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VOL. 3, NO. 1, January 2013 ISSN 2225-7217 ARPN Journal of Science and Technology

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The Occurrence of Tar Sands at Ijebu-Itele, Eastern Dahomey Basin, SW, Nigeria

1 Adewale. A Akinmosin, 2Kamal’deen,O Omosanya, 3 Tolulope Ige 1,3 Department of Geoscience, University of Lagos, Akoka

2 Department of Earth Sciences, Olabisi Onabanjo University, Ago-Iwoye, Nigeria.

ABSTRACT

This work is an addition to our previous effort on the tar sand deposits in southwestern, Nigeria. Electrical resistivity sounding and profiling using Schlumberger and Wenner array configuration were used to determine the nature and depth of occurrence of the tar sand unit. Core description of five boreholes in and around the study area was exploited for regional correlation of the tar sand horizon. The tar sand at Itele has a resistivity value of ~900Ωm to >1480Ωm with an average thickness of ~30m. Visual inspection of the cores from the study area shows that the lithology of the tar horizon is essentially muddy, non-fissile, fine-grained sand. In addition, the tar sand of the area belongs to the high resistivity facies identified by previous workers in the Ogun state axis of the tar sand belt. Keywords: Cores, electrical resistivity, tar sand, Itele. 1. INTRODUCTION The Nigerian tar sand and associated oil seepages occur in southwestern Nigeria within the Eastern Dahomey basin in an E-W belt. This region is ~120km long and ~4km to 6km wide from Edo/Ondo to Ogun state (Enu, 1985 [9]). The tar bearing horizon is found in two principal sediments designated as “X” and “Y” horizon. Previous localities studied include Idiobiolayo (Akinmosin and Shoyemi, 2010 [6]), Agbabu (Amigun et al., 2012 [7]), Imeri (Akinmosin et al., 2011 [5]), Idiopopo (Odunaike et al., 2010 [20]), Onikitibi (Akinmosin et al., 2012 [4]), and Imobi (Ikhane et al., 2011 [11]). These works used sedimentological and geophysical methods to characterize the bitumen and its host units. Controversies bothering on the nature of the tar sands in Nigeria include the facie types and the depth of the horizon bearing units. In contrast to earlier workers, twentieth century research works have revealed newer depth of occurrence and two main electrical resistivity facies. The high resistivity facies of Akinmosin et al., 2011 [5]; and low resistivity facies of less than 600 Ωm (Ikhane et al., 2011 [11]; Amigun et al., 2012; [7] Odunaike et al., 2010 [20]). The first of occurrence of tar sand/bitumen in Itele was documented by the Nigerian Bitumen Corporation; however, the area has received less attention from contemporary workers owing to the sparse accessibility to data and aversion by residents of the area. The aims of this study are a) to ascertain the occurrence of tar sand (bitumen) in the area. b) To integrate different subsurface data (resistivity and cores). c) To attempt a regional correlation of the tar sand horizon and d) to validate previous exploration activities of the Nigerian Bitumen Exploration Company in the area. The paper starts by summarising the geological setting of the Eastern Dahomey basin, the electrical and

lithological characteristic of the tar sand. We attempted a regional correlation of the tar sand horizon and also assess the depth of occurrence of the tar bearing rock at Itele. 2. REGIONAL GEOLOGY The Dahomey basin is a peri-cratonic basin developed during the initiation of rifting associated with the opening of the Gulf of Guinea in early Cretaceous to Late Jurassic (Burke et al., 1971 [8]; Klemme, 1975 [14]; Whiteman, 1982 [24]; Kingston et al., 1983 [13]). The crustal separation and thinning of the basin was accompanied by an extended period of thermal-induced basin subsidence through the mid Cretaceous to Tertiary times as the South American and the African plates entered a drift phase to accommodate the emerging Atlantic Ocean (Storey, 1995 [23]; Mpanda, 1997 [16]). The Dahomey basin is bounded on the west by the Ghana ridge, an offset extension of the Romanche Fracture Zone (RFZ), and by the Benin hinge line on its eastern boundary. The Benin hinge line supposedly defines the continental extension of the Chain Fracture Zone (CFZ), and it is a basement escarpment which separates the Okitipupa structure from the Niger delta basin. The onshore part of the basin covers a broad arc-shaped profile of ~600 km2 in extent. The onshore section of the basin attains a maximum width, along its N-S axis, some 130 km around the Nigerian – Republic of Benin border. The basin narrows to about 50 km on the eastern side where the basement assumes a convex upwards outline with concomitant thinning of sediments. Along the northeastern fringe of the basin where it rims the Okitipupa high is a brand of tar (oil) sands and bitumen seepages (Nwachukwu and Ekweozor, 1989 [19]). The lithostratigraphic unit of the eastern Dahomey basin is summarized in Table 1. The rocks of




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the study area belong to the Abeokuta group, the oldest group of sediment in the basin (Jones and Hockey, 1964 [12]). Omatsola and Adegoke (1981) [22] recognized three (3) Formations in the Abeokuta group based on lithologic homogeneity and similarity of origin. They include Ise, Afowo and Araromi Formations; as a group, they represent the thickest unit within the basin. Ise Formation: This is the oldest Formation in the Abeokuta group and it unconformably overlies the basement complex. The Ise Formation consists of conglomerate at the base, gritty to medium grained loose sand, capped by kaolinite clay (Omatsola and Adegoke 1981 [22]; Agagu 1985 [2]). The maximum thickness of the members is about 1965m and 600m penetrated by the Ise-2 well, while similar section were exposed near Ode-Remo on the Lagos-Ibadan expressway. The age is Neocomian and the unit has not been found to be bituminous both at surface and subsurface section. Afowo Formation: The Afowo Formation is composed of coarse to medium grained sandstone with variable but thick interbedded shale, siltstone and

claystone. The sandy facies are tar-bearing while the shale are organic-rich (Enu, 1985 [9]). Using palynological assemblage, a Turonian age is assigned to the lower part of this Formation, while the upper part ranges into Maastrichian. Araromi Formation: Sediments of the Araromi Formation represent the youngest topmost sedimentary sequence in the Abeokuta group. The Formation is composed of shales, fine-grained sand, thin interbeds of limestone clay and lignite bands (Omatsola and Adegoke 1981 [22]; Agagu, 1985 [2]). It is an equivalent of a unit known as Araromi shale. The age ranges from Maastrichian to Palaeocene. The Abeokuta group is overlain by the Imo (Ewekoro and Akinbo Formations- Adegoke, 1977 [1]; Jones and Hockey, 1964 [12]; Ogbe, 1972 [21]; Nton 2001 [17]; Nton and Elueze, 2005 [18]), the Oshosun (Jones and Hockey, 1964 [12]; Nton, 2001 [17]), Coastal plain sands and the Recent Alluvium (Jones and Hockey, 1964) [12].

Fig 1: a) Location of the study area highlighting the position of the Ijebu-Itele borehole and survey lines for the

geophysical investigation b) The approximate location of the different boreholes used for the regional correlation of the tar horizons. Inset is the map of Nigeria and Africa.

3. METHODOLOGY 3.1 Core Description Core samples were obtained from five (5) wells located in southwestern, Nigeria (Fig. 1b). The boreholes are from Ijebu-Itele, Ijagun (both in Ogun State), Ajegunle, and two locations in Idiobilayo (both locations are on the geographic boundary of Ogun and Ondo

State). These boreholes were drilled to depth of ~59m, ~109m, ~104m, ~45m and ~51m respectively, with core sampling interval of 3m. Core samples collected from locations other than the study area were used for regional correlation. The samples were laid out and arranged sequentially according to depth. The visual descriptions of the samples include lithology, colour, texture, grain size, sedimentary structures, and fractures.




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3. 2 Geophysical Methods In this study, two Vertical Electrical Sounding (VES) and one 2D Wenner-profiling were carried out. The ohmeter terrameter measures the variation in the electrical resistivity of the subsurface by injecting electric current through current electrodes (AB) and picking the potential difference from the potential electrodes (MN). The instrument is designed to measure the resistance; initial parameters used for the survey include a current of 5mA and four cycles for averaging the resistance value. Furthermore, two VES stations were designed using Schlumberger array over a traverse ~160m. For profiling, the Wenner electrode configuration was used with an assumption that the resistivity doesn’t change in a direction perpendicular to the survey line. The Wenner configuration entails that constant electrode spacing is maintained between the adjacent electrodes while the whole spread is transverse. The electrode spacing

between the adjacent electrodes is assigned “a” with initial spacing of 25m and subsequent spacing being multiple of 25. The maximum electrode spacing used was 100m over a survey line of 300m. The apparent resistivity was obtained from the product of the resistance and the geometric factor for the adopted electrode configuration. The value obtained were plotted against the electrode spacing (AB/2) on a log-log paper, then curved matched on a standard master curve in order to build a resistivity model for iteration on the WINRESIST software. From the final plot, the resistivity, depth and thickness of the different layers were estimated. Consequently, the apparent resistivity value was uploaded onto the RES2D INV version 3.55 software in order to produce the profiling pseudosection. The contoured pseudosection was inverted to plot the apparent resistivity against true vertical depth.

Table 1: The stratigraphic column of Eastern Dahomey Basin.

Omatsola and Adegoke (1981) Agagu (1985)

Formation Age Formation Age Formation Alluvium Recent Alluvium

Coastal Plain Sand Ilaro Ewekoro

Pleistocene- Oligocene Eocene Palaeocene

Coastal Plain Sand Ilaro Oshosun Akinbo Fm

Pleistocene- Oligocene Eocene Palaeocene

Coastal Plain Sand Ilaro Oshosun Akinbo Ewekoro

Abeokuta Maastrichtian- Neocomian

Araromi Afowo Ise

Maastrichtian- Neocomian

Araromi Afowo Ise

Precambrian Crystalline Basement Rocks 4. RESULTS 4.1 Core Logging And Regional Correlation The lithology of interest is the interval where the tar sand is present, and this is needed in order to characterise the tar sand in the study area, regionally correlate it with other places, and also integrate the core logs with the resistivity pseudo sections. At idiobilayo, two (2) wells were logged with the tar sand intercepted at depth of 12-21m, 24-27m and 39-42m respectively (Fig. 3). The tar is hosted by coarse sands, the last tar sands interval directly overlying the weathered basement. This implies that the tar sands has a combined thickness of ~15m and occurs in an interval that is ~30m thick. However, the tar sand occurs at depth of 51-57m and 69-81m at Ajegunle with thickness of 6m and 12m respectively. The first interval overlies a tar-

bearing peat. At Ijebu-Itele (Fig. 3), the tar unit is a non fissile, very-fine, muddy sandstone or shale, which occurred at three intervals 24-27m, 30-33m and 42-45m (Fig. 2). The tar horizon is ~21m thick. Conversely, east of the study area, one interval was intercepted at Ijagun. The tar sand occurs at depth of 21-27m with thickness of 6m. Lithological correction of the wells revealed that the tar bearing horizon is very sandy to the east and becomes very-fine sand and shale to the west (Fig. 3). Thus, two lithological facies are identified in the region, the sandy and very-fine to shaly tar sands. At depth, the horizons occur at shallow depth in the east (Idiobilayo 1, Idiobilayo 2 and Ijagun) and deeper levels in the west (Ijebu-Itele and Ajegunle); shallow horizons occur at less than 30m depth.




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Fig 2: a, c, d shows the exposure of the tar units along the survey line. In hand specimen, the tar sand is made of very

rounded, very-fine sands. b) Shows the layout of electrodes for the geophysical survey. The resistivity meter is ohm-meter terrameter that measures resistance averaged over 4 cycles.

Fig 3: Lithological Correlation of the tar bearing horizon across the boreholes. The tar unit occur at two principal depth,

<30m for shallow occurrence and >30m for deeper units. Lithologies of the tar bearing rock include sand and very-fine to shaly sands.




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Table 2: Inferred lithology, depth, thickness and resistivity from the VES points

Layer VES 1 Depth Thickness Resistivity Lithology

1 1 0-2.92 2.92 514.20 Sandy Topsoil 2 2.92-47.57 44.65 900.06 Sandstone (Tar Sand) 3 47.57- ∞ ∞ 650.89 Sandstone 1 2 0-3.12 3.12 610.50 Sandy Topsoil 2 3.12-45.93 42.81 1084.43 Sandstone (Tar Sand) 3 45.93-∞ ∞ 763.79 Sandstone

Fig 4: Geological model for the smooth inversion of the apparent resistivity pseudosection. The tar unit occur as seepage

on the western arm of the section and also as sand lense at the central part of the study area.

Fig 5: Geological interpretation of the robust inversion of the apparent resistivity pseudosection. The tar horizons occur as

seepages on the western and eastern arm of the pseudosection.




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Fig 6: (a) Plot of apparent resistivity versus AB/2 for VES 1 and (b) VES 2. Both curves are 3-layers and k-type curves.

Fig 7: Inferred lithosection and correlation of the two VES stations. The uppermost sandy topsoil dips to the east while the

interpreted tar horizon dips to the west. 4.2 Geophysical Data 2D Profile The resistivity pseudosections was interpreted according to the resistivity value suggested by Milsom, 2003 [15]. The apparent resistivity value was inverted in order to plot these resistivities values against true vertical depth rather than electrode spacing. The inversion procedure includes both the smooth and robust inversion (Fig 4 and 5). From the smooth inversion Pseudosection (Fig. 4), five (5) lithologic units were identified. They include the clayey topsoil, tar sand, clayey sand layer, sandy layer and sandstone with resistivities values of <137Ωm, >1480 Ωm, 137-356 Ωm,

356-572 Ωm and 572-<1480 Ωm respectively. The tar sand is thought to outcrop west of the Pseudosection and also occur as lenses at the centre of the pseudosections. Conversely, the lithology identified from the robust inverted pseudosection include the sandy topsoil, sandy layer, sandstone and tar sand with resistivity values of 470- <686 Ωm, 332-470 Ωm, 686- 1002 Ωm and >1002 Ωm respectively (Fig. 5). The tar sand is exposed at the western and eastern arm of the Pseudosection; this gave credence to the exposed section of the tar sand horizon seen on the footpath where the profiling was done (Figs. 2a, c and d).




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Vertical Electrical Sounding The plot of apparent resistivity against electrode spacing AB/2 (m) is shown in Figs 6a & b. These curves are 3-layer curve, with resistivity ρ1<ρ2> ρ3 characteristic of K-type curve. The three (3) layers interpreted at VES station 1 include the sandy topsoil, tar-bearing sandstone and sandstone with resistivity value of 514.20, 900.06 & 650.89Ωm and thickness of 2.92m, 44.65m and infinity respectively (Table 2, Fig. 7). Consequently, the lithological units inferred from VES station 2 are sandy topsoil, tar-bearing sandstone and sandstone with resistivity value of 610.50, 1084.43 & 763.79Ωm and thickness of 3.12m, 42.81m and infinity (Table 2, Fig. 7). The 2D pseudosection built from the two VES station is shown in Fig. 7. The sandy topsoil dips towards VES 2 (east) while the basal sandstone dips towards VES 1(west). From the pseudosection, the tar bearing horizon has an approximate thickness of 43m. 5. CONCLUSION The tar sand has resistivity value of 900 to 1085 Ωm which is consistent with the value from the smooth and robust inverted sections of the profiling. This result concur with those obtain from other localities along the Ogun state axis (Akinmosin et al., 2011 [5], 2012 [3]) and contrast the low resistivity obtained for the tar horizons in Ondo state at localities such as Agbabu and Idiobiolayo (Amigun et al 2012 [7]). The high resistivity value of the tar horizon in the study area represents the traditional value expected for hydrocarbon bearing-sand units. As the presence of the hydrocarbon will impede the flow of current, reduce conductivity and invariably increase the resistivity. Low resistivity values are encountered in places where the tar is thought to be associated with saline water (Halliburton, 2001[10]). The techniques in this study revealed similar lower boundary for the tar horizons. The lower limit of the units is at depth ~45m in the core log, VES results and profiling pseudosection. However, the borehole location lies in between the geophysical traverses for the VES and profiling. The thickness of the overburden is thus highest at the borehole location and lowest along the profiling traverses where some of the topsoil had been eroded, exposing the uppermost section of the tar bearing horizon. In addition, the cores from this horizon showed that the lithology of the interval varies from sandy, very-fine sand and sandy shale. This is consistent with the physical appearance of the exposed tar unit along the profiling traverse. ACKNOWLEDGEMENT Department of Earth Sciences, Olabisi Onabanjo University, Loke M.H for writing the RES2INV software and making it available for academic use. Kamal’ is grateful to the Department of Geophysics, University of Witwatersrand, South Africa for the training on acquiring and interpreting geophysical methods during the Africaaray Field School in Johannesburg in 2011.

We also acknowledge the help of Balogun Jonathan, Goodluck Ighedo, Adetoso Abiodun during the electrical resistivity survey and Mrs H.O Omosanya for the proof read of the initial draft of the Manuscript. REFERENCES [1] Adegoke, O. S., 1977: Stratigraphy and

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ABOUT THE AUTHOR Akinmosin Adewale. A, is a Lecturer at the University of Lagos, Nigeria. He is a registered member of many professional associations among which are the Nigerian Council of Mining Engineering (NMGS) and Geosciences and Council of Mining Engineers and Geoscientists (COMEG). He holds a Ph.D. degree in environmental geology from University of Ibadan, Nigeria. His research interests include Stratigraphy and Sedimentology especially on the geology of tar sands. Omosanya, Kamaldeen. O, a Lecturer in the Department of Earth Sciences, Olabisi Onabanjo University. He is a Structural Geologist, Basin analyst, and Seismic interpreter, his research interests include field and/ laboratory based project with emphasis on structural studies of sedimentary, metamorphic, and igneous rocks. He holds an MSc in Structural Geology with Geophysics from the University of Leeds and currently doing his PhD at the Cardiff University, United Kingdom. Ige, Tolulope, is a graduate of Applied Geophysics from the University of Lagos and currently works with a leading E&P Company in Nigeria.

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The Occurrence of Tar Sands at Ijebu-Itele, Eastern ...ejournalofscience.org/archive/vol3no1/vol3no1_15.pdf · The Occurrence of Tar Sands at Ijebu-Itele, Eastern Dahomey Basin, SW,