RADIOGENIC COMPONENTS OF THE NIGERIAN TAR SAND DEPOSITS

Akinmosin A.

Description

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 appropriate environmental management programs that will reduce potential negative effect of exploiting the resources 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 238 U, 232 Th and naturally occurring 40 K. 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.
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 238 U, 232 Th y 40 K. 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.

Sujets

• Gamma Ray
• Radionuclide
• Nigeria
• Método espectrométrico

Informations

• Publié par: erevistas
• Publié le: 01 janvier 2009
• Nombre de lectures: 10
• Langue: English

Extrait

EARTH SCIENCES
RESEARCH JOURNAL
Earth Sci. Res. J. Vol. 13, No. 1 (June 2009): 64-73
RADIOGENIC COMPONENTS OF THE NIGERIAN
TAR SAND DEPOSITS
1 2 2Akinmosin, A. , Osinowo, O.O. and Oladunjoye, M. A.
1Department of Earth Sciences, Olabisi Onabanjo University, Ago-Iwoye. Nigeria
2Department of Geology, University of lbadan, lbadan. Nigeria
Corresponding author Email: [e-mail]
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
238 232 40nuclides headed by U, Th and naturally occurring K. Result showed that the radiogenic composition of
-1 -1 -1the clay overburden (0.631mSvy ), shale-(0.193mSvyr ), and bituminous sand (0.446mSvyr ), are lower than
the normal background value considered harmful to man.
Key words: Spectrometric, Gamma ray, radionuclides, Nigerian.
thManuscript receiver: January 07 , 2009.
thAccepted for publication: June 15 , 2009.
64RADIOGENIC COMPONENTS OF THE NIGERIAN
TAR SAND DEPOSITS
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
238 232 40desintegración natural, encabezada por U, Th y K. Los resultados mostraron que la composición
-1 -1 -1radiogénica de la arcilla (0.631mSvy ), de las lutitas-(0.193mSvyr ), y de la arena bituminosa (0.446mSvyr ),
son inferiores al valor mínimo, considerado dañino para el hombre.
Palabras clave: Espectrometría, Rayos Gama, radio nucleídos, Nigeria.
seepage and tar sand deposits over the Okitipupa
1. Introduction
ridge in the Dahomey basin provided the initial im-
Tar sand is composed of a mixture of bitumen, which petus for oil exploration in Nigeria. From the turn
makes up about 10-20% and about 80-85% mineral of 19th century up till date, no less than over
matter including sands, clays and 4-6% water. Tar twenty groups comprising public and private ven-
sand has similar composition as the light crude (i.e. tures have shown degrees of interest. The occur-
H, C and minor amount of S and O). They are be- rence of these deposits has been known since early
lieved to have formed from biodegradation and wa- last century, however, intense investigations com-
ter-washing of light crude due to lack of cap rock. menced from mid 70’s till now. The pioneering ef-
The Nigerian Tar sand is believed to have formed in a forts were initiated by the Geological Consultancy
similar process. Trace elements occur naturally in Unit of the University of Ife (now Obafemi
rock forming minerals and ore minerals; hence they Awolowo University). The geology of these de-
can reach the environment from natural processes. posits, oil saturation and reserve estimates as well
Weathering is a physical and chemical processes that as textural characteristics of the associated sands
breakdown rocks and then release these trace ele-
has been described (Adegoke et al., 1980, and
ments naturally into the environment.
1981; Enu, 1985). The physiochemical properties
of the bitumen in relation to production and pro-The Dahomey basin (Fig. 1) is a marginal pull-
cessing have been studied (Adegoke et al., 1980;apart basin (Klemme, 1975) or Margin sag basin
Oshinowo et al., 1982; Ekweozor, 1985; Oluwole(Kingston et al., 1983), which was initiated during
the early Cretaceous separation of African and et al., 1985). The origin of the bitumen has been
South American lithospheric plates. Occurrence of considered (Coker, 1982; Ekweozor, 1985). Other
65A
e
r
o
m
g
AKINMOSIN, A., OSINOWO, O.O. AND OLADUNJOYE, M. A.
A’A Lagos Ise GbekeboAfowo Gilli-Gilli Awaizombe
Union Ojo Orimedu Bede Benin W. Ihuo
Volta Delta Oban HillsBeninFlank
0Miocene Recent
Cretaceous
Okitipupa 10000MioceneBasement complex High
Dahomey Miogeocline
20000
30000Cretaceous
Feet
n
Transitional Oceanic crustContinental crust Transitional Continental crust
C-O crust part of triple C-O crust
junction and
abakaliki armBasement
Afowo
Bede Gilli-Gilli
Prolongation of chain
and charcot F.Z.o.Volta Warri Ihuo A’according to emery
Delta et al 1978 Oban Hills
Port
Harcourt0 Delta250
Nose1500
Figure 1: East-West geological section showing the Dahomey Basin and upper part of the Niger Delta (After Whiteman,
1982)
relevant studies on the deposit include works done within the eastern Dahomey Basin. The reviewed work
by Ako et. al (1983); Ekweozor (1986 and 1990); of Omatsola and Adegoke (1981) on the Cretaceous
Ekweozor and Nwachukwu (1989); Enu (1987, stratigraphy of the Dahomey basin has recognized three
1990); Enu and Adegoke (1984). These works have formations belonging to the Abeokuta group. These
highlighted relevant aspects of the geochemical and are: the Ise Formation, consisting essentially of conti-
sedimentological characteristic of the deposit. nental sands, grits and siltstones, overlying the base-
ment complex uncomformably. Neocomian to Albian
A combination of factors has prevented the ex-
age has been assigned to this formation. Overlying the
ploitation of this resource to date; the most important
Ise Formation is the Afowo Formation, which consists
is the environmental effects that may pose treat to
of coarse to medium-grained sandstones with variable
both physical and biological components in the area
interbeds of shales, siltstones and clay. The sedi-
of occurrence. It is therefore, of utmost importance to
ments of this formation were deposited in a transi-
know the average background amounts of each metal
tional to marginal marine environment. Turonian toin the natural uncontaminated geological materials,
Maestritchtian age has been assigned to this formation.soils, and waters before assessing the contribution of
The Araromi Formation consists essentially of sand,anthropogenic sources to environmental contamina-
overlain by dark-grey shales and interbedded limestonetion that may be associated with mining project when
and marls with occasional lignite bands. The formationit eventually commences.
conformably overlies the Afowo Formation and
Maestrichtian to Paleocene age has been assigned
Stratigraphy of the Dahomey Basin (Omatsola and Adegoke, 1981).
0 1 0 1The study area lies within latitude 006 38 N- 006 40 N Overlying the Abeokuta group conformably is
0 1 0 1and longitude 004 34 E- 004 37 E (Fig. 2), and falls the Imo group, which comprises of shale, limestone
66
e
t
t
n
c
e
b
m
a
Paleocene
Eocene
se
Chain F.Z.
Basement complex
Charcot F.Z.
Basement
R. NigetRADIOGENIC COMPONENTS OF THE NIGERIAN
TAR SAND DEPOSITS
and marls. The two-lithosratigraphic units under this of the formation and the upper part is made up of me-
group are: Ewekoro Formation that consists of thick dium to coarse-grained silty sandstone (Adegoke,
fossiliferous limestone. Adegoke (1977) described 1969). The formation is Eocene in age (Agagu,
the formation as consisting of shaley limestone 1985). The sedimentation of the Oshoshun Forma-
12.5m thick which tends to be sandy a