untitled EARTH SCIENCES RESEARCH JOURNAL Earth Sci. Res. J. Vol. 13, No. 1 (June 2009): 64-73 RADIOGENIC COMPONENTS OF THE NIGERIAN TAR SAND DEPOSITS Akinmosin, A.1, Osinowo, O.O.2 and Oladunjoye, M. A.2 1Department of Earth Sciences, Olabisi Onabanjo University, Ago-Iwoye. Nigeria 2Department of Geology, University of lbadan, lbadan. Nigeria Corresponding author Email: wale.osinowo@mail.ui.edu.ng ABSTRACT A combination of factors has prevented the exploitation of the Nigerian tar sand deposits to date, among which is the environmental factor which may pose some dangers to both physical and biological components of the area of occurrence. Gamma ray spectrometric analysis was carried out on samples of bituminous sand deposits in parts of Southwestern Nigeria. The aims were to determine the presence and level of radioactivity of selected radionuclides and to assess the possible impact on the environment, and provide geochemical baseline that could be useful in planning appro- priate environmental management programs that will reduce potential negative effect of exploiting the re- sources on the environment. Twenty air-dried samples collected for this study were weighed and sealed for 28 days to enable them attain a state of secular equilibrium. They were subsequently analyzed for gamma-emitting radionuclides using Gamma-ray Spectrometer fitted with a calibrated Canberra vertical coaxial High purity Germanium Detector (HpGe) system. The radio nuclides identified with reliable regularity belong to the decay series of naturally occurring radio nuclides headed by 238U, 232Th and naturally occurring 40K. Result showed that the radiogenic composition of the clay overburden (0.631mSvy-1), shale-(0.193mSvyr-1), and bituminous sand (0.446mSvyr-1), are lower than the normal background value considered harmful to man. Key words: Spectrometric, Gamma ray, radionuclides, Nigerian. 64 Manuscript receiver: January 07th, 2009. Accepted for publication: June 15th, 2009. RESUMEN Una combinación de factores ha evitado la explotación de los depósitos de arena bituminosa de Nigeria hasta la fecha, en ellos el factor ambiental plantea algunos peligros para los componentes físicos y biológicos de la zona. Se realizó un análisis de Espectrometría de rayos gamma a las muestras de los depósitos de arena bituminosa en partes del suroeste de Nigeria. Los objetivos fueron determinar la presencia y el nivel de radiactividad de determinados radio nucleídos para evaluar el posible impacto sobre el medio ambiente, y proporcionar un referente geoquímico que sería útil en la planeación de programas de manejo ambiental que redujeran los posibles efectos negativos de la explotación de los recursos al medio ambiente. Para el desarrollo de este estudio se tomaron veinte muestras las cuales fueron pesadas y selladas durante 28 días a fin de que puedan alcanzar un estado de equilibrio secular. Un análisis de emisión de rayos gamma por radio nucleídos fue realizado posteriormente con un espectrómetro de rayos gamma equipado con un detector coaxial vertical de Germanio de alta pureza (HpGe). La radio nucleídos, identificados con una regularidad confiable, pertenecen a la serie de radio nucleídos de desintegración natural, encabezada por 238U, 232Th y 40K. Los resultados mostraron que la composición radiogénica de la arcilla (0.631mSvy-1), de las lutitas-(0.193mSvyr-1), y de la arena bituminosa (0.446mSvyr-1), son inferiores al valor mínimo, considerado dañino para el hombre. Palabras clave: Espectrometría, Rayos Gama, radio nucleídos, Nigeria. 1. Introduction Tar sand is composed of a mixture of bitumen, which makes up about 10-20% and about 80-85% mineral matter including sands, clays and 4-6% water. Tar sand has similar composition as the light crude (i.e. H, C and minor amount of S and O). They are be- lieved to have formed from biodegradation and wa- ter-washing of light crude due to lack of cap rock. The Nigerian Tar sand is believed to have formed in a similar process. Trace elements occur naturally in rock forming minerals and ore minerals; hence they can reach the environment from natural processes. Weathering is a physical and chemical processes that breakdown rocks and then release these trace ele- ments naturally into the environment. The Dahomey basin (Fig. 1) is a marginal pull- apart basin (Klemme, 1975) or Margin sag basin (Kingston et al., 1983), which was initiated during the early Cretaceous separation of African and South American lithospheric plates. Occurrence of seepage and tar sand deposits over the Okitipupa ridge in the Dahomey basin provided the initial im- petus for oil exploration in Nigeria. From the turn of 19th century up till date, no less than over twenty groups comprising public and private ven- tures have shown degrees of interest. The occur- rence of these deposits has been known since early last century, however, intense investigations com- menced from mid 70’s till now. The pioneering ef- forts were initiated by the Geological Consultancy Unit of the University of Ife (now Obafemi Awolowo University). The geology of these de- posits, oil saturation and reserve estimates as well as textural characteristics of the associated sands has been described (Adegoke et al., 1980, and 1981; Enu, 1985). The physiochemical properties of the bitumen in relation to production and pro- cessing have been studied (Adegoke et al., 1980; Oshinowo et al., 1982; Ekweozor, 1985; Oluwole et al., 1985). The origin of the bitumen has been considered (Coker, 1982; Ekweozor, 1985). Other RADIOGENIC COMPONENTS OF THE NIGERIAN TAR SAND DEPOSITS 65 relevant studies on the deposit include works done by Ako et. al (1983); Ekweozor (1986 and 1990); Ekweozor and Nwachukwu (1989); Enu (1987, 1990); Enu and Adegoke (1984). These works have highlighted relevant aspects of the geochemical and sedimentological characteristic of the deposit. A combination of factors has prevented the ex- ploitation of this resource to date; the most important is the environmental effects that may pose treat to both physical and biological components in the area of occurrence. It is therefore, of utmost importance to know the average background amounts of each metal in the natural uncontaminated geological materials, soils, and waters before assessing the contribution of anthropogenic sources to environmental contamina- tion that may be associated with mining project when it eventually commences. Stratigraphy of the Dahomey Basin The study area lies within latitude 0060381N- 0060401N and longitude 0040341E- 0040371E (Fig. 2), and falls within the eastern Dahomey Basin. The reviewed work of Omatsola and Adegoke (1981) on the Cretaceous stratigraphy of the Dahomey basin has recognized three formations belonging to the Abeokuta group. These are: the Ise Formation, consisting essentially of conti- nental sands, grits and siltstones, overlying the base- ment complex uncomformably. Neocomian to Albian age has been assigned to this formation. Overlying the Ise Formation is the Afowo Formation, which consists of coarse to medium-grained sandstones with variable interbeds of shales, siltstones and clay. The sedi- ments of this formation were deposited in a transi- tional to marginal marine environment. Turonian to Maestritchtian age has been assigned to this formation. The Araromi Formation consists essentially of sand, overlain by dark-grey shales and interbedded limestone and marls with occasional lignite bands. The formation conformably overlies the Afowo Formation and Maestrichtian to Paleocene age has been assigned (Omatsola and Adegoke, 1981). Overlying the Abeokuta group conformably is the Imo group, which comprises of shale, limestone 66 AKINMOSIN, A., OSINOWO, O.O. AND OLADUNJOYE, M. A. A Afowo Union Volta Delta Lagos Ojo Orimedu Ise Bede Gbekebo Gilli-Gilli Benin W. Benin Flank Awaizombe Ihuo Recent Miocene Cretaceous Miocene A’ Oban Hills Okitipupa HighBasement complex Dahomey Miogeocline Eo cen e Pa leo cen e Cretaceous Aeromgnet t c ba se men Ba se m en t c om pl ex 0 10000 20000 30000 Feet Continental crust Transitional C-O crust Oceanic crust part of triple junction and abakaliki arm Transitional C-O crust Continental crust Afowo Bede Basement Ba se m en t Volta Delta Prolongation of chain and charcot F.Z.o. according to emery et al 1978 Gilli-Gilli Oban Hills Ihuo A’ Ch ar co t F .Z . Warri Delta Nose Ch ain F.Z . 0 0 250 150 R . N ig e t Port Harcourt Figure 1: East-West geological section showing the Dahomey Basin and upper part of the Niger Delta (After Whiteman, 1982) and marls. The two-lithosratigraphic units under this group are: Ewekoro Formation that consists of thick fossiliferous limestone. Adegoke (1977) described the formation as consisting of shaley limestone 12.5m thick which tends to be sandy and divided it into three microfacies. Ogbe (1972) further modified this and proposed a fourth unit, which he said is Paleocene in age and associated with shallow marine environment due to abundance of coralline algae, gastropods, pelecypods, echinoid fragments and other skeletal debris. Akinbo Formation lies on the Ewekoro Formation and it comprises of shale, glauconitic rock bank, and gritty sand to pure grey sand with little clay. Lenses of limestone from Ewekoro Formation grades laterally into the Akinbo shale very close to the base. The base is characterized by the presence of a glauconitic rock. The age of the formation is Paleocene to Eocene. Overlying the Imo group is the Oshoshun For- mation; it is a sequence of mostly pale greenish-grey laminated phosphatic marls, light grey white-purple clay with interbeds of sandstones. It also consists of claystone underlain by argillaceous limestones with light grey shale at the bottom. There are inclusions of phosphatic and glauconitic materials in the lower part of the formation and the upper part is made up of me- dium to coarse-grained silty sandstone (Adegoke, 1969). The formation is Eocene in age (Agagu, 1985). The sedimentation of the Oshoshun Forma- tion was followed by a regression, which deposited the sandstone unit of Ilaro Formation (Kogbe, 1976). The sequence represents mainly coarse sandy estuarine deltaic and continental beds, which show rapid lateral facies change. The coastal plain sands are the youngest sedi- mentary unit in the eastern Dahomey basin, it prob- ably overlay the Ilaro Formation unconformably, but convincing evidence as to this is lacking (Jones and Hockey, 1964). It consists of soft, poorly sorted clayey and pebbly sands, of Oligocene to Recent age. Method of study Gamma Ray Spectrometric analysis was carried out on twenty samples. These samples were transferred from the bags into clean air-tight containers and sealed for 28 days in order for the samples to attain secular equilibrium and also to prevent escape of Ra-226 gas. Moreover, an empty clean container was RADIOGENIC COMPONENTS OF THE NIGERIAN TAR SAND DEPOSITS 67 A Legend River N Footpath Afowo Fm. Road Settlement 0 1km 4 37E0 6 40N06 40N0 4 37E 0 6 38N 0 6 38N 0 4 34E 0 4 34E 0 IDIOBILAYO IDIOPOPO Tar Sand Outcrop Figure 2: Location Map of the Study Area Showing Tar Sands Outcrop Points. equally weighed and sealed for minimum number of days to serve as the background count. Various detectors have been used to measure ra- diation energy deposited in a medium. The choice of a detector for low counting revolves around many factors, but energy resolution and counting effi- ciency are the most important. Two types of detectors used in this work are: inorganic scintillation counter sodium iodide (NaI) activated with thalium (TI), and the other is high purity germanium detector (HPGe). After a minimum of 28 days for the sealed sam- ples to attain secular equilibrium, each sample was placed on HPGe detector enclosed in a lead shield and counted for 10 hours (36 000 s). An identical but empty container that has been sealed for same 28 days was also counted for 10 hours. A standard soil sample that contains certified radioactivity concen- tration due to 238U, 232Th and 40K by weight was also counted for 10 hours. The high resolution of HPGe detector made identification of a wide spectrum of ã-ray energy in the samples possible, and the photo peaks observed with regularity in the samples were identified. The area under each peak from the spectra produced was analyzed using a spectra analysis program, SAMPO-90, which matched ã-energies at various energy levels to a library of possible radio- isotopes. The background peak height area was sub- tracted from the standard peak area in order to get net-peak area values for both the standard and the sample respectively. Results and discussion The measured radioactivity concentration (Bqkg-1) of gamma emitting radionuclides in the samples is presented in Appendix 1. It was observed that the radionuclides identified and quantified from the gamma ray spectral (Apendix2) are decay daughter products of naturally occurring radioactive element 238U and 232Th. The radioactive daughter product of 238U, are 214Bi, 214Pb and 226Ra, likewise the daughter products of 232Th are 208Ti and 228Ac. 40K that is also naturally occurring but a non series radioactive iso- tope was also detected and selected for analysis at 1460.75keV. Radionuclides were detected in all samples analyzed for, but in varying proportions. For instance, naturally but non-series 40K was detected at 1460.75keV in all the samples except in Tar L2, Tar L3, Tar sand 13, Tar sand 17 and Tar sand 18; there activity values range between 7.04 ± 0.45Bqkg-1 and 1003.91 ± 48.57Bqkg-1 The 238U series are more prominent in the sam- ples analyzed than the 232Th series.214Bi activity val- ues range between 5.02 ± 0.33Bqkg-1 and 193.01 ± 19.94Bqkg-1; 226Ra activity values range between 6.10 ± 0.84BgKg-1 and 148.07 ± 9.81BgKg-1 ; 228Ac activity values range between 2.14 ± 0.60Bqkg-1 and 53.66 ± 4.44Bqkg-1 ; 208TI values range between 1.09 ± 0.38Bqkg-1 and 86.81 ± 2.6Bqkg-1. The absorbed dose rates in air calculated from the radioactivity concentration of the various samples are given in Ta- bles 1 and 2 for the overburden, shale, and tar sand respectively. The corresponding absorbed dose rates have been calculated using the relationship derived by Beck et al (1972) which is given as: D = 0.042Ac (K) + 0.429Ac (U) +0.666 Ac (Th) Where D is in Gyhr-1 and represents the absorbed dose rate in air due to the specific activity concentra- tions of Ac (K). Ac (U) and Ac (Th) in (Bqkg -1), respec- tively. The result may be compared with those units set by the International Commission on Radiological Protection (ICRP) as shown in Table 3. It could be es- tablished that the mean dose equivalent calculated for clay overburden 0.631m Svyr-1, shale 0.193m Svyr-1 and bituminous sand 0.446m Svyr-1 are still lower than the normal background value to all humans on earth considered harmful or hazardous to the envi- ronment. Conclusion The presence of radioactive elements in an environ- ment has been a source of concern, under certain con- ditions; they can generate a significant radiation that can be injurious to health. The result of the study us- ing gamma spectrometric method of analysis indi- 68 AKINMOSIN, A., OSINOWO, O.O. AND OLADUNJOYE, M. A. RADIOGENIC COMPONENTS OF THE NIGERIAN TAR SAND DEPOSITS 69 Table 1: Absorbed Dose Rate in Tar sand Sample (nGyhr-1). Sample No. Sample Description Ac (U) Ac226 (Bqkg-1) Ac (Th) Ac228 (Bqkg-1) Absorbed Dose Rate (nGyhr-1) Dose Equivalents (mSvyr-1) S1 Tar sand 35.80 ± 6.92 14.22 ± 0.7 24.69 0.216 S6 Tar sand 20.48 ± 1.30 11.13 ±0.56 16.11 0.141 S8 Tar sand 88.4. ± 3.27 25.11 ± 0.9 54.64 0.473 S9 Slightly impregnated bituminous sand ND 2.23 ±0.28 1.48 0.013 S4 Tar sand 148.07±9.81 86.81 ± 2.6 121.33 1.06 S10 Tar sand 30.8 ± 1.16 17.76 ±0.52 24.63 0.20 S11 Tar sand ND 15.19 ±6.08 65.07 0.57 S12 Tar sand 48.80 ± 3.53 20.93 ±7.53 90.01 0.80 S13 Tar sand 6.10 ± 0.84 ND 25.86 0.23 S14 Tar sand 59.61 ± 4.97 32.40 ±9.54 130.33 1.14 S15 Tar sand 66.41 ± 5.00 53.66 ±4.44 92.37 0.81 S16 Tar sand 39.19 ± 3.11 26.52 ±5.22 78.36 0.68 S17 Tar sand 21.42 ± 1.58 16.85 ±1.56 36.30 0.32 S18 Tar sand 8.43 ± 0.76 5.97 ± 1.79 11.89 0.10 S19 Tar sand 10.73 ± 0.89 7.92 ± 1.61 14.78 0.13 S20 Tar sand 16.95 ± 1.26 12.61 ±1.37 30.01 0.26 Mean 37.20 ± 2.78 21.83 ±2.63 51.12 0.446 Table 2: Absorbed Dose Rate in Clay overburden and Shale (nGyhr-1) Sample No. Sample Description Ac (U) Ra226 Ac (Th) Ac228 Absorbed Dose Rate (nGyhr-1) Dose Equivalents (mSvyr-1) S7 Clay overburden 107.20±9.61 39.21±1.26 72.09 0.631 S6 Shale 31.49 ± 1.24 12.83±0.53 22.04 0.19 S5 Shale 10.50 ± 0.60 2.14 ± 0.60 5.92 0.05 S1 Shale 57.70 ± 1.91 21.30 ± 0.7 38.91 0.34 Mean 51.72 ± 3.43 18.87±0.77 34.74 0.303 Mean absorbed dose rate (nGyhr-1), calculated mean equivalent dose is in Svyr-1. cated that the radionuclides identified and quantified belong to the naturally occurring decay series of 238U and 232Th. The naturally occurring non-decay isotope 40K was also identified and quantified. Although the radioactivity levels of the radionuclides were found to be low. The natural gas 226Ra, that is generally associ- ated with petroleum from decay of uranium and tho- rium is noted in the bituminous sands in relatively low concentration. The specific activity of these radionuclides in the bituminous sands is not expected to cause any health hazards now, but the mode of ex- ploitation and extraction can still raise the level if not cautiously chosen and adopted in future. Open cast mining method might concentrate and lead to radia- tion of some of these elements in the overburden dur- ing excavation and thus could have severe health implication. References Adegoke, O.S. 1969. Eocene Stratigraphy of Southern Nigeria, Bulleting Bureau de Re- search Geologic ET Miners Memoirs. Vol. 69 pp 22-243. Adegoke, O.S. 1977. Stratigraphy and paleontology of the Ewekoro Formation (Paleocene) of SW Nig. Bull. 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Bitumen chemical properties and their relation to origin, 70 AKINMOSIN, A., OSINOWO, O.O. AND OLADUNJOYE, M. A. Table 3: Dose Limits and Their Biological Effects Radiation dose rate Duration of exposure Likely effects/Implication 10,000mSv/yr Short-term dose Immediate illness and subsequent death within Few weeks. 1,000mSv/yr Short-term dose Nausea and decreased white blood cell, but not death. 50mSv/yr Over 5 year The lowest dose rates where there is evidence of cancer being caused. Above this, the probability of cancer occurrence increases with dose. 2mSv/yr Normal background to all humans on earth 0.3-0.6mSv/yr Artificial sources of radiation, mostly medical equipment. Source: Uranium Information Centre, ICRP (2002). production and processing. Book of Abstracts: 21st Ann. Conf. Nig. Min. & Geosc. Soc., Jos Ni- geria, 11th-15th March 1985. Ekweozor, C.M. 1986. Characteristics of the non-asphaltene products of mild chemical degra- dation of asphaltene. Org. Geochem. 10. 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Appendix 1: Activity Concentration in Bgkg-1 (Hp Ge Detector) Sample Description K40 (1460.75Kev) U238 Th 232 Absorbed Dose Rate (nGyhr-1) Equiva- lent Dose Rate (msvyr-1) Bi214 (609.31 Kev) Pb214 (351.91 Kev) Ra226 (911.9 Kev) Ac228 (911.9 Kev) Ti208 (583 Kev) Shale L1 10.32 ± 0.78 51.69 ± 1.9 73.57 ± 7.7 57.7 ± 1.91 21.3 ± 0.7 21.3±0.76 104.71 0.917 Tar L2 ND 48.32 ± 2.0 75.52 ± 8.1 35.8 ± 6.92 14.02 ± 0.7 14.00±0.7 87.14 0.76 Tar L3 ND 20.48 ± 2.0 82.29 ± 1.3 20.48 ± 1.3 1.13 ± 0.56 11.12±0.59 67.7 0.59 Shale S4 70.19 ± 4.04 180.65 ± 5.8 193.01 ± 19.9 148.07±9.81 86.81 ± 2.6 86.81 ± 2.6 242.41 3.009 Shale S5 18.16 ± 1.14 10.50 ± 0.6 10.46 10.50 ± 0.95 2.14 ± 0.6 2.137± 1.6 17.09 0.14 Shale S6 7.04 ± 0.45 31.49 ± 1.29 35.1± 3.79 31.49 ± 1.24 12.83±0.53 12.82±0.53 59.38 0.52 Clay S7 59.903± 0.5 150.36 ± 4.9 165.88 ± 17.2 107.2 ± 9.69 39.21±1.26 19.11± 1.2 236.39 2.07 Tar sand 6 92.69 ± 57.5 88.4 ± 3.2 132.46 ± 14.4 88.4 ± 3.27 25.11 ± 0.9 25.1 ± 0.9 170 1.49 Tar sand S7 13.13 ± 0.78 ND ND ND 2.23 ± 0.28 2.23 ± 0.27 16.73 0.19 Tar sand S8 376.69±18.55 30.79 ± 1.16 42.44 ± 4.47 30.8 ± 1.16 17.16±0.59 17.16±0.58 83.3 0.73 Tar sand S9 658.72±29.77 28.66 ± 3.39 15.7 ± 2.4 ND 15.19±6.08 12.39±1.76 65.07 0.57 Tar sand10 847.22 ± 38.4 25.79 ± 3.54 12.52 ± 2.72 42.8 ± 3.53 20.93±7.53 8.54 ±1.89 90.01 0.80 Tarsand11 518.58±22.75 3.5 ±1.73 ND 6.1 ± 0.84 ND ND 25.86 0.23 Tar sand12 1003.91±48.57 39.51 ± 4.68 37.74 ± 3.96 59.61 ± 4.97 32.4 ± 9.54 11.84±2.44 130.33 1.14 Tar sand13 ND 16.75 ± 1.98 26.8 ± 0.81 66.41 ± 5.0 53.66±4.44 14.23 ± 1.0 92.37 0.81 Tar sand14 458.96 ± 20.58 23.83 ± 2.73 20.36 ± 1.99 39.19 ± 3.11 26.52±5.22 8.48 ± 1.29 78.36 0.68 Tar sand15 50.93 ± 3.28 9.68 ± 1.14 15.58 ± 0.4 21.42 ± 1.58 16.85±1.56 4.36 ± 0.36 36.30 0.32 Tar sand17 ND 2.20 ± 0.74 5.02 ± 0.33 8.43 ± 0.76 5.97 ± 1.79 1.81 ± 0.55 11.89 0.10 Tar sand18 ND 4.11 ± 0.67 5.63 ± 0.28 10.73 ± 0.89 7.92 ± 1.61 1.09 ± 0.38 14.78 0.13 Tar sand19 92.45 ± 4.69 8.28 ± 0.97 8.28 ± 0.25 16.95 ± 1.26 12.61±1.37 4.68± 0.36 30.01 0.26 Mean 213.944±12.589 34.749±2.21 47.918±2.807 40.104±2.91 21.20±2.393 13.960±1.007 82.992 0.784 Appendix 2 RADIOGENIC COMPONENTS OF THE NIGERIAN TAR SAND DEPOSITS 73 1600 1400 1200 1000 800 600 400 200 0 -200 0 1000 2000 3000 4000 5000 Channels Spectrum 1 C o u n ts /C h a n n e l 1600 1400 1200 1000 800 600 400 200 0 -200 0 1000 2000 3000 4000 5000 Channels Spectrum 7 C o u n ts /C h a n n e l