Geochemical and organic petrological study of bituminous sediments from Dahomey Basin, SW Nigeria

Accepted Manuscript Geochemical and organic petrological study of bituminous sediments from Dahomey Basin, SW Nigeria Jude Etunimonuwa Ogala, Stavros Kalaitzidis, Kimon Christanis, Omoleomo Olutoyin Omo-Irabor, Akinwale Akinmosin, Caleb Ugbade Yusuf, Nikos Pasadakis, Miltiadis Constantinopoulos, Helen Papaefthymiou PII:

S0264-8172(18)30439-2

DOI:

https://doi.org/10.1016/j.marpetgeo.2018.10.033

Reference:

JMPG 3545

To appear in:

Marine and Petroleum Geology

Received Date: 26 September 2017 Revised Date:

9 October 2018

Accepted Date: 19 October 2018

Please cite this article as: Ogala, J.E., Kalaitzidis, S., Christanis, K., Omo-Irabor, O.O., Akinmosin, A., Yusuf, C.U., Pasadakis, N., Constantinopoulos, M., Papaefthymiou, H., Geochemical and organic petrological study of bituminous sediments from Dahomey Basin, SW Nigeria, Marine and Petroleum Geology (2018), doi: https://doi.org/10.1016/j.marpetgeo.2018.10.033. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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GEOCHEMICAL AND ORGANIC PETROLOGICAL STUDY OF

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BITUMINOUS SEDIMENTS FROM DAHOMEY BASIN, SW NIGERIA

Jude Etunimonuwa OGALAa, Stavros KALAITZIDISb, Kimon CHRISTANISb,*, Omoleomo Olutoyin OMO-IRABORc, Akinwale AKINMOSINd , Caleb Ugbade YUSUFd, Nikos

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PASADAKISe, Miltiadis CONSTANTINOPOULOSe, Helen PAPAEFTHYMIOUf

Department of Geology, Delta State University, P.M.B. 1, Abraka, Nigeria

b

Department of Geology, University of Patras, 26504 Rio-Patras, Greece

c

Federal University of Petroleum Resources, Department of Earth Sciences, P.M.B. 1221, Effurun, Nigeria

d

University of Lagos, Department of Geosciences, Akoka, Lagos, Nigeria

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School of Mineral Resources Engineering, Technical University of Crete, 73100 Chania, Greece

f

Department of Chemistry, University of Patras, 26504 Rio-Patras, Greece

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*Corresponding author E-mail address: [email protected] (K. Christanis).

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Abstract The Dahomey Basin, SW Nigeria, hosts a 3000-m-thick sedimentary succession of Cretaceous and Cenozoic age, of which the Turonian-Maastrichtian Afowo Formation includes sandstones,

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arkoses, shales, shelly limestones, unconsolidated sand, clay and most importantly, bituminous (tar) sand strata. Eighteen samples were picked up from outcrops exposed by streams and channels, and were examined applying sedimentological, mineralogical, petrographical, geochemical (inorganic and organic) and radiological techniques with the aim of assessing the

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origin, the thermal maturity and the origin of the organic facies. The samples represent mostly coarse-grained sands and subordinately shales, referred usually as ‘tar sands’; they consist of mainly quartz and kaolinite, with variable amounts of mixed clay layers of illite-montmorillonite,

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mica and pyrite, probably derived from weathering of the Migmatite Gneiss Complex exposed to the north of the basin; this can also explain the relative enrichment in the natural radionuclides 40

K, 238U, 226Ra and 232Th for some samples, although the radioactivity falls within the range for

world values for soils. The clastic material of Afowo Formation was deposited in a rather oxic shallow marine environment. The particulate organic matter of the bituminous sediments comprises solid hydrocarbons, coalified (huminite) and partially oxidized (inertinite) remnants of

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terrestrial plants, and minor amounts of both terrestrial and marine liptinite macerals. The random reflectance values of the indigenous huminite population ranges between 0.40-0.45% pointing to an immature stage for oil generation. The respective reflectance of the migrabitumens ranges from 0.49-0.59% corresponding to vitrinite equivalent reflectance values of 0.70-0.76%

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falling well within the oil window. This study concludes that the hydrocarbons contained in the Afowo Formation are upwards migrating from parts of the basin where Afowo Formation has

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subsided at depths corresponding well within the oil window, and the source rock corresponds to the middle shale horizon, whereas the sandy horizons act as migration paths and/or reservoirs.

Keywords: Afowo Formation, Dahomey Basin, migrabitumen, organic geochemistry, radioactivity, tar sand

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1. Introduction Bituminous sedimentary rocks in Nigeria were first discovered in 1900; between 1908 and 1914 the Nigerian Bitumen Company had drilled several deep exploration wells in the Dahomey

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Basin especially around Lagos (Fig. 1). Nigeria's bitumen resources are estimated at 27 Gbbl of oil equivalent, while the proven reserves amount to 1.1 Gbbl (MOMSD, 2009). A pre-feasibility and scoping study of the bitumen belt was carried out by Conoco Energy Nigeria in 2002, and between 2001 and 2008, when 40 core holes were drilled by the Geological Survey of Nigeria in

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the Dahomey Basin. Since 2009, the exploitation of the Nigeria's tar sand deposits is at a standstill.

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In southwestern Nigeria extensive bituminous sediments, also referred as tar sands, outcrop solely within the Upper Cretaceous Abeokuta Group (Araromi and Afowo Formations) over a belt of 120 km long and 4-6 km wide, straddling the States of Edo, Ondo and Ogun (Enu, 1987; Fig. 2). Ekweozor and Nwachukwu (1989) and Ekweozor (1990, 1991) studied the origin and geochemical composition of these tar sands and contended that the hydrocarbons did not originate from the shales of the Afowo Formation; they also considered the bitumens as an

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alteration product of conventional oil. The Lower Cretaceous petroleum systems described by Haack et al. (2000) are thought to satisfy the requirements for the origin of oils that eventually transformed into the tar, which impregnated the sands of southwestern Nigeria. According to Haack et al. (2000), large quantity of tar hosted in the sands at the northwestern flank of the

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Niger Delta could have been derived from the Upper Cretaceous marine shales of the Araromi Formation. Coker et al. (2002) showed that the mode of occurrence of the tar sand deposits

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includes seepages from underlying sandstones, surface and near surface impregnation in sediments exposed along road cuts, cliffs, river banks, breaks of slope and flows from abandoned oil wells drilled into the oil-bearing sediments. Koledoye and Olayinka (2005) suggested that the migration towards and up the Okitipupa Ridge (Fig. 1) in the absence of trap and seal conditions allowed for tar accumulation near the surface. Applying various techniques Olabanji et al. (1994), Obiajunwa and Nwachukwu (2000), and Adebiyi et al. (2008) determined major, minor and trace element contents in tar sand samples. Fasasi et al. (2003) determined the presence and the level of natural radionuclides in bituminous sand deposits of Ondo State, whereas Akinmosin et al. (2009, 2016) concluded that the radiogenic components of all the sediments are less than 3

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the normal background values considered harmful to man and hence, the radiation is within permissible limit. Adekeye et al. (2006) assessed the hydrocarbon potential of the Upper Cretaceous to Lower Tertiary sequence in the Dahomey Basin and concluded that the shale

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facies reached maturity and generated unquantifiable amount of hydrocarbons in the subsurface. Several organic-geochemical studies have been conducted on the crude oils and source rocks of Niger Delta and Anambra Basins adjacent to the study area (e.g., Udo and Ekweozor, 1990; Akaegbobi et al., 2000; Eneogwe et al., 2000; Sonibare and Ekweozor, 2001; Eneogwe and

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Ekundayo, 2002, 2003; Akinlua and Ajayi, 2009; Ogala and Akaegbobi, 2014). However, no detailed organic petrological and geochemical studies on the tar sand deposits of the Dahomey

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Basin have been carried out up to now.

Hence, this study focuses on the organic petrographic and geochemical characteristics of the tar sand deposits with the scope of contributing to the subjects of the origin, thermal maturity and

2. Geological setting

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organic facies input.

The study area is located in the Dahomey Basin in SW Nigeria within latitudes 6o 33/ and 6o 52/N, and longitudes 3o 58/ to 5o 15/E (Figs. 1 and 2). Detailed studies on the sedimentological,

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stratigraphical, palaeontological features and the tectonic evolution of the eastern Dahomey Basin have been documented by many authors (e.g. Jones and Hockey, 1964; Reyment, 1965; Adegoke, 1969; Ogbe, 1972; Billman, 1976, 1992; Ako et al., 1981; Omatsola and Adegoke,

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1981; Enu, 1987; Enu and Adegoke, 1988; Okosun, 1990, 1998; Bankole et al., 2005). The Dahomey Basin is bounded on east by the western flank of the Niger Delta/Anambra Basins, to the south by Gulf of Guinea and to the north and west by the Basement Complex rocks of West Africa. The Dahomey Basin stretches for about 500 km from southeastern Ghana to the Okitipupa Ridge (Fig. 1); the latter consists of crystalline basement rocks and separates the Dahomey Basin from the eastern side by the lithostratigraphic units of the Anambra and Niger Delta Basins (Jones and Hockey, 1964; Fig. 1).

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The evolution of the Dahomey Basin is linked with the opening of the South Atlantic Ocean consequent upon the separation of the African from South American lithospheric plates in the Mesozoic Era (Burke et al., 1971; Murat, 1972). The Dahomey Basin is a marginal pull-apart (Klemme, 1975) or marginal sag basin (Kingston et al., 1983) formed by structures associated to

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the transcurrent movements on the oceanic fracture systems especially the Romanche, Chain and Charcot Fracture Zones (Francheteau and Le Pichon, 1972; Omatsola and Adegoke, 1981; Coker and Ejedawe, 1987; Fig. 1). According to Francheteau and Le Pichon (1972), the Romanche Fracture Zone, which passes just south of Accra in Ghana, marks the northwestern boundary of

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the Dahomey Basin. The Chain Fracture Zone intersects the African coast east of Lagos, which is at the boundary of the Dahomey Basin and the Benue Trough (Fig. 1). The Romanche and

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Chain Fracture Zones are thought to have defined the Dahomey Basin (Nwajide, 2013). These fracture zones generally appear to have acted as dams for the delta sediments. The continental extensions of the Romanche, Chain and Charcot Fracture Zones, mark the edge of where pronounced subsidence occurred from Aptian to Albian, and the limits of coastal basins, which are filled with Early Cretaceous to Holocene sediments (Fig. 3).

The Dahomey Basin hosts a 3000m-thick sedimentary succession of Cretaceous and

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Cenozoic age (Whiteman, 1982) comprising sandstones, arkoses, shales, shaly limestones, unconsolidated sand, clay and bituminous (tar) sand strata (Fig. 3a). Omatsola and Adegoke (1981) subdivided the lithostratigraphic units of the eastern Dahomey Basin into the Cretaceous Abeokuta Group consisting of Ise, Afowo and Araromi Formations, and the Cenozoic Ewekoro,

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Akinbo, Oshosun, Ilaro and Benin Formations (Fig. 3). The Ise Fm (Omatsola and Adegoke, 1981) lying unconformably on the Basement Complex of southwestern Nigeria, is the oldest

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Cretaceous formation (Neocomian, probably Valanginian-Barremian) in the Dahomey Basin. The formation comprises basal conglomerates, sandstones and mudrocks and passes upwards into medium- to coarse-grained, loose sands interbedded with kaolinitic clays. The Ise Fm is successively overlain by the tar-bearing Afowo and Araromi Formations. The Afowo Formation (Fig. 3; Omatsola and Adegoke, 1981) comprises medium- to coarsegrained sandstones interbedded with shales, siltstones and claystones. The most detailed lithological profile of the Afowo Fm is provided by Enu (1987) who describes two sandy horizons, Y and X, being separated by a black shale horizon (Fig. 3b). According to Enu (1990), 5

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the sandstone layers of the Afowo Fm around Okitipupa Ridge are tar-bearing, while the shale beds are organic-rich. The basal parts of the Afowo Fm are transitional with mixed brackish to marginal marine facies that alternate with well-sorted, sub-rounded sand, indicating a littoral or estuarine nearshore depositional environment with rapidly fluctuating shoreline (Enu, 1990;

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Elueze and Nton, 2004; Nwajide, 2013). The Afowo Fm is of Turonian-Maastrichtian age (Billman, 1976, 1992; Omatsola and Adegoke, 1981).

The Maastrichtian-Palaeocene Araromi Formation (Omatsola and Adegoke, 1981) is the

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youngest of the Cretaceous formations in the eastern Dahomey Basin (Fig. 3). The formation comprises fine- to medium-grained sandstone at the base, overlain by light grey to black

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carbonaceous shales and siltstones interbedded with thin bands of limestone, marl and lignite. The Ewekoro, Akinbo, Oshosun, Ilaro and Benin Formations succeeded the Araromi Fm in Cenozoic era (Fig. 2). A continuation of the marine transgression during the Palaeocene and up to Neogene led to the deposition of shallow marine limestones (Ewekoro Fm), shales (Akinbo Fm) (Fig. 3), clays and glauconitic shales interbedded with loose sands (Oshosun Fm), sandstones and shales (Ilaro Fm) and finally, poorly sorted sandstones with transitional to

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continental characteristics (Benin Fm) (Tattam, 1944; Jones and Hockey, 1964; Reyment, 1965; Ogbe, 1970; Okosun, 1990; 1998, Ogala et al., 2010; Nwajide, 2013).

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3. Sampling and Methods

A total of 18 outcrops exposed by streams and channels, were sampled (Fig. 2, Table 1).

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From each outcrop one sample was picked up for lab examination. Particle size distribution of the samples was macroscopically determined and also by sieving after soaking in toluene overnight. About 100 g of each sample was weighed and soaked overnight in toluene to enable loosen the bitumen from the grains before stirring and decanting the bitumen. The soaked samples were separated into three layers by virtue of their densities with bitumen/toluene mixture floating on top, followed by water, and lastly the heavy particles at the bottom of the beaker. Bitumen-free inorganic grains were obtained using the hot water/toluene

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extraction process described in detail by Enu (1985). The extracted sand grains were air-dried and used for sieving. Another part of each bulk sample was firstly dried out at a temperature of 60οC for 24 h and

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then subsequently ground to Ø < 1 mm, < 250 µm and finely, powdered (< 50 µm) in an agate mortar.

The mineralogical composition of nine particulate samples was determined on the powdered sub-samples using a Bruker D8 Advance X-ray diffractometer equipped with a Lynx-Eye®

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detector. The scanning area covered the 2θ interval between 4° and 70°, with a scanning angle step of 0.015° and a time step of 1 s. The semi-quantitative determination was performed using

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the Rietveld-based TOPAS® software applying the technique described in detail by Siavalas et al. (2009).

The elemental analyses of the samples were conducted applying various techniques. The ultimate analysis was performed on nine selected samples (Ø < 250 µm) according to ASTM D5373 (2004) using a CARLO ERBA Automatic Analyzer (EAGER 200) calibrated against the CP1 standard reference material (AgroMatTM, 2016). Furthermore, nine powdered samples were

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subjected to elemental analysis for major and minor elements using the RIGAKU NEXCG Energy Dispersive X-Ray Fluorescence Spectrometer (EDXRF). Eleven powdered samples were digested in a MILESTONE 1200 MEGA microwave oven with a mixture of nitric, perchloric

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and hydrofluoric acids and hydrogen peroxide. Minor and trace element concentrations were determined using an ELAN 6100 PERKIN ELMER Inductively Coupled Plasma-Mass Spectrometer (ICP-MS). The accuracy of the results was checked against the CP1, SARM 18 and

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SARM 19 standard reference materials (SABS, 2016). All the elemental analyses were conducted out in the following Laboratories of the University of Patras: (a) Lab of Instrumental Analysis and (b) Lab of Electron Microscopy and Microanalysis, both belonging to Faculty of Natural Sciences; and (c) Lab of Hydrogeology, Department of Geology. Polished blocks were prepared from crushed samples (Ø < 1 mm) according to International Standards (ISO 7404-2, 2014). Maceral analysis was performed in oil immersion using a LEICA DMRX coal-petrography microscope under both white incident light and blue-light excitation (ASTM D7708, 2014), following the nomenclature of the Stopes-Heerlen System as it is 7

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modified by ICCP System 1994 (ICCP, 1998; 2001; Sýkorová et al., 2005; Pickel et al., 2017). Vitrinite reflectance was measured according to ISO 7404-5 (2014). The organic geochemical analyses were performed on bulk subsamples in accordance with

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the protocol of Energy Resources Program, United States Geological Survey (USGS, 1996). The experimental session included the following processes: isolation of bitumen via Soxhlet extraction and separation of bitumen into asphaltenes and maltenes fractions. The latter fraction was then further separated with open column liquid chromatography into saturates, aromatics,

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resins and asphaltenes (SARA analysis). The saturated fraction was analyzed with gas chromatography-mass spectrometry (GC-MS) for the identification of biomarkers.

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a. Soxhlet extraction: Around 5 g of each sample was tied in a thimble, and extracted in a Soxhlet apparatus using dichloromethane (DCM) as solvent for 24 h. The solvent was removed using a rotary evaporator and the bitumen content was determined after drying under vacuum. b. Asphaltenes removal: About 50 mg of bitumen were transferred in a vial, dissolved in 0.5 ml n-pentane and then filtered through GFA paper (Whatman). The process was repeated three times and the eluate fraction, containing the maltenes, was concentrated under nitrogen. The

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asphaltenes were recovered with chloroform.

c. Open-Column Liquid Chromatography: The chromatographic column was packed with a mixture of SiO2 (230-400 mesh) and Al2O3 (70-230 mesh) 5:1 v/v, baked at 240oC for 24 h and

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de-activated with 5% and 1% H2O, respectively. The maltenes fraction, dissolved in 0.5 ml npentane, was introduced in the column. The first fraction (saturates) was eluted with 7 ml n-

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pentane, the second fraction (aromatics) with 7 ml toluene, and the final fraction (resins) with 10 ml mixture of toluene and methanol in the ratio 6/4 v/v. The collected fractions were concentrated under nitrogen and kept in vacuum chamber for 24 h. d. Gas Chromatography - Mass Spectrometry (GC-MS): GC-MS analysis of saturated hydrocarbon fractions was performed using an Agilent GC-MS HP 7890/5975C system, capillary column HP-5 5% phenyl methyl-siloxane (60 m x 250 µm x 0.25 µm). Helium was used as carrier gas. The column oven was programmed from 40oC to 200oC, at a rate of 20oC/min, and thereafter increased to 300oC with 2oC/min. The samples were injected using a 8

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split/splitless injector at 280oC (splitless mode), diluted (1/150) in ultra-pure hexane (SupraSolvR, Merck). The injected sample volume was 0.5 µl. The transfer line, MS-source and quantrupole temperatures were set at 280, 230 and 150oC, respectively. 238

U,

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Th,

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Ra,

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Κ and

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Cs in all the

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The activity concentration measurements of

samples (Ø < 1 mm) were performed using γ-ray spectrometry. The preparation of the samples for measurement, as well as the description of the γ-ray spectrometry system, the energy and efficiency calibration procedures, the photopeaks used for the analysis of radionuclides and the

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detection limits are described in detail elsewhere (Siavalas et al., 2009). The duration of counting varied from 1 to 3 days according to sample activity. For quality control purposes the IAEA-312 (226Ra, Th and U in soil) and IAEA-375 (Radionuclides and trace elements in soil) reference

values (Papaefthymiou, 2008).

4. Results

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materials were used. Results showed good agreement between measured and recommended

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4.1. Macroscopic and grain size characterisation The total thicknesses of the sampled bituminous sediment layer are presented in Table 1. Depending on the amount of bituminous matter content the samples are macroscopically

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distinguished into two groups: (a) the mostly fluidal samples of a plastic consistency; they consist of tar containing sand and silt grains, and (b) the mostly particulate samples, which are composed of sand and/or silt impregnated with tar (Table 2). The particulate samples were

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macroscopically logged (Stow, 2007); in several samples modern plant remnants were also included.

The results of particle size distribution from sieve analysis (Folk and Ward, 1957) are also presented in Table 2. The mean grain size of the sands ranges from -0.33Ø (very coarse-grained) to 3.31Ø (very fine-grained). The standard deviation values of the sands vary from 0.33 to 1.07Ø, suggesting that the sand grains are very well sorted (samples #3 and 11), moderately sorted (samples #6, 12, 13, 14, 15, 16 and 18) to poorly sorted (samples #2, 4, 10 and 17). Five (samples #2, 4, 11, 16 and 17) out of the fifteen samples display positive skewness, while the 9

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graphic kurtosis ranges from 0.74Ø (platykurtic) to 1.49Ø (leptokurtic). These results are in agreement with those reported by Enu (1985).

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4.2. Mineralogical composition

X-ray diffraction analyses on selected particulate samples (Table 3) reveal that the predominant minerals are quartz, kaolinite and illite. Sandy samples show, in general,

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enrichment in quartz, whereas silty samples in clay minerals as expected. Pyrite, copiapite (a hydrated iron sulphate originated from pyrite oxidation), aragonite, mica and brookite occur subordinately. These mineralogical assemblages are typical for clastic sediments. Similar

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mineralogical data was reported by Akinmonsin et al. (2011) who performed SEM studies.

4.3. Elemental composition

The samples display ash yields form 59-94 wt.% and hence, represent carbonaceous

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sediments according to ECE-UN (1998) classification scheme. Total organic carbon content ranges from 4.3-27.8 wt.% (on dry basis) indicating the organic-rich character of the samples (Table 4); hydrogen and nitrogen contents range from 0.6-3.77 and 0.2-0.55 wt.% (db), respectively; sulphur content is high (7 wt.%, on db) in sample #2, whereas it remains < 2.1

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wt.% in rest of the samples. All samples have H/C atomic ratios higher than 1.5 indicating hydrogen enrichment, whereas only samples #2 and 15 display O/C atomic ratio higher than 0.2

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(Table 5, Fig. 4). The high O/C ratio of these samples indicates increased input from terrestrial plant and probably reflects contamination through epigenetic roots from the plants growing on the current surface. The H enrichment might be due to either H-rich liptinite macerals or contained hydrocarbons.

The inorganic geochemical analyses reveal the predominance of SiO2 (29.7-76.1%), with Al2O3 (0.6-21.6%) and Fe2O3 (0.3-9.7%) being the secondary major oxides/elements (Table 4). Apart from TiO2 being around 1%, the elements Ca, Mg, Mn, Na and P are quite depleted with values < 1%. 10

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The trace elements like As, Ba, Co, Cr, Cu, Ga, Hf, Ni, Nb, Pb, Rb, Sr, V, Y, Zr, Zn, Ce and La, range from 5-100 mg/kg, although significant variations among the samples are noticed, whereas the elements Ag, Be, Bi, Cd, Cs, U and W occur in < 10 mg/kg (Table 4). In general, the

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obtained concentrations follow the average values reported previously by Fasasi et al. (2003). Comparing with the average values of the continental crust (Mason and Moore, 1982) it is revealed that most of the studied trace elements are depleted (Fig. 5), while elements showing significant enrichment include As and Hf (samples #18, 2, 15), Bi (#18), Cd (#14, 17), U (#12),

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4.4. Organic-petrographical composition and reflectance

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W (#2), Zn and Zr (#18).

The microscopic examination of the whole rock samples (WR) and the kerogen concentrates (KC) reveal a variety of organic components with minor variation among samples (Table 6). Modern organic matter in the form of cellulose remnants, and macerals of the huminite

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group, like textinite, ulminite and corpohuminite, occur in several samples with corpohuminite being the dominant maceral. In most of the cases corpohuminite was subordinated by suberinite indicating root systems (Plate 1). The huminite reflectance values range from 0.20-0.34% (Table 7). These assemblages of huminite macerals are regarded as epigenetic root penetrations by

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modern plants; thus they cannot be used for any rank determination of the samples. Particles of humic origin with higher than the previously mentioned reflectance, are also

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observed in most of the samples and represent two distinct populations. Particles of variable colours, cracks and halos of elevated reflectance values, are identified as reworked vitrinite particles (ASTM D7708, 2014) displaying reflectance values in the range of 0.58-0.96%. On the other hand, huminite particles being more homogeneous than the previous ones, are identified in samples #1, 2, 4, 5, 15, 18 and are regarded as the indigenous (syngenetic) huminite population, with reflectance values in the range of 0.40-0.45% (Table 7).

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Macerals of the inertinite group have a scattered distribution wherein fusinite and semifusinite are common; inertodetrinite is associated mostly with fine clay minerals, whereas funginite appears being associated mainly with corpohuminite (Table 6).

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The semiquantitative assessment shows that macerals of the liptinite group are sparse (Table 6); suberinite is the predominant maceral of this group in most of the samples, being associated with corpohuminite. Samples #1 and 3 show a relative enrichment in terrestrial liptinite macerals, i.e. cutinite, sporinite, resinite, than the rest samples. Alginite is rarely identified, only

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in three samples (#2, 3 and 8). Apart from alginite and liptodetrinite, both deposited syngenetically with the sands, the rest liptinite macerals occur mostly in the corpohuminite-rich samples being obviously of epigenetic origin, related to the growth of terrestrial plants on the

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layers sampled.

Zooclasts and oil drops within the clay matrix are also included in several samples. The main minerals that could be identified are quartz, clays, pyrite (both of massive and framboidal forms), and marcasite (in samples #15 and 16).

The predominant organic phases in the studied samples are solid hydrocarbons, both in the

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form of migrabitumen and pyrobitumen (Plate 1). Migrabitumens are the main organic phase and along with clay minerals constitute the matrix of the studied sediments. Microgranular migrabitumens are predominant, although homogenous migrabitumens with smooth texture are

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also common, surrounding quartz grains, filling pores or the spaces among clay minerals (Plate 1). Pyrobitumens are mostly fragmented < 50 µm within the mineral matrix. Occasionally, larger pyrobitumen particles are encountered as fracture infillings, and in these cases they are

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interrelated to massive pyrite crystals. The mean random reflectance values of the solid migrabitumens (BRo%) reveal a relative consistent variation (Table 7). BRo values from 0.24-0.37% are measured for microgranular migrabitumens, often related to clayey matrix and pore fillings. Homogeneous and with lowrelief surfaces migrabitumens display reflectance BRo values in the range of 0.49-0.59%. The reflectance of pyrobitumens (PBRo) varies between 1.56 and 5.1%.

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4.5. Organic geochemical composition 4.5.1. Bitumen and maltenes composition The results from SARA analysis of the bituminous sediment samples are summarized in

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Table 8. The Soxhlet analysis reveals an average bitumen saturation of 35% w/w, ranging from 0.2-95.6%. According to Ibisi (2006), the values of bitumen saturation at the same location samples may also show a similar wide range (13.4-99.7% w/w). As an average the bitumen content of the analyzed samples is lower compared to the reported values (Tissot, 1984) for tar

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sand samples from Athabasca, Canada (41%) and Eastern Venezuela (48%).

Most of the samples show high maltenes content (67-80% w/w), except the samples #10 and

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11, which contain about 50% w/w maltenes. These values are in accordance with Ibisi (2006). The liquid chromatography proved that in almost all the samples aromatic hydrocarbons are dominant. Exceptions are samples #2 and 11, in which NSO content and saturated hydrocarbons, respectively, appear at highest concentration. The SARA composition of the analyzed samples

4.5.2. Biomarker analysis

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does not correlate with the samples locations, probably indicating variable parent oil generations.

As the analyzed bitumens are considered as the end products of hydrocarbons migration from sources/reservoirs towards the surface, the analysis aimed to collect information about the

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type of the source, the conditions of the deposition, the achieved thermal maturity level and, if possible to reveal the processes that altered the originally produced oils.

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4.5.2.1. n-alkanes and isoprenoids

The concentrations of n-alkanes and isoprenoid compounds (pristane and phytane) were determined from the m/z=85 fragmentogram of the GC-MS analysis (total ion scanning) using relative response factors (RRF) of the individual components. Three representative chromatograms are shown in Figure 6. All the samples, with the exception of sample #2, show the characteristic hump of heavily biodegraded oils. As a measure of the biodegradation level the concentrations of the identified n-alkanes in the original bitumen samples were calculated (Table 9); samples #1 and 2 are the most and the least biodegraded ones, respectively. The absence of 13

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the specific lighter n-alkanes according to Wenger et al. (2002) classifies the studied bitumens as “heavily” biodegraded. Specific geochemical indices calculated from the n-alkanes and isoprenoids are summarized

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in Table 9. The values of the Pr/Ph ratio, as well as the Pr/nC17 and Ph/nC18 of the sample #2 may be attributed to a marine shale source (Didyk, 1978). The CPI values (~1) verify the high biodegradation level, previously mentioned for all the samples with the exception again of sample #2. The high CPI value of this sample is an indication of terrigenous organic matter input

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in the source. The same pattern is observed for the OEP 27-31 index pointing to an anoxic or hypersaline environment (Scalan and Smith, 1970; Singh et al., 2017). The R22 ratio (Ten Haven et al., 1985) also differentiates sample #2 from the rest samples. It can be explained as an

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indication of a pronounced contribution of terrigenous organic matter for this specific sample. Summarizing it can be concluded that all the analyzed bitumens, with the exception of sample #2, have been formed in similar environments and have undergone similar transformations mainly due to biodegradation. Sample #2 seems to have been formed under both marine and terrestrial organic-matter input.

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4.5.2.2. Hopanes

The main geochemical indices calculated from the peak areas of hopanes (m/z 191) and steranes (m/z 217-218) are presented in Table 9. The interpretation of the reported values was

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conducted mainly based on Peters et al. (2005) and Peters and Moldowan (1993). The missing values are due to the lack of the respective compounds’ peaks, which can be explained as the result of the already documented biodegradation of the bitumens, which among others resulted in

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relative high concentrations of tricyclic terpanes. Thermal maturity parameters The characteristic thermal maturity indicator, the Ts/(Ts+Tm) index shows more or less similar values, pointing to a common maturity level of the samples. Under this assumption, the significant variation that the ratio of tricyclic terpanes to hopanes (Tricyclics/Hopanes; see Table 9) exhibits within the samples, may be interpreted as a reflection of a variable organic matter input from terrestrial to marine. The moretane index (moretane/C30 hopane) differentiates mainly 14

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samples #2 and 12, as being the less thermally mature, which is in accordance with the respective maturity level estimation from the n-alkanes concentrations. Source related parameters

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The (19tri+20tri)/ 23tri ratio of the tricyclic terpanes shows a pronounced terrestrial input for sample #1 compared to the rest samples (Tao et al., 2015). A comparative examination of the (19tri+20tri)/23tri and 23tri/21tri ratios reveals a grouping tendency of the samples into two groups according to the land plant input, consisting of the samples #4, 7, 9, 11, 16 and #3, 10, 13,

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15, 17, respectively. The 24tetra/26tri (C24 tetracyclic, Des-E-hopane)/C26 tricyclic terpane) ratio values are low, pointing to a marine/deltaic shale source rock. In addition, the 26tri/25tri (C26

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tricyclics/c25 tricyclic terpanes) ratio exhibits values at the edge of a lacustrine and marine shale source (Zumberge, 1983).

Oleanane index (oleanane/C30 hopane) determined for several samples (Table 9), indicates that the parent oils of the analyzed bitumens are derived at least in part from terrigenous organic matter of Upper Cretaceous or younger age (Peters and Moldowan, 1993). The presence of tricyclic terpanes in all the analyzed samples confirms the age determination (Peters et al., 2005).

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High Gammacerane index (gammacerane/C30 hopane) values for the samples #13, 15, 18 may be

4.6. Radioactivity

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attributed to a high hypersaline depositional environment for the source rock.

(238U,

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Table 10 presents the measured activity concentration values of the natural radionuclides 226

Ra,

232

Th and

40

K) and the man-made

137

Cs in the examined samples. The activity

concentrations of 238U and 226Ra range from 6.4-222 and 5.9-179 Bq kg-1, respectively. The 232Th activity concentrations range from 1.4 to 54.5 Bq kg-1, whereas those of 40K from <20 Bq kg-1 in four samples up to 120 Bq kg-1 (sample #1). The

238

U,

226

Ra and

232

Th activity values are

comparable to those found by the other researchers in bituminous sand samples obtained from the same area (Fasasi et al., 2003; Akinmosin et al., 2009; 2016). High variations in 40K activity concentrations (below detection limit up to 1004 Bq kg-1) are reported by Akinmosin et al. (2009). These high variations in

40

K activities could be attributed to differences in the 15

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mineralogical composition throughout the bituminous sediment deposit in the examined area. In most cases, the concentrations of

232

Th proved being lower than those of

238

U and

226

results are consistent with those reported by Akinmosin et al. (2016). In general, the and

232

Ra; these

238

U,

226

Ra

Th activity concentration values fall within the range of world values for soil, whereas

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those of 40K are below the world range for soil in all samples examined (140-850 Bq kg-1, mean value 400 Bq kg-1) (UNSCEAR, 2000). As expected, the man-made 137Cs was detected only in a small number of the examined samples (#2, 4, 5, 7). The

238

U/226Ra ratio ranges from 0.62 to

2.52, showing that there is no significant disturbance in radioactive equilibrium within the 238

U/232Th ratios widely vary

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uranium series in the examined samples. On the other hand, the

between 0.63 and 18.2 indicating higher activity concentration values of 238U as compared to that

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of 232Th.

5. Discussion

5.1. Host rock features and provenance of sands

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Using the ratios proposed by Herron (1988), the geochemical classification of the studied samples (Fig. 7) reveals a significant variation on the lithotypes, ranging from Fe-shale (samples #2, 15, 18) and shale (#10), to Fe-sand (#3, 12), arenite (#11), and sublitharenite (#14, 17). This variation indicates that the studied samples represent both the sandy and the shale horizons

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described by Enu (1987, see Fig. 3b).

In order to estimate the weathering of the rocks the samples derived from, the Chemical

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Index of Alteration (CIA) was calculated (Table 5; see Nesbitt and Young, 1982). All the samples reveal values higher than 80 indicating significant weathering in the source area; hence, aluminosilicates are mostly represented by kaolinite as a by-product of K-feldspars weathering (Piñán-Llamas and Escamilla-Casas, 2013; Armstrong-Altrin and Machain-Castillo, 2016). Using IBM SPSS Statistics factor analysis (Davis, 1986) was conducted in order to assess the mode of occurrence and interrelationships among the minor and trace elements contained in the studied samples. A 4-factor statistical model was selected in R-type analysis representing 92.3% of the total variance of the eigenvalues, with all factors displaying a bipolar mode. The 16

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positive pole of the 1st factor shows strong grouping of the major elements Al, Fe, K, Mg, Na, with Ag, Ba, Be, Bi, Co, Cr, Cs, Cu, Ga, Mn, Ni, Pb, Sr, Ti, V, W, Y, Zn, Ce, La, and their affiliation to the aluminosilicate minerals illite and kaolinite (Fig. 8a). The elements Nb and Ti show a moderate correlation to this aluminosilicate grouping. Sulphur and pyrite display also

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high positive loadings; however, this correlation indicates the co-occurrence of sulphides in samples enriched in aluminosilicates. The negative pole of the 1st factor represents mostly the fraction of bitumens and secondary the predominance of quartz (correlated very well with Si) (Fig. 8a). This correlation is reflected to the more sandy samples #11, 12, 14, 17, suggesting that

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these lithologies are the most enriched in bitumens. The aluminosilicate enrichment is evidently displayed in the shaly sample #2, whereas the rest samples are more representative by the

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correlations of the negative pole (Fig. 9a). The negative pole of the 2nd factor is also grouping Si with quartz, whereas the positive pole groups the elements Ca, As, Ti, Hf, Nb, Y, Zr (Fig. 8a); since the latter grouping characterises shale sample #18, it probably indicates affiliation of these elements within an aluminosilicate-like mica, which only occurs in sample #18 (Table 3). The positive pole of the 3rd factor strongly correlates bitumen, C and P, grouping mainly the organic matter and indicating the organic affiliation of P, whereas the negative pole represents again Si.

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From the factor scores it can be interpreted that samples #2, 3 and 10 are the most enriched in primary organic matter. The 4th factor displays high negative loadings for the contents of U and Cd and the activity concentration of

238

U as well; hence, it can be regarded as a measure of the

radioactive elements occurrence (Fig. 8b), which is evident for sample #12 (Fig. 9b).

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Fasasi et al. (2003) attributed the radioactivity of the bituminous sands to the presence of zircon, sphene and tourmaline, identified by means of SEM examination. The X-ray diffraction

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analysis does not reveal the presence of these minerals (Table 3), probably due to resolution limits. However, the activity concentration of 232Th correlates positively to Zr concentration (Fig. 10) indicating the co-occurrence of Th to Zr-bearing minerals. Moreover, factor analysis provides a clear affiliation of K with the clay fraction (Fig. 8a) and hence, the variability of 40K radioactivity values (Table 10), as also reported by Akinmosin et al. (2009), is related to the relative enrichment in kaolinite and illite minerals. Uranium distribution is more complex probably following an intermediate affiliation to both organic matter and some heavy minerals, not identifiable by using XRD analysis only.

17

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In order to assess the provenance of the samples several geochemical indices can be taken into account. The Al2O3/TiO2 ratio is commonly used to discriminate among mafic to felsic parental rocks (e.g. Hayashi et al., 1997; Moosavirad et al., 2011); values <8 point to mafic, whereas > 21 to felsic igneous rocks. The Al2O3/TiO2 ratios for the studied samples range from

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9.6-18.2 (Table 5) indicating an intermediate igneous provenance. The discriminant plot proposed by Roser and Korsch (1988) is also applied to investigate the source of the sand. Based on Figure 11a most of the samples fall within the quartzose sedimentary provenance field and only sample #2 in the mafic igneous provenance. Additionally, the TiO2 vs. Zr diagramme sample #2 falls within the intermediate field (Fig. 11b).

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proposed by Hayashi et al. (1997) indicates mostly felsic igneous provenance and again only

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The evaluated geochemical indices and particularly the enrichment in Zr show, in general, a felsic to intermediate provenance for the studied samples, being in agreement with the dominant exposure of the Migmatite Gneiss Complex, which consists of igneous and metamorphic rocks, to the north of the studied area (Fig. 2). Additionally, the suggestion for sedimentary provenance obtained from Figure 11a, is most probable an indication of significant reworking of the sediments before settled to their final environment; this is also supported by the predominant

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occurrence of kaolinite as an indication of severe alteration of K-feldspars and the very minor amounts of mica, as well as the described above CIA values. Enu (1985) reached similar conclusions by additionally identifying sillimanite and andalusite, zircon and tourmaline in the samples he studied. Furthermore, the origin of the sand from the Migmatite Gneiss Group can

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justify the occurrence of natural radionuclides.

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In terms of palaeo-redox conditions during the deposition, the Ni/Co ratio can be applied as a proxy (Jones and Manning, 1994); the Ni/Co ratio in the studied samples ranges between 0.55.96 (Table 5) indicating predominantly an oxic environment (<5) for most of the samples; only for sample #10 suboxic conditions are inferred. The overall lithological, mineralogical and geochemical data of the studied sandy layers point to a shallow marine environment, in proximity to the shoreline, where siliciclastic material of terrigenous origin deposited under oxic conditions for sandy horizons, whereas the shale horizons were most probably deposited in a low-energy environment under reduced oxygen

18

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supply. These conditions are in agreement to the transitional brackish to marine near-shore environment proposed by Coker et al. (1983) and Enu (1985). Vertical and lateral variations of the hydrodynamic regime and the water-column depth resulted in alternations of sand and shale

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layers, as well as fluctuations in the type and quantity of primary organic matter deposition.

5.2. Source of tar and maturation level

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In the studied samples the organic matter can be distinguished into three main categories: (i) syngenetic organic particles of mostly humic origin (vitrinite, inertinite) plus some of the liptinite macerals, mostly alginite, (ii) epigenetic humic matter (macerals of huminite), mostly resembling

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root systems along funginite and terrestrial liptinite macerals, mostly suberinite, and (iii) solid hydrocarbons, which are the prevailing organic phase. The enrichment of H for most of the samples, as pointed out by the H/C vs. O/C plot (Fig. 4), coincides with the solid hydrocarbons being predominant.

The semi-quantitative maceral evaluation indicates that in the majority of the samples most

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of the synsedimentary maceral fraction is “transported” (e.g. vitrinite, inertinite, sporinite, cutinite) and only one sample (#2) contains significant synsedimentary indigenous fraction like alginite and liptodetrinite, coinciding with the Pr/Ph ratio. The organic petrographical data is in agreement with the sedimentary and geochemical character of the samples, the majority of them

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being sand deposited in an oxic-suboxic environment under relative high hydrodynamic regime favouring the transportation of organic fragments of terrestrial origin. Hence, the studied

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lithologies, in terms of type of organic matter, do not satisfy the conditions to be source rocks, let alone to have provided the extensive tars, apart from sample #2, which is a shale, containing liptinitic material capable to generate hydrocarbons under certain conditions of maturation. The most accurate parameter to determine the maturation of sedimentary rocks is the vitrinite reflectance (Taylor et al., 1998; ASTM D7708, 2014); however this is not always possible due to the lack of organic matter or more precisely, of indigenous vitrinite. Numerous studies have been conducted to overcome the issue, particularly in source rocks with abundant

19

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solid hydrocarbons and various relationships/equations appear in the literature (e.g., Jacob, 1989; Landis and Castaño, 1995; Petersen et al., 2013). Among the contained organic particles in the studied samples the maturity of the bituminous

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sediments is provided by the indigenous vitrinite particles that show no reworking and oxidation effects (ASTM D7708, 2014). Such particles were identified in five samples with mean random vitrinite reflectance ranging from 0.40-0.45% (Table 7) and indicating an immature stage and a maximum burial depth of few to several hundred meters in a normal geothermal gradient (Tissot

SC

and Welte, 1984).

The migrabitumen reflectance values range from 0.23-0.59% for both the microgranular and

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the homogenous variations; however, the “dirty” nature (inhomogeneous) of the microgranular ones makes them inappropriate for maturity estimations. The homogenous migrabitumens fall within a narrower reflectance range from 0.49-0.59% in eight samples (Table 7). These values correspond to vitrinite equivalent reflectance values of 0.70-0.76%, well within the oil generation window, and under conventional conditions, at burial depths below 2 km (Tissot and Welte, 1984).

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The petrographical constituents and the contradicting reflectance values among the primary organic matter and the solid bitumens indicate that the contained solid hydrocarbons were not expelled from the indigenous organic matter, but rather were migrated to the host sandy

EP

formation. Hence, it is suggested that the hydrocarbons occurring at the studied sites are the result of migration, either from another area and/or another formation.

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The under question source rock reached the maturity stage of early to main oil window as revealed from both reflectance values and the organic geochemical indices (Tables 7, 9). The organic geochemical data indicates that the hydrocarbons originate from source rocks with variable characteristics. Both terrigenous and marine precursors of organic matter are inferred (Table 9); the deposition took place under relatively anoxic conditions in different palaeoenvironments with variable salinity, as 24tetra/26tri ratio and OEP 27-31 index (Scalan and Smith, 1970; Singh et al., 2017) indicated, ranging from lagoonal to open marine ones. These variations could represent vertical variations of a single formation or lateral transitions of

20

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the source rock’s palaeoenvironments, caused by transgressive and regressive cycles in the epicontinental setting of Nigeria during Cretaceous (Adegoke, 1977, within Enu, 1987). Synthesizing all the presented data and the respective interpretations it is evident that the

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source rock of the solid hydrocarbons is different from the sandy lithologies hosting tar nowadays. The lack of a qualified formation beneath Afowo Fm to act as a source rock, since ISE Fm corresponds to continental sandstones and conglomerates deposited above the crystalline basement (Omatsola and Adegoke, 1981), leaves one alternative. Afowo Fm as a whole is the

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source and the host one; however, only the shale horizon occurring in the middle part of this formation (Fig. 3b) acted as a source rock. In the examined sites, only sample #2 seems to correspond to the shale horizon, whereas the rest samples are transitional to or represent the

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sandy ones.

The tectonic evolution of Dahomey and its adjacent basins caused the development of a series of ridges and grabens with variable subsidence both in an East-West, as well as NorthSouth direction (Omatsola and Adegoke, 1981; Coker et al., 1983). The result was Afowo Fm to reach depths from few hundred meters in the central part of the study area (north of Araromi

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Obu, see Fig. 2) to >3 km southwards off shore in Ise Graben (Omatsola and Adegoke, 1981; Coker and Ejedawe, 1987). This variation in the subsidence is confirmed from the reflectance values presented in this paper. The low maturity indicated by the indigenous VRo of 0.45% corresponds to parts of the Afowo Fm that never went deep enough and extended in the ridges or

EP

the shallow grabens (Fig. 12). The hydrocarbons, on the other hand, are sourced from the parts of the Afowo Fm that reached the oil window maturation stage of EqVRo 0.76%, where the shale

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horizon being rich in alginite and other liptinite macerals generated hydrocarbons, which thereafter migrated to the upper parts within the sandy lithologies. The intense biodegradation recognised in the studied outcrops is coinciding to the distant migration of the hydrocarbons.

6. Conclusion Bituminous sediments of the Afowo Formation outcrop in significant part of the Dahomey Basin. These sediments represent shales up to coarse-grained sands, referred usually as “tar 21

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sands”, and consist of mainly quartz and kaolinite, with variable amounts of mixed clay layers of illite-montmorillonite, micas and pyrite. The mineralogical assemblages, as well as the geochemical composition of the sediments

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indicate intermediate to felsic igneous provenance, pointing to the Migmatite Gneiss Complex, exposed to the north, as the parental material. This metamorphic terrain suffered intense weathering during the Cretaceous and clastic material was transported and deposited in an oxic shallow marine environment, forming the Afowo Formation. The origin from the Migmatite Gneiss Group provides an explanation for the relative enrichment in the natural radionuclides K, 238U, 226Ra and 232Th for some samples, although in general the radioactivity falls within the

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40

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range for world values for soils.

Afowo Fm was formed during Cretaceous in a transitional brackish to marine near-shore environment, where siliciclastic material of terrigenous-origin deposited under oxic (sandy horizons) to suboxic/anoxic conditions (shale horizon). The contained synsedimentary organic matter comprises both terrestrial and marine organic matter, with the latter being more profound in clay-rich lithologies (shales).

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The particulate organic matter of the Dahomey bituminous sediments consists of solid hydrocarbons, coalified (huminite) and partially oxidized (inertinite) remnants of terrestrial plants, and minor amounts of both terrestrial and marine liptinite macerals. The mean random

EP

reflectance of the indigenous huminite population ranges between 0.40-0.45%. The respective reflectance of the homogenous migrabitumens ranges from 0.49-0.59% corresponding to vitrinite

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equivalent reflectance values of 0.70-0.76%, well within the oil window. The hydrocarbons hosted in the Dahomey bituminous sands, seem to have been originated from the Afowo Formation, in which the middle shale horizon acted as a source rock and the upper sandy horizon as migration path and/or reservoir. Biodegradation of the hydrocarbons is intense and related to the distance from the part of the basin that was affected by significant tectonic subsidence. Further work is needed in order to define precisely the lateral characteristics and the migration pathways of the tar in Afowo Fm. 22

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Acknowledgements The authors are also grateful to Prof. Dr. Nikos Lambrakis, Laboratory of Hydrogeology, Dept.

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of Geology, University of Patras, for performing the ICP-MS determinations; Ms. Vaya Xanthopoulou, Laboratory of Electron Microscopy and Microanalysis, Faculty of Natural Sciences, University of Patras, for conducting the XRF analysis; Mr. Dimitrios Vachliotis, Laboratory of Instrumental Analysis, Faculty of Sciences, University of Patras, for performing

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the ultimate analysis; and Ms. Rafaella Ioannou, Dept. of Geology, University of Patras, for

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laboratory assistance.

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Reflectance

of

Vitrinite.

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Nesbitt, H.W., Young, G.M., 1982. Early Proterozoic climates and plate motions inferred from major element chemistry of lutites. Nature 299, 715-717.

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Nwajide, C.S., 2013. Geology of Nigeria's sedimentary basins. CSS Bookshops Limited, Lagos, Nigeria, 565 pp.

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Whiteman, A., 1982. Nigeria: its petroleum geology, resources and potential. Graham and Trotman, London. Wright, J.B., Hastings, D.A., Jones, W.B., Williams, H.R., 1985. Geology and mineral resources

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of West Africa. George Allen and Unwin, London. Zumberge, J.E., 1983. Tricyclic diterpane distributions in the correlation of Palaeozoic crude oils from the Williston Basin. In: Bjoroy, M., Albrecht, C., Cornford, C., et al. (Eds), Advances

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in Organic Geochemistry. John Wiley and Sons, New York, pp. 738-745.

33

ACCEPTED MANUSCRIPT Fig. 1: Regional geological map of western Africa showing the location of the Dahomey Basin (modified after Wright et al., 1985). Fig. 2: Geological map of Dahomey Basin showing sampling locations (modified after Billman, 1992). Fig. 3: Stratigraphic column of the Dahomey Basin (after Omatsola and Adegoke, 1981, and

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Enu, 1987, modified; not to scale).

Fig. 4: H/C vs. O/C diagram of the Nigerian bituminous sediment samples (after van Krevelen, 1993).

continental crust values (Mason and Moore, 1982).

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Fig. 5: Multi-element diagram for trace element concentrations normalized against upper

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Fig. 6: GC-MS (TIC) chromatograms from samples #1, 2 and 3.

Fig. 7: Geochemical classification of the studied sediment samples after the log(SiO2/Al2O3) vs. log(Fe2O3/K2O) diagram proposed by Herron (1988).

Fig. 8: Scatter plot of the 4-factor model loadings showing grouping of elements and minerals (KL: kaolinite, Mcl: clay minerals, Py: pyrite, Qz: quartz).

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Fig. 9: Scatter plot of the 4-factor model scores showing grouping of samples. Fig. 10: Correlation between Zr content and bituminous sediments.

232

Th activity concentration in the Nigerian

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Fig. 11: a) Discriminant plots for provenance signatures of the Nigerian samples (after Roser and Korsch, 1988); for discriminant factors see Table 6. b) TiO2 vs. Zr scatter plot (after

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Hayashi et al., 1997).

Fig. 12: Schematic illustration of the Afowo Fm, maturation history and HC migration paths; tectonic evolution based on Omatsola and Adegoke (1981), modified.

ACCEPTED MANUSCRIPT

Igborisa / Ondo

Altitude a.s.l. (m) 38

Thickness of outcrop (m) ~0.3

Thickness of sampled layers (m) ~0.3

N06o 39’ 08’’

E004o 58’ 25.57’’

2

Oke-idebi / Ondo

71

~0.3

~0.3

N06o 38’ 40.2’’

E004o 39’ 26.2’’

3

Ladan 1 / Ondo

62

~0.3

~0.3

N06o 38’ 59.9’’

E004o 42’ 37.8’’

4

Lomitile / Ogun

32

~0.3

~0.3

N06o 41’ 53.8’’

E004o 14’ 22.6’’

5

Ijuoke / Ondo

64

~0.3

~0.3

N06o 38’34.2’’

E004o 38’ 55.6’’

6

Yegbata 2 / Ondo

40

~0.3

~0.3

N06o 38’ 22.0’’

E004o 42’ 06.6’’

7

Labora / Ogun

21

~0.3

~0.3

N06o 41’ 50.4’’

E004o 14’ 43.4’’

8

Ladawo / Ondo

49

~0.3

~0.3

N06o 38’ 51.2’’

E004o 40’ 38.6’’

9

Oke-oyinbo / Ogun

22

~0.3

~0.3

N06o 41’ 56.2’’

E004o 16’ 19.9’’

9a

Oke-oyinbo / Ogun

22

~0.3

~0.3

N06o 41’ 48.2’’

E004o 16’ 39.2’’

10

Yegbata / Ondo

19

~0.3

~0.3

N06o 38’ 31.4’’

E004o 42’ 00.7’’

11

Ladan 2 / Ondo

48

~0.3

~0.3

N06o 39’ 09.3’’

E004o 42’ 32.8’’

12

Imerin Ijebu / Ogun

64

1.0

0.5

N06o 46’ 48.0’’

E003o 58’ 39.4’’

13

Trianga / Ogun

44

4.0

3.0

N06o 41’ 47.1’’

E004o 12’ 12.7’’

14

Lobuko / Ondo

26

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Table 1: Location of the sampling sites of the Nigerian bituminous sediments

~0.3

~0.3

N06o 39’ 59.7’’

E004o 37’ 03.2’’

15

Orisumbare / Ogun

20

0.9

0.9

N06o 42’ 24.1’’

E004o 2’ 26.9’’

16

Onikitingbi / Ogun

19

3.0

0.2

N06o 40’ 27.2’’

E004o 18’ 23.1’’

17 18

Gbegude / Ondo Idiobilayo / Ondo

37 58

5.0

3.2 0.5

N06o 40’ 04.5’’ N06o 38’ 36.3’’

E004o 25’ 14.9’’ E004o 34’ 47.4’’

5.0

GPS coordinates

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1

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Location / State

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Sample

ACCEPTED MANUSCRIPT

Table 2: Macroscopic features of the studied bituminous sediments Macroscopic description

Consistency

Colour

tar

viscous

black

2

silt

particulate

black

3

sand

particulate

dark brown

4

sand

particulate

black

5

tar

viscous

black

6

sand

particulate

dark brown

7

tar

viscous

black

8

tar

viscous

dark brown

9

tar

viscous

black

10

sand

particulate

dark brown

11

sand

particulate

black

12

sand

particulate

black

13

silt

particulate

black

14

sand

particulate

dark brown

15

silt

particulate

16

sand

17 18

remains

Mz

SD

Sk

Ku

Lithology

0.431

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1

Grain size analysis

Plant

1.006

0.204

1.158

coarse-grained sand

0.926

0.920

-0.325

1.846

coarse-grained sand

0.998

1.070

-0.439

1.248

coarse-grained sand

2.956

0.330

0.037

0.741

fine-grained sand

1.155

0.940

-0.416

1.263

medium-grained sand

abundant

1.411

0.841

-0.181

0.972

medium-grained sand

abundant

1.222

0.903

-0.183

1.342

medium-grained sand

black

1.085

0.779

-0.334

0.891

medium-grained sand

particulate

dark grey

-0.329

0.931

0.479

1.168

coarse-grained sand

silt

particulate

black

0.443

1.061

0.258

1.138

coarse-grained sand

silt

particulate

dark grey

2.127

0.727

-0.149

1.389

fine-grained sand

2.013

abundant

0.953

fine-grained sand

0.325

-0.036

0.737

fine-grained sand

abundant

abundant

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EP

AC C

2.983

0.049

SC

abundant

1.047

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Sample

Mz: mean grain size (in mm); SD: standard deviation; Sk: skewness; Ku: kurtosis in phi (Ø) units

ACCEPTED MANUSCRIPT Table 3: Mineralogical composition of selected particulate samples Quartz Kaolinite Illite Pyrite Aragonite Mica Chlorite Copiapite Brookite (in wt.% of crystalline phases) 70.6

25.7

3

33.5

65.5

10

64.0

26.5

11

97.0

12

95.5

2.5

14

85.0

10.5

3.0

15

33.5

41.5

24.5

17

93.5

6.5

18

59.0

38.0

3.7 1.0

8.0

1.5 1.8

1.2

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2

2.0

1.5

0.5

0.5

AC C

EP

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2.0

SC

Sample

ACCEPTED MANUSCRIPT Table 4: Element composition and ash yield of the Nigerian bituminous sediment samples

12.68 2.36 0.45 7.09 21.59 0.08 9.69 0.46 0.13 0.06 0.09 29.73 1.28

18.37 2.44 0.32 0.07 4.10 0.07 1.52 0.07 0.03 0.02 0.09 44.34 0.43

27.84 3.77 0.55 0.17 8.06 0.07 2.37 0.44 0.07 0.04 0.09 29.41 0.79

7.84 0.97 0.18 0.04 0.59 0.05 0.15 0.04 0.01 0.01 0.03 62.75 0.04

1.5 84.8 78.5 2.7 1.5 0.8 20.2 69.7 2.7 26.8 21.8 180.8 6.0 33.4 29.0 31.8 18.3 32.4 99.1 0.8 6.0 44.0 162.0 224.0 179.0 103.0 59.5

0.4 <0.1 6.9 0.1 0.2 <0.1 0.9 <0.1 <0.1 <0.1 0.8 8.5 6.0 3.8 14.0 2.3 0.3 5.5 7.7 0.8 <1 11.0 21.1 209.0 37.0 19.0 78.1

1.6 <0.1 41.3 0.5 0.5 0.1 2.4 4.1 1.2 <0.1 5.6 24.4 11.0 14.3 25.0 7.9 7.5 10.4 21.2 1.7 <1 20.0 22.7 410.0 53.0 38.0 61.7

0.9 0.1 16.8 0.1 0.1 <0.1 0.7 10.0 0.1 0.0 1.1 3.2 1.0 2.9 7.0 1.8 1.1 12.4 6.1 0.2 1.0 3.0 7.2 36.0 18.0 16.0 90.0

a c

Samples 12

14

15

17

18

4.33 0.60 0.20 0.32 1.93 0.02 0.76 0.02 0.01 0.01 0.02 76.14 0.11

8.43 1.10 0.26 0.03 4.43 0.03 0.30 0.18 0.01 0.01 0.03 63.36 0.25

14.09 1.80 0.31 1.80 12.42 0.09 3.48 0.13 0.03 0.03 0.05 46.74 1.15

7.60 1.00 0.16 0.32 1.77 0.05 0.66 0.12 0.03 0.01 0.06 67.61 0.11

13.37 1.69 0.31 2.08 11.78 0.08 3.42 0.12 0.03 0.02 0.05 45.85 1.20

0.2 <0.1 8.0 0.2 <0.1 2.7 1.0 2.4 0.1 <0.1 0.4 4.8 2.0 0.5 7.0 2.1 <0.1 7.1 <0.1 6.3 <1 5.0 15.5 70.0 2.0 5.0 93.6

0.2 <0.1 8.0 0.2 0.1 2.8 1.0 2.4 0.1 <0.1 0.4 4.8 6.0 0.5 14.0 2.1 <0.1 7.1 <0.1 1.6 1.0 11.0 15.5 210.0 32.0 9.0 89.5

0.7 25.2 23.3 0.4 0.1 0.1 1.1 <0.1 0.1 <0.1 2.5 17.3 13.0 4.1 24.0 7.7 3.0 5.8 3.8 1.9 2.0 30.0 18.0 468.0 77.0 48.0 75.6

0.7 <0.1 26.5 0.2 <0.1 1.0 1.1 <0.1 0.3 0.5 0.6 8.2 2.0 4.2 7.0 2.7 2.4 11.2 2.1 3.7 <1 5.0 8.8 62.0 22.0 16.0 90.9

0.6 389.2 36.7 1.2 0.1 0.3 9.4 11.1 0.4 1.5 7.6 60.0 17.0 15.6 26.0 14.1 2.3 16.1 37.3 3.5 3.0 31.0 90.0 617 90.0 57.0 78.8

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11

SC

10

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M AN U

3

AC C

in % Ca Ha Na Sa Al2O3b CaO b Fe2O3b K2Ob MgOb Na2Ob P2O5b SiO2b TiO2b in mg/kg Agc Asc Bac Bec Bic Cdc Coc Crc Csc Cuc Gac Mnc Hfb Nic Nbb Pbc Rbc Src Vc Uc Wb Yb Znc Zrb Ceb Lab Ash %

2

EP

Elements

: concentrations determined by using Carlo Erba; b: concentrations determined by using XRF spectrometry; : concentrations determined by using ICP-MS.

ACCEPTED MANUSCRIPT Table 5: Geochemical indices applied to evaluate the Nigerian samples

H/Ca O/Ca CIAb Al2O3/TiO2 DF1c DF2d Ni/Co

2 2.24 1.06 97.3 16.8 8,43 -6,66 1.65

3 1.59 0.03 96.3 9.6 -6,28 -6,70 4.22

10 1.58 0.13 93.7 10.2 -4,37 -5,97 5.96

11 1.48 0.09 86.4 15.2 -8,70 -6,79 4.14

a

Sample 12 14 1.67 1.57 0.16 0.06 97.2 95.2 18.2 17.4 -7,54 -6,83 -6,84 -6,25 0.50 0.50

15 1.54 0.34 98.0 10.8 -1,07 -6,19 3.73

17 1.58 0.01 91.3 16.6 -7,85 -6,72 3.82

18 1.52 0.21 98.1 9.8 -1,60 -6,20 1.66

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Index

AC C

EP

TE D

M AN U

SC

: Atomic ratios after van Krevelen (1993); oxygen was calculated by difference [O = 100 – (C + H + N + S + Ash)]dry base b : CIA = [(Al2O3)/(Al2O3 + CaO + Na2O + K2O)] x 100 (after Nesbitt and Young, 1982) c : Discriminant function 1: = (-1.773 x TiO2) + (0.607 x Al2O3) + (0.76 x Fe2O3) + (-1.5 x MgO) + (0.616 x CaO) + (0.509 x Na2O) + (-1.224 x K2O)+(-9.09) d : Discriminant function 2: = (0.445 x TiO2) + (0.07 x Al2O3) + (-0.25 x Fe2O3) + (-1.142 x MgO) + (0.432 x CaO) + (0.509 x Na2O) + (1.426 x K2O)+(-6.861) (after Roser and Korsch, 1988)

ACCEPTED MANUSCRIPT

Table 6: Semi-quantitative organic-petrographical composition of the Nigerian bituminous sediments 3 WR

4 KC

5 KC

6 WR

7 KC

8 KC

9 KC

10 KC ++

+

+ + +

+++

+

+ + ++

++ ++ +

+ + +

++ +

++ ++ +

+

++

+

+ +

+

+

++ ++

+++

+++ +

++

+++ ++

+

++ +

+++ ++

+

+

+ +

+

+++ +

++ +

++

+ + + +

++

++

++ +

++ +

+

TE D

+++ +++

++ ++ + ++

EP

+

12 WR

13 KC

14 WR

+ + ++

+ ++

+++ +

+

+ +++

+ +

+ +

++

15 WR

16 WR

++ +

+

17 WR

+

+

++

+

++

18 WR

+ ++

+

+

+

++

+ + + +

++

++

++

M AN U

+++

11 WR

RI PT

2 WR

SC

1 KC

AC C

Sample Particle type Huminite Textinite Ulminite Corpohuminite Vitrinite Telo-/Detrovitrinite Reworked Vitrinite Inertinite Fusinite Semifusinite Funginite Inertodetrinite Liptinite Cutinite Resinite Sporinite Suberinite Alginite Liptodetrinite Solid Hydrocarbons Migrabitumen Pyrobitumen Other Zooclasts Oil drops Minerals Pyrite Marcasite Clays Quartz

+

+

++ +

++ +

+ +++

+

+

++

++ + ++ ++

+

+ ++ ++

++ +++

+++

+

+

++ +

+ +++

+++ ++

+

++ + ++

+: minor, ++: common, +++: abundant, KC: kerogen concentrate, WR: whole rock

++

++

++ + + +

++ + +

ACCEPTED MANUSCRIPT Table 7: Mean random reflectance values (%Ro) measured on various components in the Nigerian bituminous sediments

1 2

0.53 0.05 0.34

0.04

3

4.22

Std HRo

Std

VRo Std RVRo Std EqVRo

1.61 0.21

0.04

0.45 0.10

0.73

0.44 0.10 0.82 0.10

0.61

4.86 0.52 0.05

2.80

0.91 0.22

5.10

0.10

4

0.26

0.01 0.56 0.02

5

0.28

0.01

6

0.37

0.01

0.20 1.83

7

0.57 0.01

8

0.59 0.04

9

0.28

0.02

10

0.26

0.01 0.49 0.04

0.01

0.01

0.57

5.10

0.10

4.36

0.74 0.21 0.24

3.80

0.59

12

2.89

0.35

0.10 0.24

14

0.27

0.01 0.53 0.02

5.10

0.30

15

0.26

0.04 0.52 0.01

4.40

0.69 0.34

0.03

16

0.24

0.01

4.37

0.60 0.31

0.01

4.61

0.41

TE D 3.99

0.38

0.63 0.75 0.76

0.70

5.10

0.01 0.31 0.04

0.57

0.01

0.01

0.23

0.40 0.05 0.58 0.01

0.57

0.25

18

0.74

0.04

13

17

0.41 0.06 0.69 0.11

M AN U

11

0.72

RI PT

BmRo Std BhRo Std PBRo

SC

Sample

0.03

0.60 0.01

0.55

0.77 0.01 0.65 0.01

0.43 0.02 0.96 0.07

0.55

0.59

AC C

EP

Bm: microgranular migrabitumen; Std: standard deviation; Bh: homogenous migrabitumen; PB: Pyrobitumen; H: huminite; V: vitrinite; RV: reworked vitrinite; EqVRo: vitrinite equivalent reflectance according to Jacob (1989), EqVRo = 0.618 x BRo + 0.4

ACCEPTED MANUSCRIPT Table 8: SARA results of the Nigerian bituminous sediment samples

Sample

Bitumen

Maltenes

Saturates

Aromatics

NSO

(in weight %) 75.23

76.24

28.15

48.97

22.87

2

0.20

77.30

32.99

13.40

53.61

3

33.68

68.09

26.18

4

46.17

76.18

26.32

5

55.60

76.57

24.35

6

39.26

63.55

23.58

7

95.62

77.01

28.17

43.34

28.48

8

57.44

72.13

25.94

47.37

26.69

9

62.96

79.27

27.53

46.82

25.65

10

33.97

56.39

25.00

34.38

40.63

11

14.05

51.38

45.13

30.09

24.78

12

7.87

80.65

33.04

40.29

26.67

13

49.11

67.96

25.38

35.98

38.64

14

13.03

65.28

29.10

41.14

29.77

15

14.39

80.60

30.95

48.50

20.55

16

15.45

79.66

33.52

40.34

26.14

17

16.69

69.08

31.83

40.51

27.65

18

7.27

78.34

32.45

41.89

25.66

48.36

25.45

47.37

26.32

51.29

24.35

41.87

34.55

SC

M AN U

TE D

EP AC C

RI PT

1

ACCEPTED MANUSCRIPT

Table 9: Geochemical indices of the Nigerian bituminous sediment samples

0.17 0.06 0.37 0.07 1.05 1.16 2.27 0.80

1.08 0.99 1.15 1.02 1.21 1.11 245 258 0.19 0.90 1.30 0.11 0.68 6.30

6

1.07 1.06 1.11 1.06 1.23 0.94 218 598 0.00 3.22 3.97 0.23 0.64 1.80

0.85 3.25 0.35 0.07

7

0.93 1.00 1.36 268

8

9

1.13 1.09 1.14 1.24 1.00 1.14 362 148

10

2.94

11

12

13

14

RI PT

5

1.14 1.19 1.08 177

SC

4

1.35 1.96 1.47 74

1.19 1.07 1.04 1.22 1.02 1.01 1.07 1.00 0.79 339 300 2496 3.88 2.38 3.44 5.86 4.05

9.79 2.72 6.75 2.81 2.85 4.70 0.38 0.68 0.06 0.18 0.56 0.17 0.16 0.55 0.21 0.45 0.40 0.22 0.17 0.07 0.45 0.08 0.34 0.13 0.15 0.25 1.28 0.12 0.64 0.28 0.44 0.60 0.87 0.16 1.8 4.4 5.4 4.1 2.2 6.5 2.1 2.21 1.22 0.94 0.61 0.67 1.96 0.61 0.77 0.59 0.09 0.06 0.55 0.17 0.20 0.10 4.71 16.52 5.03 15.65 4.66 4.30 12.07 12.16 0.19 0.07 0.72 0.21 0.07 0.16 0.20 0.09 0.07 1.10 0.85 0.93 0.95 1.37 1.15 0.89 1.19 1.66 0.87 0.81 0.75 0.64 0.87 0.88 0.82 0.73 0.82 0.81 0.88 0.83 0.29 0.54 0.43 0.32 0.26 0.40 0.37 0.33 0.28 0.30 0.57 0.80 0.85 0.73 1.23 2.03 1.05 1.29 0.87 0.20 1.38 0.16 0.59 2.43 1.64 0.91 2.15

EP

1.83 1.38

3

M AN U

2 1.19 0.51 0.27 1.32 3.85 1.83 4.80 1.86 0.68 71 14294 0.04

0.98

TE D

1

AC C

Index / Sample Pr/Ph Pr/nC17 Ph/nC18 CPI OEP 27-31 R22 n-alkanes (ppm) 23tri/C30Hop 29tri/C30Hop (28+29)tri/C30Hop 24tetra/C30Hop Tricyclics/Hopanes 19tri/20tri 19tri/23tri (19tri+20tri)/23tri (19tri+20tri)/(23tri+24tri) 22tri/21tri 23tri/21tri 24tri/23tri 24tetra/23tri 25tri/24tetra 24tetra/26tri 26tri/25tri ETR Ts/(Ts+Tm) Ts/C30Hop Tm/C30Hop C29nor/C30Hop Moretane Oleanane Gammacerane

0.70

15

0.99 1.09 0.98 244 0.29 0.26 0.39 0.07 0.30 0.32 0.04 0.16 0.09 0.19 1.5 0.77 0.24 3.29 0.31 0.97 0.51 0.35 0.37 0.69 1.61 0.48 0.06 2.66

16

17

18

1.25 0.96 1.02 1.53 1.04 1.10 1.13 1.55 0.97 199 121 589 1.15 0.25 1.01 0.23 1.50 0.35 0.15 0.06 2.18 1.56 0.28 0.39 0.33 0.30 0.07 0.14 0.04 0.25 0.56 0.15 0.12 0.32 0.09 0.41 0.17 0.18 4.7 1.5 1.6 1.12 0.75 0.75 0.23 0.13 0.26 5.29 5.33 2.98 0.22 0.17 0.34 0.85 1.13 0.99 0.69 0.77 0.53 0.53 0.38 0.34 0.45 0.32 0.72 0.61 0.43 1.37 0.42 0.43 0.06 0.57 2.31

ACCEPTED MANUSCRIPT Table 10: Activity concentrations of natural radionuclides and 137Cs in the Nigerian bituminous sediment samples 238

Sample

226

U

232

Ra

40 137 Th K Cs (in Bq kg-1d.w. ± 1stot)

238

U/226Ra

238

U/232Th

222±17

179±11

54.5±3.0

120±21

bdl

1.24

4.07

2

10.0±1.8

5.0±0.4

4.4±0.4

bdl

0.66±0.1

1.99

2.30

3

24.4 ±5.4

25.9±1.8

19.8±1.6

27.4±17.5

bdl

4

69.7± 7.5

68.2±4.2

33.5±1.9

34.6±13

5

32.4±6.3

30.2±2.5

24.8±2.5

6

20.8±6.9

33.4±2.4

7

20.0±8.0

8

RI PT

1

1.23

0.69±0.17

1.02

2.08

70.8±20

3.3±0.6

1.07

1.31

23.0±1.8

bdl

bdl

0.62

0.90

17.6±1.2

2.7±0.6

43.5±13

0.40±0.1

1.14

7.41

38.6±5.8

17.6±1.6

14.2±1.4

23.3±16

bdl

2.19

2.71

9

19.1±4.5

20.4±1.5

28.4±2.0

26.1±17

bdl

0.94

0.67

10

33.2±7.0

35.1±2.3

28.8±1.9

59.4±20

bdl

0.95

1.15

11

6.4±4.0

5.9±0.5

1.4±0.5

bdl

bdl

1.08

5.66

12

110±8.5

135±14

6.05±1.1

31.2±15

bdl

0.82

18.2

13

39.4±6.2

25.1±1.7

12.8±1.2

37.0±15

bdl

1.57

3.08

14

23.0±4.6

23.0±1.7

36.5±2.2

54.7±16

bdl

1.0

0.63

15

56.1±6.9

49.6±3.2

42.3±2.7

bdl

bdl

1.13

1.33

16

36.0±5.9

34.7±2.4

28.7±2.0

32.2±16

bdl

1.04

1.25

17

21.4±5.0

8.5±1.3

9.2±0.9

32.0±13

bdl

2.52

2.33

51.2±2.6

27.0±15

bdl

1.0

1.13

M AN U

TE D

EP

AC C

18

58.0±7.2

58.0±4.8

SC

0.94

bdl: below detection limit; dl for 40K: 20 Bq kg-1; dl for 137Cs: 0.4 Bq kg-1 stot: Combined standard uncertainty

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ACCEPTED MANUSCRIPT

Plate 1. Photomicrographs of selected particles taken under immersion white light, in oil immersion and total magnification of 500x: a) Corp: corpohuminite; b) microgranular (Bm) and homogenous (Bh) migrabitumens within a clayey matrix; c) microgranular (Bm) and homogenous (Bh) migrabitumens surrounding quartz (Qz) grains; d) homogenous (Bh) migrabitumen interrelated with quartz grains; e) Pyrobitumen (PB) particles; f) pyrobitumen (PB) fracture-filling texture.

ACCEPTED MANUSCRIPT • The Turonian-Maastrichtian Afowo Formation, SW Nigeria, includes - among other sedimentary rocks - bituminous (tar) shale and sand strata outcropping at several places. • The inorganic clastic matter derived from weathering of the metamorphic rocks exposed to the north of the basin. • The particulate organic matter comprises mainly solid hydrocarbons migrating from deeper

RI PT

source rocks, as well as remnants of terrestrial plants, and minor amounts of both terrestrial and marine liptinite macerals.

• Biomarker analyses along vitrinite reflectance data indicate that the shale horizon of the

AC C

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Afowo Fm represents the source rock, while the sandy ones the reservoirs of the tar.