International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 11 No: 01 61 Structural Control For Bitumen Seepages in Imeri, Southwestern Nigeria Akinmosin, A**, Omosanya, K.O*, Ariyo, S.O*, Folorunsho, A.F**, and Aiyeola, S.O* kamaloomosanya@yahoo.com *Department of Earth Sciences, Olabisi Onabanjo University, Ago-Iwoye, Nigeria. **Geosciences Department, University of Lagos, Nigeria; Abstract-- Electrical resistivity survey was done to investigate the role of fault as barrier or conduit for bitumen seepages found in Imeri, Southwestern Nigeria. Two (2) Vertical electrical sounding (1D) with a maximum electrode spacing(ab) of 700m using schlumberger array, and an electrical resistivity imaging (ERI) along the same profile using wenner array were used in this research work; the results were interpreted using WINKLINK and DIPROWIN respectively. The bitumen horizon was encountered at depth of ~34.56m and ~20.99m with resistivity value of 2206 and 4398Ωm in VES1 and 2. The resultant lithological logs shows that layers are displaced relative to one another, with younger lithological units placed on top of older ones, this suggests the presence of a Normal fault; the fault has an estimated throw of ~28.1m. The 2D survey did not probe as deep as the VES but the result identified the bitumen horizon and possible fault zones. The bitumen seepages in Imeri were influenced by the presence of a fault and absence of a good cap rock. The fault is thought to serve as a conduit for the migration of the bitumen to the surface. Further fault seal analyses may incorporate the determination of the Shale Gouge Ratio (SGR) along the fault zones. Index Term-- Resistivity, Bitumen, Seepages, Fault, Conduit. I. INTRODUCTION Faults play an important role in controlling the accumulation and migration of hydrocarbons. They can act as barriers to fluid flow i.e. seal reservoirs and stop migration of the hydrocarbon, and can also provide lateral and vertical migration paths for fluid. The general characteristic of oil sands (Bitumen) reservoirs is that they lack appropriate cap rock to keep the hydrocarbon in place which usually is responsible for their biodegradation. Other than this, wherever oil seepages are recorded the question in the mind of most structural geologists is to what is responsible for the seepage, whether there is a fault in place to structurally keep the hydrocarbon in the reservoir or the ability of such faults to act as conduits for the passage of the Hydrocarbon to the surface of the earth. Oil sand (tar sand) and active oil seepages occur in southwestern Nigeria within the Eastern Dahomey (Benin) basin, a marginal pull apart (Klemme, 1975) or marginal sag (Kingston et al., 1983) basin. The oil sand outcrops in an E-W belt, approximately 120km long and 4-6km wide, extending from Edo/Ondo-Ogun States (Enu, 1985). Occurrence of the seepage and tar sand deposit over the Okitipupa ridge in the Dahomey basin provided the initial impetus for oil exploration in Nigeria. Studies carried out on the series of outcrop sections, cores and drilled cuttings obtained from different exploration Campaign have shown the presence of two horizon-bearing sediments designed as X and Y horizon, (Fig. 1.0). The X-horizon, being the shallower of the two deposits, constitutes the prominent outcropping unit in most areas, though significantly eroded in the north western part of the basin. The thickness varies from 9m to about 22m with an average of 15m. The Y- horizon is the prominent outcropping sequence in the northwestern part of the structure where X has been largely eroded. Its thickness varies widely from 3m in the east to 22.6m in the west with an average of about 12m. Up till date, no concrete research has been done to investigate the geologic factor responsible for the seepages recorded in the tar sands outcrops. This study is aimed at identifying the subsurface lithologies, correlate the geo-electric sections, ascertain the depth at which the bitumen occur and validate the presence/absence of a fracture(fault) which is traditionally thought to be responsible for the seepage. The study area lies within latitude 06 0 46 I -06 0 49 I N and longitudes 003 0 58 I -003 0 6 E, on an area extent of approximately eighteen square kilometers (~18 sq. km), a sedimentary terrain in the Dahomey Basin of Southwestern, Nigeria. Imeri is almost a flat plain ground with extremely low relief with elevation between 100ft and 250ft above sea level. The drainage pattern is sub-dendrite-like with an average annual temperature of 31 0 C and mean annual rainfall of 1200mm to 2300mm. Vegetation include shrubs, grasses and forest with agriculture being the predominant occupation of the dwellers.
International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 11 No: 01 62 Meters 70 WEST 60 50 40 30 20 10 0 H OR IZ ON X OIL SHALE HORIZON Y BH 21 LEGEND Silt Shale Lateritic top soil Tar bearing sand Limestone BH 11 BASEMENT COMPLEX BH 20 HORIZONTAL SCALE 1 : 10,000 BH 7 EAST Fig. 1. Schematic geological east-west cross-section showing the stratigraphic sequence and the tar sand horizons in some boreholes. (Modified from Ekweozor et al.,1989) 003 58 E 003 60 E 06 49 N AFOWO FORMATION ARAROMI FORMATION II. REGIONAL GEOLOGY The Benin (Dahomey) Basin constitute part of a system of West African peri-cratonic (margin sag) basin (Klemme 1975; Kingston et al 1983) developed during the commencement of the rifting, associated with the opening of the Gulf of Guinea, in the Early Cretaceous to the Late Jurassic (Burke et al, 1971;Whiteman, 1982). The crustal separation, typically preceded by crustal thinning, was accompanied by an extended period of thermally induced basin subsidence through the Middle Upper Cretaceous to Tertiary times as the South American and the African plates entered a drift phase to accommodate the emerging Atlantic Ocean (Storey, 1995; Mpanda, 1997). On the western part, the basin is bounded by the Ghana Ridge, which is presumably an offset extension of the Romanche Fracture Zone while the Benin Hinge Line, a Basement escarpment which separates the Okitipupa Structure from the Niger Delta basin binds it to the east. The Benin Hinge Line supposedly defines the continental extension of the Chain Fracture Zone, Figure 2.0. The onshore part of the basin covers a broad arc-shaped profile of about 600 km 2 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). The lithostratigraphic units of the Cretaceous to Tertiary sedimentary sequence of eastern margin of Dahomey basin is summarized in Table I. + 06 46 N 0 1km Fig. 2. Topographical Map of the Study Area and the VES Points.
International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 11 No: 01 63 T ABLE I STRATIGRAPHIC COLUMN OF THE EASTERN DAHOMEY BASIN (MSMD, 2006) III. MATERIALS AND METHODS Resistivity geophysical technique was used for this research work due to its ability to investigate near-surface phenomenon (Bitumen seepages are near-surface phenomenon). This method has found application in the investigation of a number of near surface measurement which include hydrogeology, engineering survey, mapping of subsurface fracture, determination of depth to bedrock and a lot more (Singh et al 2006, Loke and Baker, 1996b). The resistivity measurements were made by injecting current through two current electrodes and measuring the resulting voltage difference at two potential electrodes. The instrument used in this instance is an ABEM SAS-1000 Terrameter; this device was used to measure the resistance of the ground which is the ratio of voltage to current. Electrical Resistivity Imaging (ERI) with wenner array and two (2) Vertical Electrical sounding (VES) with schlumberger array were used for this research work (Fig 2); both with a maximum electrode spacing(ab) of 700m. The two VES points were sounded along the 2D resistivity profile at the centre of Imeri Township beside the abandoned water borehole drilled previously for the municipal water supply; VES 1 on a profile of 100m, and VES 2 200m along the 2D profile. In the VES survey, the potential e lectrodes remain fixed and the current electrodes were expanded geometrically about the centre of the spread. The field apparent resistivities were plotted against electrode configuration (AB/2) on a log-log graph paper to determine the geometry of the curves for the VES points. The resultant curves were curve-matched using the approach of Zohdy (1965) in order to determine the number of layers, resistivity, and depth of each of the layer. Computer iteration of the curves was done using WINKLINK software program. This software has the advantage of plotting raw data directly without the rigor of pre-iteration curve-matching. However, the process was careful monitored in order to obviate ambiguity common with such iteration. (Folorunsho, 2009) The interpretation of data was done by inverse numerical modeling techniques (inversion) because the calculated resistivity value is not the true resistivity value of the subsurface. For the ERI, one wenner profile was carried out within the study area. This was done at an electrode spacing a equal to 10m with n =1, 2, 3, and 4. The profile was chosen along the area where the bitumen seepage was cited and previously reported. The current and potential electrodes were kept at equal interval to one another with the two potential electrodes in the middle. After the whole profile is exhausted, the electrode spacing was increased to 2a. This is the second level of a = 10m. The same process was repeated up to the 4 th level i.e. 4a= 40m electrode spacing. A 2-D model for the subsurface, which consist of a large number of rectangular blocks was used to interpret the data from the 2-D imaging survey, this was done using the software DIPRO for windows version 4.0. The program was used to determine the resistivity of the individual blocks so that the calculated apparent resistivity values agree with the measured value from the survey.
VES NO International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 11 No: 01 64 Layer T ABLE II SUMMARY OF VES RESULTS IN THE STUDY AREA. Resistivity Thickness(m) Depth (m) Lithology (Ωm) 1 1 320 1.32 1.32 Top Soil 2 184 12.25 13.57 Clayey 3 141 20.99 34.56 4 4398 - - Bituminous 2 1 198 0.37 0.37 Top Soil 2 634 6.13 6.50 s 3 2206 14.49 20.99 Bituminous sand 4 700 65.56 86.55 intercalated with shale 5 293 - - IV. RESULTS AND DISCUSSIONS N.B: - Means infinity The result from the VES shows that geoelectric layers from VES 1 and 2 are 4 and 5 layers respectively. The inferred lithological composition is shown in the last column of table 2. The bitumen horizon is located at depth of 34.56 m from VES1 and depth of 20.00m from VES 2, with approximate thickness of 14.59m in VES 2. For VES1, the bitumen horizon has a resistivity of 4398Ωm and 2206Ωm for VES 2. These values are very unusual, but characteristics of bitumen found in this area, the lower unit of the bitumen horizon in VES 1 is infinity, because it is the limit of probe of the VES at this Point. Overburden thickness above the bitumen layer in VES1 is ~34.56m and ~6.50m in VES 2; with the topsoil being thinner in VES2 than VES1. In VES2, the bituminous sand is underlain by an 86.55 m thick sand that is intercalated with shale, the implication is that this unit is comprised of couplets of permeable and impermeable layers. Fig. 3. Lithological correlation across the two (VES) points; the last lithological units in the two sections are actually inferred to infinity.
International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 11 No: 01 65 Imeri Profile (Field Data Pseudosection) Imeri Profile (Theoretical Data Pseudosection) Imeri Profile (2-D Resistivity Structure) Cl ayey Clayey Bituminous Cl ayey San d 0Depth (m) 60 Imeri Cross Section (Inferred lithology and Structure) Cl ayey The distribution of resistivity along the pseudosection (Fig.4) is not uniform; the area with green- yellow color code in the profile depicts fairly high resistivity value and is interpreted as the Lateritic soil (110-250Ωm). The blue area shows low resistivity (<110Ωm) and is thought to be the fault zone; the Bituminous sand has resistivity value of > 2200Ωm, see the Fig. 4. Interpreted 2D resistivity model of the subsurface beneath traverse 2. Clayey Bituminous Clayey area shown as red and beyond and are contoured from depth of >10m to infinity, while areas in green contour represent clayey sands, the sands are contoured in yellow. The ERI is 2 dimensional and it represents a vertical slice through an area rather than a one-dimensional figure produced from the VES result, its major limitation is the depth of investigation.
International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 11 No: 01 66 Fig. 5. Geoelectric Section from the Vertical Electrical Sounding at Imeri, SW, Nigeria. Having juxtaposed the lithology inferred from the VES results, it is geologically sensible to suspect the presence of a fault in an area where you witness an omission/repetition of stratigraphy; emplacement of older/younger rock on top of younger/older rock (Fig.3&5) In this instance the stratigraphy of the Dahomey basin portends that the Bitumen is found in the Afowo Formation, relative to the position of VES2, the bitumen horizon is located at a deep depth under VES1; the implication is that younger rocks have been place on top older rocks/lithological units. A normal fault/half graben is suspected to be present in between the two VES points which caused the displacement of the Bitumen horizon down (the hanging wall) relative to the Bitumen Horizon under VES2 (the footwall). (Fig.5) The estimated throw along the fault is approximately 28.1m. It is common sense not to suspect a thrust fault since there is no repetition of stratigraphy beneath any of the VES position. The big question now is that, is the fault sealing? Fault seal analysis falls into two broad categories: Juxtaposition seal; a situation where the juxtaposition of permeable rocks against non-permeable rocks provides a sealing mechanism; Movement on a fault surface displaces the stratigraphical units of the hanging-wall relative to the footwall. This can control hydrocarbon trapping, analysis of the petrophysical properties of the lithologies across a fault surface for a given amount of deformation can be used to determine cross-fault spill points. Membrane seals are provided as a result of pressure difference across a fault plane, in which case the fault itself must provide a barrier to hydrocarbon migration. Fault throw are very good discriminant of sealing and non sealing faults. Faults will act as good seals, in that the rock would have slipped over long distance from zones of high permeability to low permeability and would have smeared enough shale along its path. Likewise a small throw may mean a reservoir juxtaposing against itself and portends a sealing mechanis m. Therefore, the likelihood of sealing appears to
International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 11 No: 01 67 increase with fault throw, usually a limit of 100m is chosen for large throw that will act as good seal, while 20m is the threshold for s mall throws or offset. In this instance the fault throw is greater than 20m, it is possible that the fault is not actively involved in the seepage of the bitumen, however, the Bitumen horizon is juxtaposed against a y Shale unit, which means the Shale Gouge Ratio will be lower unlike when it juxtaposed against a shaly unit. The fault is thought not to be sealing; its juxtaposition against the shaly unit and the fact that the Bitumen Horizon in the footwall lacks a perfect cap rock to keep the bitumen in place all supports a model of a non-sealing fault. CONCLUSION It has been established that fault/ fracture plays important role in hydrocarbon seepages, they can act barrier or conduit to the escape of hydrocarbon from a reservoir. Not only this, a suitable cap rock is also a prerequisite in the ability of a fault seal. The Bitumen seepage in Imeri is influenced by the presence of a fault and absence of good cap rock. The fault and the incompetent cap rock serves as conduits for the migration of the Bitumen to the surface. The fault could have been sealing if it had been juxtaposed against a shale/mudstone, in addition to the amount of throw along the fault being greater than 20m. A better understanding of the fault seal capability can be done with emphasis on Shale Gouge Ratio (SGR) in order to fully understand the ability of the shaly sand juxtaposing the bitumen horizon in sealing the hydrocarbon from escaping to the surface. This work shows that Electrical resistivity could be used to investigate near surface phenomenon such as bitumen seepages and fractures. 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(1965): The Auxilliary Point Method of Electrical Sounding Interpretation and its Relationship to the Dar Zarrouk Parameters. Geophysics 30: 644-650. ABOUT THE AUTHORS 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 area is in 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 a Seismic interpreter, He his research interests includes field and/ laboratorybased project with emphasis on structural studies of sedimentary, metamorphic, and igneous rocks; geologic structures; focusing on the use of geophysical, geochemical, geochronological, and sedimentological data in solving geological problems. He holds an MSc in Structural Geology with Geophysics from the University of Leeds. Ariyo, S.O, A Lecturer in the department of Earth Sciences, Olabisi Onabanjo University. His research interests covers Resistivity Geophysical exploration technique, Mineralogy, and Hydrogeology. He has over ten years experience in the teaching of Geophysics and water exploration. He is a consultant on borehole geophysics. Folorunsho, A.F, is a Lecturer at the University of Lagos, Nigeria. He is a registered member of many professional associations among which is the Nigerian Council of Mining Engineering and Geosciences. He is currently pursuing his Ph. D in the same University. Aiyeola Sholabomi O. is a geologist practicing with one of the geological institutions in Nigeria. He holds a Bachelor of Science degree in geology.