ORIGINAL PAPER. Keywords Cretaceous Lamja Formation. Hydrocarbon generation potential. Total organic carbon. Introduction

Transcription

1 DOI /s ORIGINAL PAPER Organic geochemical characteristics of Cretaceous Lamja Formation from Yola Sub-basin, Northern Benue Trough, NE Nigeria: implication for hydrocarbon-generating potential and 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 characterisation of the Cretaceous Lamja Formation shale and coal sediments from the Yola Sub-basin, Northern Benue Trough, northeastern Nigeria, has been undertaken to provide an overview on the origin, richness, hydrocarbon generation potential and paleodepositional conditions. This study is based on geochemical analyses of whole rock (total organic carbon content, pyrolysis, bitumen extraction and biomarker distributions) and vitrinite measurements. The total organic carbon (TOC) contents of the Lamja Formation range from 0.8 to 63 % and 0.8 to 1.16 % for coal and shale samples, respectively, with an average TOC value of %. The hydrogen index of these samples ranges from 93.1 to 228 mg hydrocarbon (HC)/g TOC. The kerogen is predominantly type III with a significant mixture of type II kerogens, indicative of mainly gas with limited liquid hydrocarbon-generating potential. The analysed Lamja Formation samples have vitrinite reflectance in the range of %R o and pyrolysis temperature at B. M. Sarki Yandoka: W. H. Abdullah : K. A. Mustapha : A. K. Adegoke Department of Geology, University of Malaya, Kuala Lumpur, Malaysia B. M. Sarki Yandoka (*): M. B. Abubakar National Centre for Petroleum Research and Development, ATBU, Bauchi, Nigeria bmsydgombe@yahoo.com M. H. Hakimi Geology Department, Faculty of Applied Science, Taiz University, 6803 Taiz, Yemen A. K. Adegoke Department of Geology, Ekiti State University, P.M.B. 5363, Ado Ekiti, Nigeria maximum (T max ) in the range of C which indicate that the samples are thermally mature and entered early mature to peak oil window stage. The molecular geochemical biomarkers are characterised by dominant odd carbon numbered n-alkanes in the range of n-c 23 to n-c 33,moderatelyhigh pristane/phytane (Pr/Ph) ratios ( ), very high C 27 17α(H)-22,29,30-trisnorhopane/C 27 18α(H)-22,29,30- trisnorneohopane (Tm/Ts) ratios (>10) and high concentrations of regular sterane C 29, indicating suboxic to oxic conditions, typical of delta plain/coastal marine environment of deposition with prevalent contribution of land plants and minor aquatic organic matter input. The occurrence of oleanane in the analysed samples is also a strong indicator of a terrestrial angiosperm plant source input and the presence of marine influence. Keywords Cretaceous Lamja Formation. Hydrocarbon generation potential. Total organic carbon Introduction The Yola Sub-basin in the Northern Benue Trough of Nigeria is one of the hydrocarbon exploration frontier basins where to date minimal data is available for adequate assessment of its hydrocarbon potential. It is part of the West and Central African Rift System (Fig. 1a) from which several petroleum exploration successes have been recorded in the Muglad Basin of Sudan and Doba and Termit basins of Chad and Niger Republics. In view of the exploration successes in these basins within the same rift trend, the Northern Benue Trough of Nigeria has attracted the attention of many petroleum

2 Fig. 1 a Regional tectonic map of western and central African rifted basins showing b the Nigerian Benue Trough and study area (adapted after Abubakar 2006, 2014) researchers and explorers. Three exploratory wells were drilled in the Gongola Sub-basin of the Northern Benue Trough from 1999 to 2003 and an estimated reserve of 33 billion cubic feet of gas was encountered in Kolmani River-1 well (Obaje et al. 2004; Abubakar 2014). However, there is no reported drilled well or core in the Yola Sub-basin, and thus, there is poor knowledge on the organic facies variation and distributions in the Yola Sub-basin (Fig. 1b). Preliminary geochemical studies have previously been undertaken on some formations from Yola Sub-basin (e.g. Akande et al. 1998; Obaje et al. 2006), but detailed organic geochemical investigation on the origin, type, richness, paleodepositional conditions as well as the assessment of the hydrocarbon potential and thermal maturation of the organic matter is lacking. More so, the earlier interpretations had been based primarily on the pyrolysis method and have not examined the source inputs, paleodepositional conditions and thermal maturation from biomarker parameters. Studies have shown that pyrolysis methods have their constraints against organically lean sediments because they are more prone to mineral matrix effects (Peters 1986; Espitalié et al. 1980). The present study focuses on the organic geochemical characteristics of the Cretaceous Lamja Formation sediments from the Yola Sub-basin, so as to provide an overview on the organic matter type, richness, source inputs, hydrocarbon potential, paleodepositional conditions as well as the thermal maturation. Since such studies of this kind have not been previously conducted, selected outcrop samples were collected from various stratigraphic intervals (Fig. 2) and detailed investigations were performed. This study is expected to contribute to petroleum source rock prediction and assessment, which in turn will help in risk reduction for hydrocarbon exploration campaign in the Northern Benue Trough. Geology and stratigraphy The Benue Trough is one of the major rift basins formed from the tension generated by the separation of the African and South American plates (Abubakar 2014). It is a NE SW trending, intra-continental, Cretaceous sedimentary basin in Nigeria that extends about 1000 km in length and 50 km in width (Fig. 1b). Several authors have presented tectonic models for the genesis of the Benue Trough (Abubakar 2014). King (1950)proposed tensional movement resulting in a rift, while Stoneley (1966) proposed a graben-like structure. The rift rift failed (RRF) triple junction model leading to plate dilation and opening of the Gulf of Guinea was proposed by Grant (1971). Olade (1975) considered the Benue Trough as the third failed arm or aulocogen of a three-armed rift system related to the development of hotspots. Benkhelil (1982, 1989) and Guiraud and Maurin (1992) considered

3 Fig. 2 Lithologic description of Yola Sub-basin successions and sedimentary log of the studied sediments of the Lamja Formation with location of the collected shale and coals samples wrench faulting as the dominant tectonic process during the Benue Trough evolution and defined it as a set of juxtaposed pull-apart basins. The Benue Trough is geographically sub-divided into Southern, Central and Northern portions (Nwajide 2013). The Northern Benue Trough is made up of two major sub-

4 basins: the N S trending Gongola Sub-basin and the E W trending 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 geology and stratigraphy of the Northern Benue Trough. The stratigraphic succession in the Yola Sub-basin of the Northern Benue Trough (Fig. 2) comprises the continental Lower Cretaceous Bima Formation, the Cenomanian transitional marine Yolde Formation, the marine late Cenomanian Numanha Shales and Lamja formations (Carter et al. 1963; Abubakar 2006; SarkiYandokaetal.2014). The Lamja Formation was earlier described as carbonaceous beds by Carter et al. (1963) and conformably overlies the Numanha Shales (Nwajide 2013; Abubakar 2006, 2014). It consists of a crystalline and shelly limestone, siltstone and yellowish to whitish fine-grained well-bedded sandstone, dark grey shale and dark coals (Fig. 2) deposited in a relatively shallow marine environment (Carter et al. 1963; Nwajide 2013). This formation terminates the sedimentation of the Yola Sub-basin in the Northern Benue Trough and was dated Santonian (Carter et al. 1963). Volcanic plugs of Tertiary age have been reported as intrusions in the Yola Sub-basin by Carter et al. (1963) and Wright (1976) (Fig. 1b) although not significantly affecting the sediments of the Lamja Formation in the region. Sampling and experimental methods A total of 18 outcrop samples were collected from shale and coal intervals within the Lamja Formation from the Yola Sub-basin that represent different sedimentary facies (Fig. 2). Since weathering is always a factor of concern for organic geochemical and petrographic studies of outcrop sediments, the weathered rock surfaces were removed by digging to approximately 0.5 m in each sampling point. Prior to analysis, the samples were scrubbed and exhaustively cleaned with distilled water to remove traces of surficial dirt and plant growth and then dried at 35 C for 12 h. All of the samples were selected for organic geochemical and petrographic analyses, which were carried out at the Department 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 was performed on 15 and 80 mg crushed coal and shale samples, respectively, which were heated to 600 C in a helium atmosphere and measured several parameters such as S 1, S 2 and temperature of maximum pyrolysis yield (T max ) (Table 1). Total organic carbon (TOC) content was determined using a Multi EA2000 Analyser. Pyrolysis data are reported in this paper as characterising, respectively, the organic richness, kerogen type, petroleum generation potential of the organic matter and its thermal maturity (Espitalié et al. 1980; Espitalié et al. 1985; Peters and Cassa 1994). Following the pyrolysis, the samples were selected for further molecular geochemical analyses and vitrinite reflectance measurements.tgroup For molecular geochemical analyses, about 30 g for shale and 12 g for coal were subjected to bitumen extraction with Soxhlet apparatus for 72 h using an azeotropic mixture of dichloromethane (DCM) and methanol (CH 3 OH) (93:7). The extracts were separated into saturates, aromatics and nitrogen, sulfur and oxygen (NSO) compounds by liquid (column) chromatography. The saturated hydrocarbon fractions were dissolved in hexane and analysed by a gas chromatography-mass spectrometry (GC- MS) on a HP 5975B MSD mass spectrometer with a gas chromatograph attached directly to the ion source (70 ev ionisation voltage, 100 ma filament emission current, 230 C interface temperature). Some selected saturated fractions were subsequently analysed using gas chromatography-doublet mass spectrometry (GC-MS/MS) on Agilent 7000B Triple quad, fitted with a fused silica capillary column (60 m 0.25 mm I.D., 0.25 μm film thickness). Helium was the carrier gas at 30 psi constant pressure, and the column was heated from 150 to 300 C at 2 C/min, with a final holding temperature at 300 C for 30 min. Vitrinite reflectance measurements were performed on polished blocks using Leica DM6000M microscope under a monochromatic light at 546 nm and using an optical sapphire glass standard having a reflectance of % 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 organic matter from Lamja Formation shales and coals were evaluated using pyrolysis S 2 yield, TOC content and bitumen extraction data (e.g. extractable organic matter (EOM) and hydrocarbon yields) (Table 1). The shales have relatively low to fair TOC content ( wt%) and EOM yield ( ppm), whereas the coaly sediments as expected contain higher TOC content in the range wt% and extractable bitumen in the range ,987 ppm. The extractability and, hence, the proportion of organic carbon content of Lamja shale and coal sediments are high enough to classify them as possessing good to excellent source rock generative potential (Tissot and Welte 1984; Peters and Moldowan 1993; Peters and Cassa 1994; Hunt 1995). In addition to the determination of TOC and EOM content, the amount of hydrocarbon yield (S 2 ) generated during pyrolysis is also a useful parameter to evaluate the generation potential of source rocks (Peters 1986; Bordenave et al. 1993). In the analysed samples, the hydrocarbon (S 2 ) yield ranges from 0.82 to mg hydrocarbon (HC)/g rock for all lithologies (Table 1). The pyrolysis S 2 yield for shales ranges from 0.82 to 1.41 mg HC/g rock, while for coals, it ranges from to mg HC/g rock (Table 1).

5 Table 1 Bulk geochemical data and ratios based on the distributions of n-alkanes and isoprenoids of the analysed Lamja shale and coal sediments Sample ID Lithology %R o TOC and pyrolysis data (SRA) Bitumen extraction and chromatographic fractions (ppm of whole rocks) n-alkanes and acyclic isoprenoids S 1 (mg/g) S 2 (mg/g) T max ( C) HI PI TOC (wt %) EOM (ppm) Sat (ppm) Arom (ppm) NSO (ppm) HC (ppm) Pr/ Ph Pr/n- C 17 Pr/n- CPI WI C 18 LSS2 Shale LSS5 Shale LSS8 Shale LSS55 Shale LSS1A Coal , LSS1B Coal , LSS3A Coal LSS3B Coal LSS6A Coal LSS6B Coal LSS7A Coal , LSS7B Coal , LSS9A Coal , LSS9B Coal , LSS10A Coal , LSS10B Coal LSS13A Coal , LSS13B Coal S 1 volatile hydrocarbon (HC) content (mg HC/g rock), S 2 remaining HC generative potential (mg HC/g rock), HI hydrogen index=s 2 100/TOC (mg HC/g TOC), TOC total organic carbon (wt%), T max temperature at maximum, Pr pristane, Ph phytane, EOM bitumen extraction (ppm), Sat saturation fractions, Arom aromatic fractions, HC hydrocarbon fractions = (saturation + aromatic), CPI carbon preference index (1): {2 (C 23 +C 25 +C 27 +C 29 )/ (C [C 24 +C 26 +C 28 ]+C 30 )}, %R o vitrinite reflectance, WI waxiness index (n-c 21 n-c 31 )/ (n-c 15 n-c 20 ) The hydrocarbon yields (S 2 ) are in agreement with TOC content, indicating that the Lamja coal and shale sediments have fair to excellent source rock generative potential based on the classification given by Peters and Cassa (1994)(Fig.3). The coal samples can act as very promising source rock for hydrocarbon generation as reflected by the high S 2 and high TOC (wt%) content (Fig. 3). Organic matter quality To interpret the organic data in terms of paleoenvironmental changes and quality, information about the composition which discriminates between marine and terrigenous sources is necessary (Stein 1991). Kerogen typing is also considered to produce different types of hydrocarbons. Generally, type I and II kerogens commonly derived from lacustrine and marine source rocks are capable of generating liquid hydrocarbons (Hakimi et al. 2011). Type III kerogen is mostly composed of woody materials and gas prone, and type IV is composed primarily of inert materials and has no potential of generating hydrocarbons (ibid). Based on the pyrolysis data, the kerogen classification diagrams were constructed using hydrogen index (HI) versus T max as carried out by previous workers (e.g. Mukhopadhyay et al. 1995). The pyrolysis data (HI against T max )(Fig.4) indicated that the analysed Lamja Formation samples generally plot in the maturezoneofmixedtypeii III kerogens grading to type III kerogen (Fig. 4). This corresponds to their HI values in the range of mg HC/g TOC (Table 1). Most samples are plotted in the type III field in this diagram, while some coal samples plotted within the mixed type II III kerogens (Fig. 4). These suggest that the Lamja Formation sediments can be expected to generate mainly gas with limited capability to generate liquid hydrocarbons. Molecular geochemistry The gas chromatography-mass spectrometry (GC-MS) analysis was performed on the saturated hydrocarbon fraction for the analysed Lamja coal and shale samples. The normal alkanes and acyclic isoprenoids ratios were determined based on m/z 85 of GC-MS fragmentation (Fig. 5) and the calculated ratios were tabulated in Table 1. Distributions of tricyclic terpanes, hopanes and steranes were performed on m/z 191 and m/z 217 (Fig. 6), respectively, and determined based on the retention time and comparison with published works (e.g. Peters et al. 2005; Korkmaz and Kara Gülbay 2007; Hakimi

6 Fig. 3 Relationship between remaining hydrocarbon potential (S 2 ) and total organic carbon (TOC, wt%) for the analysed Lamja coal and shale samples and Wan Hasiah 2013). C 30 tetracyclic polyprenoids 21R (Ta) and 21S (Tb) isomers were determined on m/z and 27-norcholestanes on m/z from GC-MS/MS transitions and interpreted based on the published work of Holba et al. (2003). The C 30 tetracyclic polyprenoid (TPP) compounds were calculated as follows: [TPP ratio=(2 peak Ta)/(2 peak Ta)+ 20R steranes]. The biomarker peaks are shown in the Appendix and the calculated ratios were tabulated in Table 2. n-alkanes and acyclic isoprenoids The chromatograms of the analysed Lamja Formation shale and coal display prominent saturated hydrocarbon distributions between n-c 12 n-c 33 n-alkanes and dominant pristane (Pr) over phytane (Ph) isoprenoid hydrocarbons (Fig. 5). The n-alkane distribution shows a bimodal distribution with predominance of high molecular weight compounds (n-c 23 n- C 29 ), which support high terrigenous land-derived organic matter contribution with small aquatic organic matter input (Ebukanson and Kinghorn 1986; Murray and Boreham 1992). The distribution is depleted in the n-c 12 n-c 19 range and shows an odd predominance of the heavier members (n- C 25 +) which gave moderate carbon preference index (CPI) values in the range of (Table 1). Acyclic isoprenoids occur in a significant amount (Fig. 5). Pristane generally occurs in relatively high concentrations compared to phytane, possessing Pr/Ph ratios in the range of (Table 1), which suggest that the Lamja Formation shale and coal sediments were deposited under suboxic to relatively oxic conditions (Peters and Moldowan 1993). Furthermore, there were higher amounts of isoprenoids pristane compared to n- alkanes (Fig. 5), thus giving distinctively high pristane/n-c 17 and low phytane/n-c 18 ratios in the range of and , respectively (Table 1). Waxiness index was also calculated to provide some insights into the source input of the organic matter (Table 1). This index may be used to determine the amount of land-derived organic materials in the sediments (Peters et al. 2005). Lamja sediments generally contain variable waxy ratios in the range of (Table 1). Terpane and sterane biomarkers Terpane and sterane biomarkers were measured from m/z 191 to m/z 217 mass fragmentograms, respectively (Fig. 6). The m/z 191 fragmentograms of the saturated hydrocarbon fractions of all the analysed shale and coal samples show moderate abundances of pentacyclic and tricyclic terpanes with a significant occurrence of C 24 tetracyclic terpanes (Fig. 6). The relative abundance of C 29 to C 30 hopane is generally similar in most of the studied samples with C 29 /C 30 ratiosrangingfrom0.93to1.53(table2). The predominance of C 29 hopane is frequently associated with carbonate-rich, but this is not always the case (Waples and Machihara 1991), although the enhanced norhopane input may also be associated with land plant input (Rinna et al. 1996)which is the case for the samples analysed in this current study. The Tm (C 27 17α(H)-22,29,30-trisnorhopane) predominates over Ts (C 27 18α(H)-22,29,30-trisnorneohopane) with Tm/Ts ratios ranging from 2.6 to 29 (Table 2). The 18α(H)-oleanane, which is an important land plant-derived biomarker (Peters et al. 2005;Peters and Moldowan 1993), was identified in low proportion in almost all of the analysed samples (Fig. 6a). The studied samples display variable oleanane index (oleanane/c 30 hopane) in the range of (Table 2). Extended hopanes are dominated by the C 31

7 Fig. 4 Plot of hydrogen index (HI) versus pyrolysis T max for the analysed Lamja coal and shale samples, showing kerogen quality and thermal maturity stages homohopane and decreasing towards the C 34 homohopane (Fig. 6a). The αβ-hopanes are more prominent than the βαhopanes, while the S-isomers are more dominant than the R- isomers among the homohopanes (C 31 C 34 ). The concentration of C 26 tricyclic terpanes is much less than that of C 24 tetracyclics in most of the analysed samples in this study (represented by C 24 Te/C 26 T) (Table 2). Selected ratios of other tricyclic terpanes were also calculated and give some insights into the source of organic material (Peters et al. 2005; Alias et al. 2012) as discussed in the section Organic matter source inputs and paleodepositional conditions. The 17β,21α(H)-moretane was also detected in all the samples though in relatively low concentrations (Table 2). The steranes are another group of important biomarkers that are derived from sterols found in higher plants and algae but rare or absent in prokaryotic organisms (Volkman 1986). The m/z 217 mass fragmentograms of all the analysed samples are dominated by steranes over diasteranes with C 29 sterane being the predominant component (Fig. 6b). The relative proportions of each of the regular steranes (C 27,C 28 and C 29 ) can vary greatly from sample to sample, depending upon the type of organic matter input to the sediment. Relative abundances of C 27,C 28 and C 29 regular steranes and the ratios of C 29 /C 27 regular sterane, diasterane/sterane and two thermal maturity based on sterane ratios are calculated and the results are given in Table 2. The Lamja shale and coal extracts show a high proportion of C 29 ( %) compared to C 27 ( %) and C 28 ( %) steranes as shown in Table 2. Thermal maturity of organic matter In this study, a number of data types were used to assess the level of thermal maturity of organic matter in the Cretaceous LamjacoalandshalesedimentsoftheYolaSub-basin, Northern Benue Trough. The maturity data include mean vitrinite reflectance (%R o ), T max values and biomarker

8 Fig. 5 Mass fragmentograms m/z 85 of saturated hydrocarbons of some of the studied Lamja shale and coal samples maturity ratios (Tables 1 and 2), which suggested that the Lamja samples are thermally mature. This is consistent with vitrinite reflectance and T max values of coal and shale sediments (Table 1). The mean reflectance of vitrinite particles ranges from 0.57 to 0.82 % (Table 1), corresponding to early mature to peak oil window maturity. This is in good agreement with the T max values ( C), as illustrated by a good correlation between T max and %R o (R 2 =0.68) (Fig. 7). The biomarker maturity parameters of the coal and shale extracts (Table 2) suchasc 32 hopane 22S/(22S+22R), moretane/hopane, C 29 sterane 20S/(20S+20R) and ββ/ (ββ+αα) ratios also were used as maturity indicators (Mackenzie et al. 1980; Waples and Machihara 1991). The ratios of 22S/(22S+22R) for C 32 hopanes are between 0.57 and 0.64 (Table 2), suggesting that they have reached equilibrium and have reached oil window maturity (Seifert and Moldowan 1986). The ratios of C 29 sterane 20S/(20S+20R), maturity ratios of the extracted samples, are (Table 2), equivalent to the onset of oil generation (Seifert and Moldowan 1978, 1981). The ββ/(ββ+αα) sterane ratios

9 Fig. 6 The m/z 191 mass fragmentograms (a)andm/z 217 mass fragmentograms (b) of saturated hydrocarbon fractions of some of the studied Lamja shale and coal samples also increased with increasing maturity (Seifert and Moldowan 1981). The Lamja coal and shale extracted samples have ββ/(ββ+αα) sterane ratios in the range of 0.43 to 0.59 (Table 2). These values are mostly either at or close to thermal equilibrium and support a maturation level indicating onset to peak the oil window generating range. This is supported by moretane/hopane ratios consistent with low relative abundance of C 30 moretane (Fig. 6a). Moretane converts to C 30 hopane with increasing thermal maturity (Seifert and Moldowan 1986), and thus, moretane decreases as thermal maturity increases. The Lamja coal and shale samples have moretane/hopane ratio in the range of , which suggests that the extracted samples are thermally mature (Mackenzie et al. 1980; Seifert and Moldowan 1986).

10 Table 2 Selected biomarker parameters of the Lamja samples illustrating source organic matter and depositional environments as well as thermal maturity of the analysed samples Sample ID Lithology Terpanes (m/z 191) Steranes and diasteranes (m/z 217) Hopanes/ (hopanes + 20R Hopane Tricyclic (T) and Tetracyclic (Te) terpanes C29 20S/ 20S+ 20R Csterane 29/C30 C31 22R/ C30H Ol/ C30 C32 22S/ (22S+22R) MC30/ HC30 Tm/ Ts C21T/ C23T C 22 T/ C 21 T C 24 T/ C 23 T C 24 Te/ C 26 T C 23 T/ C 24 T C 23 T/ C 24 Te C29 ββ/ ββ+ αα Regular steranes (%) C 27 C 28 C 29 C 29 /C 27 Diasterane/ sterane steranes) TPP ratios LSS2 Shale LSS5 Shale LSS8 Shale LSS55 Shale LSS1A Coal LSS1B Coal LSS3A Coal LSS3B Coal LSS6A Coal LSS6B Coal LSS7A Coal LSS7B Coal LSS9A Coal LSS9B Coal LSS10A Coal LSS10B Coal LSS13A Coal LSS13B Coal C 29 /C 30 C 29 norhopane/c 30 hopane, C 30 M/C 30 H C 30 moretane/c 30 hopane, Ol/C 30 H oleanane/c 30 hopane, Ts (C 27 18a(H)-22,29,30-trisnorneohopane), Tm (C 27 17a(H)-22,29,30-trisnorhopane), TPP ratio (2 peak Ta)/(2 peak Ta)+ 20R steranes

11 Fig. 7 Cross-plots of pyrolysis T max versus vitrinite reflectance (%R o ) of the Lamja coal and shale samples, showing good relationship between them Overall, the biomarker maturity ratios indicate that the coal and shale extract samples have been exposed to a thermal maturity level of early to peak oil window stage of petroleum generation (Fig. 8). Organic matter source inputs and paleodepositional conditions In this study, biomarker distributions have been used to describe source input of organic matter and depositional environment conditions of the Lamja shale and coal sediments. The paleodepositional environment was primarily examined through the use of biomarkers such as n-alkanes, acyclic isoprenoids, sterane and terpane distributions (Figs. 5 and 6), and parameters were calculated from their distributions (Tables 1 and 2). The n-alkane distributions are consistent with a dominant source of terrestrial higher plants although receiving a minor aquatic organic matter as indicated by a predominance of odd carbon number alkanes to even carbon number alkanes in the gas chromatograms of the analysed Fig. 8 Cross-plot of two biomarker parameters sensitive to thermal maturity of the Lamja coal and shale extracts which shows that most of the samples plot in the area of early mature to peak oil window maturity (modified from Peters and Moldowan 1993)

12 samples (Fig. 5). The long-chain n-alkanes (>n-c 23 ) are characteristic biomarkers for higher terrestrial plants (Eglinton and Hamilton 1967), whereas the short-chain n-alkanes (<n-c 20 ) are predominantly found in algae and microorganisms (Cranwell 1977; Peters et al. 2005). The long-chain n-alkanes of the sediments are also characterised by variable odd over even predominance (CPI according to Bray and Evans 1961). Relatively higher CPI values (>1.0) are obtained from the Lamja coal and shale extracts (Table 1). These CPI values are accompanied by the presence of significant long-chain compound alkanes (+n-c 23 ), thus supporting a terrigenous organic matter input deposited under relatively oxic conditions (Akinlua et al. 2007; Meyers and Snowdon 1993). The pristane which is much higher than phytane (Fig. 5) also supported this high terrigenous organic matter input (Didyk et al. 1978; Peters et al. 2005). The Pr/Ph ratio is one of the most commonly used geochemical parameters and has been widely invoked as an indicator of the redox conditions in the depositional environment and source input of organic matter (Powell and McKirdy 1973; Didyk et al. 1978; Tissot and Welte 1984; Chandra et al. 1994; Large and Gize 1996). Organic matter originating predominantly from terrestrial plants 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 3.0 suggest intermediate conditions (suboxic conditions) (Peters and Moldowan 1993). The Pr/Ph ratios for the Lamja coal and shale samples are in range of (Table 1), thus showing that the high terrigenous organic matter input was deposited under suboxic to relatively oxic conditions. Furthermore, pr/n-c 17 and ph/n-c 18 ratios suggest a significant contribution of terrigenous organic matter with a small amount of aquatic organic matter input that was preserved under suboxic to relatively oxic conditions (Fig. 9). The relative distribution of C 27,C 28 and C 29 regular steranes is graphically represented in the form of a ternary regular steranes diagram in Fig. 10 (adapted after Huang and Meinschein 1979). This diagram has often been employed to represent the relative proportions of the C 27,C 28 and C 29 regular steranes and can provide valuable paleoenvironmental information. Dominance of C 27 steranes would indicate a preponderance of marine phytoplankton, whereas a dominance of C 29 would indicate a strong land plant terrestrial contribution and C 28 steranes might indicate a heavy contribution by lacustrine algae. Based on this ternary classification, the analysed Lamja coal and shale samples contain a high contribution of terrestrially derived organic matter with minor aquatic organic matter contributions (Fig. 10), thus displaying a strong predominance of C 29 steranes (Table 2). The C 29 /C 27 regular sterane ratio in the samples that ranges from 1.46 to 6.44 further supported the above interpretation. Applying the oleanane parameter to indicate angiosperm input in rocks of Late Cretaceous or younger age to the coal and shale sediments of the Lamja shows that the Lamja samples have measurable amounts of oleanane (Fig. 6a) which are a strong indicator of terrestrial angiosperm plant as initially reported by Ekweozor and Telnaes (1990). The presence of oleanane suggests a probable marine influence as indicated by an earlier work of Murray et al. (1997) and as recently reported for Sabah Tertiary coals (Alias et al. 2012). The Lamja sediments have relatively low C 24 /C 23 and low C 22 /C 21 tricyclic terpane values (Table 1), indicating deltaic mixed organic matter with a major contribution of terrigenous organic matter and minor aquatic organic matter input. The deltaic depositional environmental Fig. 9 Phytane to n-c 18 alkane (Ph/n-C 18 ) versus pristane to n- C 17 alkane (Pr/n-C 17 ), showing depositional conditions and type of organic matter of the analysed Lamja samples

13 saline lacustrine environments (Holba et al. 2003; Dzouetal. 1999). TPP ratios of the analysed Lamja coal and shale samples are in the range , indicating that the samples were deposited in deltaic environment as indicated by the plot of TPP against hopanes/hopanes+ 20R sterane ratios (Fig. 12). This deltaic depositional environment setting is in agreement with the earlier works of Carter et al. (1963) and Abubakar (2006). Hydrocarbon generation potential Fig. 10 Ternary diagram of regular steranes (C 27 C 29 ) indicating the relationship between sterane compositions and organic matter input, showing that the analysed Lamja coal and shale extracts are composed of mixed organic matter (modified after Huang and Meinschein 1979) conditions of the Lamja sediments have also been interpreted using C 31-22R-hopane/C 30 -hopane ratio (Table 2). This ratio is generally higher than 0.25 for marine environments while lower than 0.25 for lacustrine settings (Peters et al. 2005). However, the ratios vary in the analysed sediments of Lamja formations (Table 2), suggesting that the Lamja sediments were deposited in deltaic environments (Fig. 11). This also concurs with the finding based on C 30 TPP ratios (Table 2). C 30 TPPs are prominently observed in samples derived from low salinity, i.e. fresh to brackish lacustrine environments, and are generally present in low levels in samples derived from saline, i.e. marine and Kerogen type and the associated hydrocarbons that might be generated were characterised based on organic geochemical data in relation to source of organic matter input. Several studies have shown that there is a direct correlation between pyrolysis data (HI) and hydrocarbon generation potential (e.g. Bordenave et al. 1993; Hunt 1995). Samples that contain type III kerogen would be expected to generate gas with HI values less than 200 mg HC/g TOC, whereas samples with HI values higher than 200 mg HC/g TOC can generate oil although their main generation products are gas and condensate. Moreover, samples that have hydrogen index greater than 300 mg HC/g TOC can generate oil (Bordenave et al. 1993; Hunt1995). In this respect, most of the Lamja Formation samples under the current investigation have HI lower than 200 mg HC/g TOC and can generate gas if they are subjected to sufficient burial and heating. This is suggested by the abundance of type III kerogens (Fig. 6). In contrast, the analysed Lamja samples with HI values higher than 200 mg HC/g TOC can generate limited liquid hydrocarbons. This is as indicated by organic matter input and depositional environment conditions. The Lamja sediments contain a high contribution of land plants Fig. 11 Cross-plot of hopane ratios versus pristane/phytane of the analysed Lamja samples (modified after Peters et al. 2005)

14 Fig. 12 Cross-plot TPP ratios versus hopane/(hopane+ 20R steranes) indicating deltaic depositional environment of Lamja coal and shale sediments (modified after Holba et al. 2003) and minor aquatic organic matter input that were deposited under oxic to relatively suboxic conditions, typical of delta plain/coastal marine environment of deposition. The high-gas generation potential of Lamja sediments was due to the high contribution of land plant inputs that were deposited under oxic conditions, whereas the limited liquid hydrocarbons are attributed to a minor aquatic organic matter input. Therefore, a high prospect for major and minor gas is anticipated from the Lamja Formation sediments in the Yola Sub-basin of the Northern Benue Trough. Conclusions An integrated geochemical and molecular characterisation of the Cretaceous Lamja Formation sediments from Yola Subbasin in the Northern Benue Trough was performed in order to determine the organic matter richness, source inputs, paleodepositional conditions and hydrocarbon generation potential and has consequently revealed the following: 1. The Lamja Formation shales and coals possessed poor to occasionally very good source generative potential. Coals have better generative potential compared to the shales, on account of more organic matter richness and very high yield of EOM. 2. High-gas generation potential is anticipated from the Lamja Formation sediments with HI values generally below 200 mg HC/g TOC (type III kerogen). Coals have fairly mixed type II III kerogens are and expected to generate limited components of liquid hydrocarbons. The land-derived kerogen with minor aquatic organic matter input is supported by biomarker distributions. 3. The pyrolysis T max,%r o and biomarker maturity parameters of the Cretaceous Lamja Formation sediments from the Yola Sub-basin indicate their maturities as early to peak oil generation window. 4. The biomarkers of the saturated hydrocarbon fractions indicate that the Lamja Formation shales and coals are likely to have been deposited in a delta plain marine environment under oxic to relatively suboxic conditions. This suggests that most of the sediments were not preserved in low bottom oxygen levels (anoxia condition) and this may have reduced their potential to generate liquid hydrocarbons. Consequently, the Lamja Formation shales and coals from the Yola Sub-basin have high prospect for gas generation although minor oil can be expected. Acknowledgments This study received support from the National Centre for Petroleum Research and Development, Energy Commission of Nigeria (NNBT Project Research Fund), for funding the field trips and sample collection. The research was also funded by the University of Malaya IPPP Research Fund (PG B). Grateful acknowledgement is given to Miss Hajah Zaleha Abdullah, Mr. Mohamed Zamri Rashid and Mrs. Maisarah Binti Yusoff(Geology Department) for analytical assistance.

15 Appendix Table 3 Peak assignments for saturated hydrocarbon fractions in the m/ z 191 mass fragmentograms Peak no. References Compound abbreviation C 21 C 21 tricyclic (cheilanthane) Tri C 21 C 22 C 22 tricyclic (cheilanthane) Tri C 22 C 23 C 23 tricyclic (cheilanthane) Tri C 23 C 24 C 24 tricyclic (cheilanthane) Tri C 24 C 24 C 24 tetracyclic Tetra C 24 C 25 C 25 tricyclic (cheilanthane) Tri C 25 C 26 C 26 tricyclic (cheilanthane) Tri C 26 Ts 18α(H),22,29,30-trisnorneohopane Ts Tm 17α(H),22,29,30-trisnorhopane Tm C29N 17α,21β(H)-norhopane C 29 hop C30 17α,21β(H)-hopane Hopane C30M 17β,21α(H)-moretane C 30 Mor C31S 17α,21β(H)-homohopane (22S) C 31 (22S) C31R 17α,21β(H)-homohopane (22R) C 31 (22R) C32S 17α,21β(H)-homohopane (22S) C 32 (22S) C32R 17α,21β(H)-homohopane (22R) C 32 (22R) C33S 17α,21β(H)-homohopane (22S) C 33 (22S) C33R 17α,21β(H)-homohopane (22R) C 33 (22R) Abubakar MB (2006). 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