Arab J Geosci DOI 10.1007/s12517-014-1713-3
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 43.87 %. 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 0.57–0.82 %Ro and pyrolysis temperature at B. M. Sarki Yandoka : W. H. Abdullah : K. A. Mustapha : A. K. Adegoke Department of Geology, University of Malaya, 50603 Kuala Lumpur, Malaysia B. M. Sarki Yandoka (*) : M. B. Abubakar National Centre for Petroleum Research and Development, ATBU, Bauchi, Nigeria e-mail:
[email protected] 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 (Tmax) in the range of 435–451 °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-C23 to n-C33, moderately high pristane/phytane (Pr/Ph) ratios (1.77–4.16), very high C27 17α(H)-22,29,30-trisnorhopane/C 27 18α(H)-22,29,30trisnorneohopane (Tm/Ts) ratios (>10) and high concentrations of regular sterane C29, 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
Arab J Geosci
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
Arab J Geosci
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-
Arab J Geosci
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; Sarki Yandoka et al. 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.
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 (CH3OH) (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 (GCMS) 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 0.589 % in oil immersion, following the procedures outlined by Taylor et al. (1998).
Sampling and experimental methods Results and discussion 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 (n-C23) are characteristic biomarkers for higher terrestrial plants (Eglinton and Hamilton 1967), whereas the short-chain n-alkanes (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-C23), 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 (
