquid hydrocarbons.

Molecular geochemistry

The gas chromatography-mass spectrometry (GC-MS) analy-sis was performed on the saturated hydrocarbon fraction forthe analysed Lamja coal and shale samples. The normalalkanes and acyclic isoprenoids ratios were determined basedonm/z 85 of GC-MS fragmentation (Fig. 5) and the calculatedratios were tabulated in Table 1. Distributions of tricyclicterpanes, hopanes and steranes were performed on m/z 191and m/z 217 (Fig. 6), respectively, and determined based onthe retention time and comparison with published works (e.g.Peters et al. 2005; Korkmaz and Kara Gülbay 2007; Hakimi

Table 1 Bulk geochemical data and ratios based on the distributions of n-alkanes and isoprenoids of the analysed Lamja shale and coal sediments

SampleID

Lithology %Ro TOC and pyrolysis data (SRA) Bitumen extraction and chromatographicfractions (ppm of whole rocks)

n-Alkanes and acyclicisoprenoids

S1(mg/g)

S2(mg/g)

Tmax

(°C)HI PI TOC

(wt %)EOM(ppm)

Sat(ppm)

Arom(ppm)

NSO(ppm)

HC(ppm)

Pr/Ph

Pr/n-C17

Pr/n-C18

CPI WI

LSS2 Shale 0.78 0.22 0.82 445 93.2 0.21 0.88 896 186 375 335 561 1.77 0.90 0.60 1.01 1.44

LSS5 Shale 0.76 0.23 1.38 445 129 0.14 1.07 988 231 372 385 603 4.16 0.85 0.21 1.08 0.81

LSS8 Shale 0.82 0.35 1.04 451 90.0 0.25 1.16 1167 278 448 441 726 3.60 1.52 0.38 1.08 1.39

LSS55 Shale – 0.33 1.41 446 130 0.20 1.08 1031 258 444 329 702 1.80 0.93 0.47 1.03 1.51

LSS1A Coal 0.61 2.78 115.48 438 228 0.02 50.70 13,561 2874 4780 5907 7654 2.29 3.14 0.98 1.12 2.34

LSS1B Coal 0.58 2.63 116.1 437 197 0.02 58.84 12,963 3120 4301 5542 7421 2.18 2.92 1.04 1.15 2.43

LSS3A Coal 0.64 0.76 47.31 438 98.3 0.02 48.15 5876 1421 1611 2844 3032 2.28 2.14 0.98 1.16 2.62

LSS3B Coal 0.62 0.72 45.49 438 101 0.03 45.23 5763 1842 1836 2085 3678 2.82 3.67 0.87 1.13 2.33

LSS6A Coal 0.68 0.52 56.15 442 100 0.01 56.17 4021 1011 1112 1898 2123 2.75 3.33 0.95 1.11 2.36

LSS6B Coal 0.63 0.77 55.17 443 95.0 0.01 58.10 3876 986 1557 1333 2543 3.48 5.40 0.82 1.18 2.56

LSS7A Coal 0.60 2.51 100.67 437 209 0.02 48.28 11,531 3682 4442 3407 8124 2.38 2.69 0.96 1.14 2.41

LSS7B Coal 0.57 2.64 102.59 438 151 0.03 67.85 17,987 3968 5853 8166 9821 2.44 3.33 0.98 1.18 2.27

LSS9A Coal 0.67 1.88 92.42 439 188 0.02 49.15 10,412 3012 3777 3623 6789 2.27 2.83 1.15 1.25 2.78

LSS9B Coal 0.66 2.34 96.98 438 167 0.02 58.32 14,876 3687 5544 5645 9231 2.08 2.72 1.06 1.12 1.98

LSS10A Coal 0.61 2.89 125.53 436 194 0.02 64.67 13,987 3014 5961 5012 8975 1.82 2.19 1.25 1.15 2.89

LSS10B Coal 0.65 0.66 59.85 445 111 0.01 53.88 5709 1324 1710 2675 3034 3.75 5.83 0.88 1.12 2.41

LSS13A Coal 0.64 2.61 100.79 437 158 0.03 63.98 12,976 3254 4711 5011 7965 2.97 3.79 0.95 1.20 2.56

LSS13B Coal 0.68 2.17 94.1 438 152 0.02 62.12 9876 2654 4318 2904 6972 2.49 5.74 0.82 1.17 2.27

S1 volatile hydrocarbon (HC) content (mg HC/g rock), S2 remaining HC generative potential (mg HC/g rock), HI hydrogen index=S2×100/TOC (mgHC/g TOC), TOC total organic carbon (wt%), Tmax temperature at maximum, Pr pristane, Ph phytane, EOM bitumen extraction (ppm), Sat saturationfractions, Arom aromatic fractions, HC hydrocarbon fractions = (saturation + aromatic), CPI carbon preference index (1): {2 (C23+C25+C27+C29)/(C22+2 [C24+C26+C28]+C30)}, %Ro vitrinite reflectance, WI waxiness index—∑ (n-C21–n-C31)/∑ (n-C15–n-C20)

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and Wan Hasiah 2013). C30 tetracyclic polyprenoids 21R (Ta)and 21S (Tb) isomers were determined on m/z 414–259 and27-norcholestanes on m/z 358–217 from GC-MS/MS transi-tions and interpreted based on the published work of Holbaet al. (2003). The C30 tetracyclic polyprenoid (TPP) com-pounds were calculated as follows: [TPP ratio=(2×peakTa)/(2×peak Ta)+∑20R steranes]. The biomarker peaks areshown in the Appendix and the calculated ratios were tabu-lated in Table 2.

n-Alkanes and acyclic isoprenoids

The chromatograms of the analysed Lamja Formation shaleand coal display prominent saturated hydrocarbon distribu-tions between n-C12–n-C33 n-alkanes and dominant pristane(Pr) over phytane (Ph) isoprenoid hydrocarbons (Fig. 5). Then-alkane distribution shows a bimodal distribution with pre-dominance of high molecular weight compounds (n-C23–n-C29), which support high terrigenous land-derived organicmatter contribution with small aquatic organic matter input(Ebukanson and Kinghorn 1986; Murray and Boreham 1992).The distribution is depleted in the n-C12–n-C19 range andshows an odd predominance of the heavier members (n-C25+) which gave moderate carbon preference index (CPI)values in the range of 1.01–1.25 (Table 1).

Acyclic isoprenoids occur in a significant amount (Fig. 5).Pristane generally occurs in relatively high concentrations com-pared to phytane, possessing Pr/Ph ratios in the range of 1.77–4.16 (Table 1), which suggest that the Lamja Formation shale andcoal sediments were deposited under suboxic to relatively oxicconditions (Peters and Moldowan 1993). Furthermore, therewere higher amounts of isoprenoids pristane compared to n-

alkanes (Fig. 5), thus giving distinctively high pristane/n-C17

and low phytane/n-C18 ratios in the range of 0.85–5.83 and0.21–1.25, respectively (Table 1). Waxiness index was alsocalculated to provide some insights into the source input of theorganic matter (Table 1). This index may be used to determinethe amount of land-derived organic materials in the sediments(Peters et al. 2005). Lamja sediments generally contain variablewaxy ratios in the range of 0.81–2.89 (Table 1).

Terpane and sterane biomarkers

Terpane and sterane biomarkers were measured from m/z 191 tom/z 217 mass fragmentograms, respectively (Fig. 6). The m/z191 fragmentograms of the saturated hydrocarbon fractions of allthe analysed shale and coal samples show moderate abundancesof pentacyclic and tricyclic terpanes with a significant occurrenceof C24 tetracyclic terpanes (Fig. 6). The relative abundance of C29

to C30 hopane is generally similar in most of the studied sampleswith C29/C30 ratios ranging from 0.93 to 1.53 (Table 2). Thepredominance of C29 hopane is frequently associated with car-bonate-rich, but this is not always the case (Waples andMachihara 1991), although the enhanced norhopane input mayalso be associated with land plant input (Rinna et al. 1996) whichis the case for the samples analysed in this current study. The Tm(C27 17α(H)-22,29,30-trisnorhopane) predominates over Ts (C27

18α(H)-22,29,30-trisnorneohopane) with Tm/Ts ratios rangingfrom 2.6 to 29 (Table 2). The 18α(H)-oleanane, which is animportant land plant-derived biomarker (Peters et al. 2005; PetersandMoldowan 1993), was identified in low proportion in almostall of the analysed samples (Fig. 6a). The studied samples displayvariable oleanane index (oleanane/C30 hopane) in the range of0.04–0.11 (Table 2). Extended hopanes are dominated by the C31

Fig. 3 Relationship betweenremaining hydrocarbon potential(S2) and total organic carbon(TOC, wt%) for the analysedLamja coal and shale samples

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homohopane and decreasing towards the C34 homohopane(Fig. 6a). The αβ-hopanes are more prominent than the βα-hopanes, while the S-isomers are more dominant than the R-isomers among the homohopanes (C31–C34). The concentrationof C26 tricyclic terpanes is much less than that of C24 tetracyclicsin most of the analysed samples in this study (represented byC24Te/C26T) (Table 2). Selected ratios of other tricyclic terpaneswere also calculated and give some insights into the source oforganic material (Peters et al. 2005; Alias et al. 2012) asdiscussed in the section “Organic matter source inputs andpaleodepositional conditions.” The 17β,21α(H)-moretane wasalso detected in all the samples though in relatively low concen-trations (Table 2).

The steranes are another group of important biomarkersthat are derived from sterols found in higher plants and algaebut rare or absent in prokaryotic organisms (Volkman 1986).Them/z 217 mass fragmentograms of all the analysed samplesare dominated by steranes over diasteranes with C29 sterane

being the predominant component (Fig. 6b). The relativeproportions of each of the regular steranes (C27, C28 andC29) can vary greatly from sample to sample, depending uponthe type of organic matter input to the sediment. Relativeabundances of C27, C28 and C29 regular steranes and the ratiosof C29/C27 regular sterane, diasterane/sterane and two thermalmaturity based on sterane ratios are calculated and the resultsare given in Table 2. The Lamja shale and coal extracts show ahigh proportion of C29 (48.1–68.9 %) compared to C27 (10.7–32.8 %) and C28 (17.2–29.9 %) steranes as shown in Table 2.

Thermal maturity of organic matter

In this study, a number of data types were used to assess thelevel of thermal maturity of organic matter in the CretaceousLamja coal and shale sediments of the Yola Sub-basin,Northern Benue Trough. The maturity data include meanvitrinite reflectance (%Ro), Tmax values and biomarker

Fig. 4 Plot of hydrogen index(HI) versus pyrolysis Tmax for theanalysed Lamja coal and shalesamples, showing kerogen qualityand thermal maturity stages

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maturity ratios (Tables 1 and 2), which suggested that theLamja samples are thermally mature. This is consistent withvitrinite reflectance and Tmax values of coal and shale sedi-ments (Table 1). The mean reflectance of vitrinite particlesranges from 0.57 to 0.82 % (Table 1), corresponding to earlymature to peak oil window maturity. This is in good agree-ment with the Tmax values (436–451 °C), as illustrated by agood correlation between Tmax and %Ro (R

2=0.68) (Fig. 7).The biomarker maturity parameters of the coal and shaleextracts (Table 2) such as C32 hopane 22S/(22S+22R),

moretane/hopane, C29 sterane 20S/(20S+20R) and ββ/(ββ+αα) ratios also were used as maturity indicators(Mackenzie et al. 1980; Waples and Machihara 1991). Theratios of 22S/(22S+22R) for C32 hopanes are between 0.57and 0.64 (Table 2), suggesting that they have reached equilib-rium and have reached oil window maturity (Seifert andMoldowan 1986). The ratios of C29 sterane 20S/(20S+20R),maturity ratios of the extracted samples, are 0.32–0.44(Table 2), equivalent to the onset of oil generation (Seifertand Moldowan 1978, 1981). The ββ/(ββ+αα) sterane ratios

Fig. 5 Mass fragmentograms m/z 85 of saturated hydrocarbons of some of the studied Lamja shale and coal samples

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also increased with increasing maturity (Seifert andMoldowan 1981). The Lamja coal and shale extracted sam-ples have ββ/(ββ+αα) sterane ratios in the range of 0.43 to0.59 (Table 2). These values are mostly either at or close tothermal equilibrium and support a maturation level indicatingonset to peak the oil window generating range. This is sup-ported by moretane/hopane ratios consistent with low relative

abundance of C30 moretane (Fig. 6a). Moretane converts toC30 hopane with increasing thermal maturity (Seifert andMoldowan 1986), and thus, moretane decreases as thermalmaturity increases. The Lamja coal and shale samples havemoretane/hopane ratio in the range of 0.10–0.36, which sug-gests that the extracted samples are thermally mature(Mackenzie et al. 1980; Seifert and Moldowan 1986).

Fig. 6 The m/z 191 mass fragmentograms (a) and m/z 217 mass fragmentograms (b) of saturated hydrocarbon fractions of some of the studied Lamjashale and coal samples

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Tab

le2

Selected

biom

arkerparametersof

theLam

jasamples

illustratingsource

organicmatteranddepositio

nalenvironmentsas

wellasthermalmaturity

oftheanalysed

samples

Sam

ple

IDLith

ology

Terpanes

(m/z191)

Steranesanddiasteranes(m

/z217)

Hopanes/

(hopanes

+∑20R

steranes)

TPP

ratio

sHopane

Tricyclic(T)andTetracyclic

(Te)

terpanes

C29

20S/

20S+

20R

C29

ββ/

ββ+

αα

Regular

steranes

(%)

C29/C

27

Diasterane/

sterane

Csterane

29/C30

C31

22R/

C30H

Ol/

C30

C32

22S/

(22S

+22R)

MC30/

HC30

Tm/

TsC21T/

C23T

C22T/

C21T

C24T/

C23T

C24Te/

C26T

C23T/

C24T

C23T/

C24Te

C27

C28

C29

LSS

2Sh

ale

1.18

0.31

0.06

0.58

0.36

11.3

1.86

0.46

0.47

3.75

2.14

0.33

0.43

0.56

20.2

22.1

57.7

2.85

0.60

0.72

0.24

LSS

5Sh

ale

1.05

0.19

0.05

0.61

0.26

2.7

1.76

0.34

0.76

2.21

1.31

0.90

0.42

0.56

21.1

20.2

54.1

2.34

0.68

––

LSS

8Sh

ale

1.44

0.18

–0.61

0.24

11.5

1.92

0.24

0.24

1.65

2.16

0.39

0..41

0.54

32.8

19.1

48.1

1.46

0.57

0.71

0.26

LSS

55Sh

ale

1.03

0.13

0.11

0.63

0.10

2.6

1.40

0.28

0.65

3.33

1.54

1.0

0.38

0.58

28.3

17.2

54.5

1.93

1.20

––

LSS

1ACoal

1.26

0.22

0.08

0.58

0.27

262.33

0.58

1.0

3.29

1.02

0.13

0.41

0.43

18.5

21.5

60.0

3.24

0.54

––

LSS

1BCoal

1.09

0.24

0.06

0.57

0.24

192.35

0.57

1.03

3.28

1.03

0.13

0.39

0.58

18.0

23.5

58.5

3.25

0.55

0.77

0.24

LSS

3ACoal

1.28

0.20

0.06

0.60

0.23

252.23

0.64

1.02

2.43

0.80

0.15

0.38

0.54

15.5

20.0

64.5

4.16

0.56

––

LSS

3BCoal

1.27

0.22

0.04

0.62

0.23

282.33

0.71

1.0

2.33

1.03

0.14

0.37

0.52

15.0

20.5

64.5

4.30

0.46

0.73

0.26

LSS

6ACoal

1.37

0.23

0.04

0.59

0.25

232.34

0.72

1.06

2.55

1.04

0.11

0.32

0.59

14.4

19.1

66.5

4.62

0.53

––

LSS

6BCoal

1.41

0.26

0.04

0.61

0.26

182.0

0.67

1.0

2.38

1.01

0.11

0.36

0.58

15.3

18.0

66.7

4.36

0.58

––

LSS

7ACoal

1.20

0.24

0.08

0.59

0.24

292.67

0.56

1.07

2.67

1.05

0.13

0.42

0.55

15.4

22.3

62.3

4.05

0.52

–

LSS

7BCoal

1.18

0.23

0.07

0.58

0.25

242.0

0.58

0.93

2.70

1.00

0.14

0.41

0.50

17.2

19.7

63.1

3.67

0.49

0.72

0.23

LSS

9ACoal

1.20

0.20

0.07

0.61

0.23

191.91

0.46

0.94

3.39

1.07

0.19

0.40

0.51

14.6

23.8

61.6

4.22

0.50

––

LSS

9BCoal

1.19

0.23

0.08

0.64

0.27

272.0

0.45

0.80

3.37

1.25

0.18

0.42

0.55

14.8

23.4

61.8

4.18

0.58

––

LSS

10A

Coal

1.03

0.22

0.06

0.58

0.32

291.36

1.09

1.27

3.33

1.12

0.14

0.44

0.58

15.8

29.9

54.3

3.44

0.59

––

LSS

10B

Coal

1.51

0.19

0.05

0.61

0.23

192.4

0.50

1.2

3.35

0.83

0.16

0.32

0.57

16.5

28.6

54.9

3.33

0.60

0.78

0.25

LSS

13A

Coal

1.53

0.21

0.09

0.60

0.22

212.0

0.61

1.11

2.1

1.02

0.12

0.39

0.51

10.7

20.4

68.9

6.44

0.51

––

LSS

13B

Coal

0.93

0.18

0.07

0.62

0.23

221.8

0.67

1.20

2.90

0.88

0.14

0.37

0.57

14.7

18.5

66.8

4.54

0.39

––

C29/C

30C29norhopane/C30hopane,C

30M/C

30HC30moretane/C30hopane,O

l/C30Holeanane/C

30hopane,Ts(C

2718a(H)-22,29,30-trisnorneohopane),Tm(C

2717a(H)-22,29,30-trisnorhopane),TP

Pratio

(2×peak

Ta)/(2×peak

Ta)+

∑20Rsteranes

Arab J Geosci

Overall, the biomarker maturity ratios indicate that the coaland shale extract samples have been exposed to a thermalmaturity level of early to peak oil window stage of petroleumgeneration (Fig. 8).

Organic matter source inputs and paleodepositionalconditions

In this study, biomarker distributions have been used to de-scribe source input of organic matter and depositional

environment conditions of the Lamja shale and coal sedi-ments. The paleodepositional environment was primarily ex-amined through the use of biomarkers such as n-alkanes,acyclic isoprenoids, sterane and terpane distributions (Figs. 5and 6), and parameters were calculated from their distributions(Tables 1 and 2). The n-alkane distributions are consistentwith a dominant source of terrestrial higher plants althoughreceiving a minor aquatic organic matter as indicated by apredominance of odd carbon number alkanes to even carbonnumber alkanes in the gas chromatograms of the analysed

Fig. 7 Cross-plots of pyrolysisTmax versus vitrinite reflectance(%Ro) of the Lamja coal and shalesamples, showing goodrelationship between them

Fig. 8 Cross-plot of twobiomarker parameters sensitive tothermal maturity of the Lamjacoal and shale extracts whichshows that most of the samplesplot in the area of early mature topeak oil window maturity(modified from Peters andMoldowan 1993)

Arab J Geosci

samples (Fig. 5). The long-chain n-alkanes (>n-C23) are char-acteristic biomarkers for higher terrestrial plants (Eglinton andHamilton 1967), whereas the short-chain n-alkanes (<n-C20)are predominantly found in algae and microorganisms(Cranwell 1977; Peters et al. 2005). The long-chain n-alkanesof the sediments are also characterised by variable odd overeven predominance (CPI according to Bray and Evans 1961).Relatively higher CPI values (>1.0) are obtained from theLamja coal and shale extracts (Table 1). These CPI valuesare accompanied by the presence of significant long-chaincompound alkanes (+n-C23), thus supporting a terrigenousorganic matter input deposited under relatively oxic condi-tions (Akinlua et al. 2007; Meyers and Snowdon 1993). Thepristane which is much higher than phytane (Fig. 5) alsosupported this high terrigenous organic matter input (Didyket al. 1978; Peters et al. 2005). The Pr/Ph ratio is one of themost commonly used geochemical parameters and has beenwidely invoked as an indicator of the redox conditions in thedepositional environment and source input of organic matter(Powell and McKirdy 1973; Didyk et al. 1978; Tissot andWelte 1984; Chandra et al. 1994; Large and Gize 1996).Organic matter originating predominantly from terrestrialplants would be expected to contain high Pr/Ph ratio of >3.0(oxidising conditions), while low values of Pr/Ph ratio (<0.6)indicate anoxic conditions, and values between 1.0 and 3.0suggest intermediate conditions (suboxic conditions) (PetersandMoldowan 1993). The Pr/Ph ratios for the Lamja coal andshale samples are in range of 1.82–4.16 (Table 1), thus show-ing that the high terrigenous organic matter input was depos-ited under suboxic to relatively oxic conditions. Furthermore,pr/n-C17 and ph/n-C18 ratios suggest a significant contributionof terrigenous organic matter with a small amount of aquatic

organic matter input that was preserved under suboxic torelatively oxic conditions (Fig. 9). The relative distributionof C27, C28 and C29 regular steranes is graphically representedin the form of a ternary regular steranes diagram in Fig. 10(adapted after Huang andMeinschein 1979). This diagram hasoften been employed to represent the relative proportions ofthe C27, C28 and C29 regular steranes and can provide valuablepaleoenvironmental information. Dominance of C27 steraneswould indicate a preponderance of marine phytoplankton,whereas a dominance of C29 would indicate a strong landplant terrestrial contribution and C28 steranes might indicatea heavy contribution by lacustrine algae. Based on this ternaryclassification, the analysed Lamja coal and shale samplescontain a high contribution of terrestrially derived organicmatter with minor aquatic organic matter contributions(Fig. 10), thus displaying a strong predominance of C29

steranes (Table 2). The C29/C27 regular sterane ratio in thesamples that ranges from 1.46 to 6.44 further supported theabove interpretation. Applying the oleanane parameter toindicate angiosperm input in rocks of Late Cretaceous oryounger age to the coal and shale sediments of the Lamjashows that the Lamja samples have measurable amounts ofoleanane (Fig. 6a) which are a strong indicator of terrestrialangiosperm plant as initially reported by Ekweozor andTelnaes (1990). The presence of oleanane suggests a probablemarine influence as indicated by an earlier work of Murrayet al. (1997) and as recently reported for Sabah Tertiary coals(Alias et al. 2012). The Lamja sediments have relatively lowC24/C23 and low C22/C21 tricyclic terpane values (Table 1),indicating deltaic mixed organic matter with a major contri-bution of terrigenous organic matter and minor aquatic organ-ic matter input. The deltaic depositional environmental

Fig. 9 Phytane to n-C18 alkane(Ph/n-C18) versus pristane to n-C17 alkane (Pr/n-C17), showingdepositional conditions and typeof organic matter of the analysedLamja samples

Arab J Geosci

conditions of the Lamja sediments have also beeninterpreted using C31-22R-hopane/C30-hopane ratio(Table 2). This ratio is generally higher than 0.25 formarine environments while lower than 0.25 for lacustrinesettings (Peters et al. 2005). However, the ratios vary in theanalysed sediments of Lamja formations (Table 2), suggest-ing that the Lamja sediments were deposited in deltaicenvironments (Fig. 11). This also concurs with the findingbased on C30 TPP ratios (Table 2). C30 TPPs are promi-nently observed in samples derived from low salinity, i.e. freshto brackish lacustrine environments, and are generally presentin low levels in samples derived from saline, i.e. marine and

saline lacustrine environments (Holba et al. 2003; Dzou et al.1999). TPP ratios of the analysed Lamja coal and shalesamples are in the range 0.23–0.27, indicating that the sampleswere deposited in deltaic environment as indicated by the plotof TPP against hopanes/hopanes+∑20R sterane ratios(Fig. 12). This deltaic depositional environment setting is inagreement with the earlier works of Carter et al. (1963) andAbubakar (2006).

Hydrocarbon generation potential

Kerogen type and the associated hydrocarbons that might begenerated were characterised based on organic geochemicaldata in relation to source of organic matter input. Severalstudies have shown that there is a direct correlation betweenpyrolysis data (HI) and hydrocarbon generation potential (e.g.Bordenave et al. 1993; Hunt 1995). Samples that contain typeIII kerogen would be expected to generate gas with HI valuesless than 200 mg HC/g TOC, whereas samples with HI valueshigher than 200 mg HC/g TOC can generate oil although theirmain generation products are gas and condensate. Moreover,samples that have hydrogen index greater than 300 mg HC/gTOC can generate oil (Bordenave et al. 1993; Hunt 1995). Inthis respect, most of the Lamja Formation samples under thecurrent investigation have HI lower than 200 mg HC/g TOCand can generate gas if they are subjected to sufficient burialand heating. This is suggested by the abundance of type IIIkerogens (Fig. 6). In contrast, the analysed Lamja sampleswith HI values higher than 200 mg HC/g TOC can generatelimited liquid hydrocarbons. This is as indicated by organicmatter input and depositional environment conditions. TheLamja sediments contain a high contribution of land plants

Fig. 11 Cross-plot of hopaneratios versus pristane/phytane ofthe analysed Lamja samples(modified after Peters et al. 2005)

Fig. 10 Ternary diagram of regular steranes (C27–C29) indicating therelationship between sterane compositions and organic matter input,showing that the analysed Lamja coal and shale extracts are composedof mixed organic matter (modified after Huang and Meinschein 1979)

Arab J Geosci

and minor aquatic organic matter input that were depositedunder oxic to relatively suboxic conditions, typical of deltaplain/coastal marine environment of deposition. The high-gasgeneration potential of Lamja sediments was due to the highcontribution of land plant inputs that were deposited underoxic conditions, whereas the limited liquid hydrocarbons areattributed to a minor aquatic organic matter input. Therefore, ahigh prospect for major and minor gas is anticipated from theLamja Formation sediments in the Yola Sub-basin of theNorthern Benue Trough.

Conclusions

An integrated geochemical and molecular characterisation ofthe Cretaceous Lamja Formation sediments from Yola Sub-basin in the Northern Benue Trough was performed in order todetermine the organic matter richness, source inputs,paleodepositional conditions and hydrocarbon generation po-tential and has consequently revealed the following:

1. The Lamja Formation shales and coals possessed poor tooccasionally very good source generative potential. Coalshave better generative potential compared to the shales,on account of more organic matter richness and very highyield of EOM.

2. High-gas generation potential is anticipated from theLamja Formation sediments with HI values generallybelow 200 mg HC/g TOC (type III kerogen). Coals havefairly mixed type II–III kerogens are and expected togenerate limited components of liquid hydrocarbons.The land-derived kerogen with minor aquatic organicmatter input is supported by biomarker distributions.

3. The pyrolysis Tmax, %Ro and biomarker maturity param-eters of the Cretaceous Lamja Formation sediments fromthe Yola Sub-basin indicate their maturities as early topeak oil generation window.

4. The biomarkers of the saturated hydrocarbon fractionsindicate that the Lamja Formation shales and coals arelikely to have been deposited in a delta plain marineenvironment under oxic to relatively suboxic conditions.This suggests that most of the sediments were not pre-served in low bottom oxygen levels (anoxia condition)and this may have reduced their potential to generate liquidhydrocarbons. Consequently, the Lamja Formation shalesand coals from the Yola Sub-basin have high prospect forgas generation although minor oil can be expected.

Acknowledgments This study received support from the National Cen-tre for Petroleum Research and Development, Energy Commission ofNigeria (NNBT Project Research Fund), for funding the field trips andsample collection. The research was also funded by the University ofMalaya IPPP Research Fund (PG140-2012B). Grateful acknowledgementis given to Miss Hajah Zaleha Abdullah, Mr. Mohamed Zamri Rashid andMrs. Maisarah Binti Yusoff(Geology Department) for analytical assistance.

Fig. 12 Cross-plot TPP ratiosversus hopane/(hopane+∑20Rsteranes) indicating deltaicdepositional environment ofLamja coal and shale sediments(modified after Holba et al. 2003)

Arab J Geosci

Appendix

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Table 3 Peak assignments for saturated hydrocarbon fractions in them/z 191 mass fragmentograms

Peak no. Compound abbreviation

C21 C21 tricyclic (cheilanthane) Tri C21

C22 C22 tricyclic (cheilanthane) Tri C22

C23 C23 tricyclic (cheilanthane) Tri C23

C24 C24 tricyclic (cheilanthane) Tri C24

C24 C24 tetracyclic Tetra C24

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C26 C26 tricyclic (cheilanthane) Tri C26

Ts 18α(H),22,29,30-trisnorneohopane Ts

Tm 17α(H),22,29,30-trisnorhopane Tm

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C30 17α,21β(H)-hopane Hopane

C30M 17β,21α(H)-moretane C30Mor

C31S 17α,21β(H)-homohopane (22S) C31(22S)

C31R 17α,21β(H)-homohopane (22R) C31(22R)

C32S 17α,21β(H)-homohopane (22S) C32(22S)

C32R 17α,21β(H)-homohopane (22R) C32(22R)

C33S 17α,21β(H)-homohopane (22S) C33(22S)

C33R 17α,21β(H)-homohopane (22R) C33(22R)

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