Annals of Earth Science and Geophysics

Application of Thorium Normalization Technique to the Airborne Radiometric Data of Some Parts of the Central Benue Trough (CBT) Nigeria, for Possible Hydrocarbon Accumulation

Abstract

A new hydrocarbon exploratory technique namely; Thorium Normalization is applied on the aero geophysical data (radiometric) of some parts of the central Benue Trough to delineate areas of possible hydrocarbon accumulation. This technique is considered very significant because, it identifies areas with probable hydrocarbon presence in sedimentary basins. Separation of radioelement measurements and estimation of the characteristics statistics over each lithological units were determined. The statistical process carried out on the radioactive elements of thorium, potassium and uranium (eTh, %K and eU) of the area displayed low coefficient of variability (CV%) for the three radioelements, depicting higher degree of homogeneity. The delineation of radioactive anomalies (DRAD), showed results ranging from -0.94 to 5.72% while the potassium relative deviation (KD) obtained values ranging from 0.84 to -5.60%. The areas identified with positive DRAD and negative KD values include the southwestern, northern and northwestern parts of the study area. These areas are considered favourable zones for hydrocarbon generation and accumulations, if other conditions are met. The results of this study present a preliminary knowledge/information that can act as a guide for hydrocarbon exploration in the study area on a reconnaissance basis.

Keywords

KD, DRAD, Thorium normalization, Petroleum accumulation, Radioactive elements

Introduction

Hydrocarbon exploration campaign in the Benue Trough started years ago, when an aggressive geophysical investigation was extended to the inland basins in search of hydrocarbon deposits to improve the country’s reserve. The Central Benue Trough has not received much attention like the Lower Benue Trough by researchers, despite the early speculations of hydrocarbon presence in the area. Several geophysical studies have been carried out within the Benue Trough and surrounding environ to investigate hydrocarbon potentials based on magnetic and gravity data interpretations, using different techniques [1-7]. Their works were done on the basis of determining depths of sedimentary thickness. The seismic method remains unsurpassed in terms of hydrocarbon prospecting, but the high cost of this technique limits its utilization. Hence, the use of other geophysical methods like gamma-ray spectrometric data can be employed in early stages of hydrocarbon exploration [8]. Current advancement, development and improvements on airborne geophysical data acquisition, processing, analyzing and interpretations have provided new possibilities in defining petroleum systems of any area of interest. Hydrocarbon deposits are characterized by high naturally gamma-ray radiations. This creates opportunity for the employment of aero-radiometric data to delineate places with possible hydrocarbon accumulations. In mapping radiometric signatures associated with hydrocarbon accumulations in sedimentary basins Saunders, et al. [8] developed one of the best models known as the ‘Simplified Thorium Normalization technique’. Earlier on the investigations of Saunders, et al. [9] had employed Thorium content as a lithologic control to explain optimal uranium and potassium value for samples. The fundamental assumption of the technique emanates from the fact that anything carried out to influence the equivalent Thorium (eTh) apparent concentration will also affect potassium and uranium concentrations in the same vein and predictable ways [10]. The scholarly works of Saunders, et al. [8]; El Sadek [11]; Al-Alfy [12] and Nigm, et al. [13] asserted that, if hydrocarbons are not available, the naturally occurring radioactive elements of Uranium (eU), Potassium (%K) and Thorium (eTh) should maintain natural and constant proportions. This new exploratory technique has demonstrated helpful guide for early stage delineation for petroleum accumulations. This new exploratory technique is further being employed for hydrocarbon investigations by several researchers as a guide for hydrocarbon exploration [9-17]. Sequel to these previous investigations, the present study attempts the use of simplified thorium normalization technique as a reconnaissance tool for the identification of areas with potentials for hydrocarbon exploration in the Central Benue Trough, Nigeria.

Location and Geology of the Area of Study

The area of investigation falls within the Central Benue Trough and is bounded by latitudes 7°00' and 8°30' North, and longitudes 8°00' and 9°30 ^′ East (Figure 1). The study area covers approximately landmass of about 27,225 km ² , and comprises parts of Benue, Nasarawa and little parts of Taraba states of the north cental Nigeria. The Benue Trough is an extensive Cretaceous folded rift basin that extends for about 100 km from the Niger Delta and trending NE-SW towards the Chad Basin [18,19]. The Benue Trough is part of the more extensive Central African Rift system that cuts across most of central Africa and connects with the South Atlantic Ocean through Nigeria [20]. The origin of the basin is related with the separation of the African tectonic plate from the South American tectonic plate. The sedimentary basin fill includes the Nkporo, Agwu, Lafia-Wukari, Eze-Aku (1&2), Asu-River and Bima formations. The formations consists of shale, mudstone, limestone, sandstone, sandy shale, siltstone, feldspathic sandstone, calcerious sandstone, black shale, shelly limestone and sandstone intercalations. They are also exposures of basement rocks which intruded into the sedimentary beds as seen in Figure 2. These rocks are rhyolite, biotite granite, quartzite, porphyritic granite and undifferentiated schists, including phyllites, banded gneiss/biotite, migmatite, porphyroblastic gneiss, silicified large quartz veins and sheared rocks.

Materials and Methods

Source of data acquisition

Nine (9) half degree by half degree high-resolution aero-radiometric data, with sheets numbers 230, 231, 232, 250, 251, 252, 270, 271 and 272 were procured from Nigeria Geological Survey Agency (NGSA) Abuja for interpretation and analysis for this research project. The airborne data were acquired as part of the airborne survey carried in Nigeria by Fugro airborne surveys Canada, for the NGSA between year 2005 to 2009. In acquiring the data, a flight altitude of 100 m, along flight line spacing of 500 m and tie line spacing of 2000 m was used. This digital data acquired was made available on scale of 1:100,000 and 55 × 55 km per sheets. The Softwares used for the study are the Oasis Montaj 8.4 and ArcGis 10.5.

Methodology

Simplified thorium normalization technique

The Normalizing of Thorium (eTh) concentrations is expected to attenuate subsurface rock units, which in turn affect the environment. According to Saunders, et al. [8], these behavioural similarities enables the engagement of thorium measurements to predict occurrence of potassium and uranium by ascertaining their general relationships. Also, important variations among the predicted concentrations of potassium, uranium, and measured values is expected to be responsible for factors than soil moisture, vegetation, lithology, shielding (counting geometry) and by determining this these secondary effects, possible petroleum accumulations could be inferred [8].

Employing the Saunders, et al. [8] method, equivalent concentrations of potassium (%K) and uranium from the aero-radiometric spectral profiles of the research area were normalized to Thorium (eTh) data by firstly; plotting the field measure (%) Ks versus eThs and eUs versus eThs values for all readings. Hence, different linear logarithm and second-order curve fitting techniques were tested and sequel to the processes, the simplest effective equations (1a) and (1b) relating with these variables were ascertained to be linear and moving through the origin (source). The slopes of the lines were determined by the ratios of the mean (%) Ks to mean of eThs, or mean eUs to the mean eThs. The equations applied by Saunders, et al. [8] relating to the variables are presented below as:

$$K_i=\frac{Mean{K}_s}{MeaneT{h}_s}\times T{h}_s\text{ (1a)}$$

$$e{U}_i=\frac{Meane{U}_s}{MeaneT{h}_s}\times T{h}_s\text{ (1b)}$$

Where K _i  is the ideal equivalent Thorium defined potassium value from the station (readings) with an actual Thorium value of eThs. The U _i  is the ideal equivalent thorium defined equivalent uranium value of Ths for that station.

Engaging the method discussed above, the equations were estimated directly from the data and a quick field assessment may be done without producing the plots and restoring to curve fitting. The deviations of the actual values from the estimated values for each reading were obtained by adopting the equations of Saunders, et al. [8]:

$$KD\%=\frac{K_s-K_i}{K_s}\text{ (2)}$$

$$eUD\%=\frac{e{U}_s-e{U}_i}{e{U}_s}\text{ (3)}$$

Where %Ks and eUs are measured Potassium and Uranium values at the stations. eUD% and KD% are relative deviations of Uranium (eU) and Potassium (%K) expressed as a fractions of the station (reading) values. Saunders, et al. [8] asserted that, experience have proven KD% yielding smaller negative values and eUD% yielding small negative or at times positive values over hydrocarbon accumulations. Based on these two relationships, Saunders, et al. [8] introduced a new parameter, known as DRAD (delineation of radioactive anomalies), which is expressed mathematically as;

$$DRAD=eUD\%-KD\%\text{ (4)}$$

Where positive DRAD values depicts favourable indications for subsurface petroleum accumulations in an area [8].

Statistical assessment of the profile data

A statistical assessment was carried out on the radioactive elements of (eTh, K and eU) for each lithology unit with respect to geology map (Figure 2) of the study area. This statistical assessment depended solely on the application of the coefficient of variability (CV) as presented in equation (5). If the coefficient of variability (CV%) is less than 100% for a certain variable in the research area the variables tend to show a normal distribution.

$$CV\%=\left(\frac{SD}{X}\right)100\text{ (5)}$$

Where, SD is standard deviation and X is arithmetic mean.

Therefore, lower coefficient of variability (CV %) represents a higher degree of homogeneity.

Results and Discussion

The three radioactive elememts (K, eTh and eU) maps (Figure 3, Figure 4 and Figure 5) presented high, low (intermediate) and very low concentraions levels which relates to the lithologic units over the area of study. These radiometric composite maps also emphasized the nature of distribution of the radioactive elements and geologic features within the study area. The higher concentrations of potassium (Figure 3), ranges from 1 to 2.3% is recorded to the north eastern, central, southeastern, and top northwestern parts of the study area. The low (intermediate) K-concentrations with values from 0.2 to 0.9% is observed at the SW, NW and southern parts of the area while the very low K-concentrations ranging from 0.1 to 0.2% occurs more at NNW parts of the study area around Doma, Gidan Rai, Amaku, Lafia, Agyaragu and Ogbabu. The equivalent uranium (eU) and thorium (eTh) concentration maps (Figure 4 and Figure 5) shows that, the distributions of the eTh and eU concentrations is variable, widely spread and distributed all over the various lithological units of the study area. High, low and very low concentrations of eTh and eU are observed all over the study area. This effect may be associated with the abundant nature of uranium and immobile nature of thorium in the earth crust. The high concentrations of both radioactive elements are recorded at the SE, NW, NE, SW areas. The very low concentrations are more pronounced at the northwestern and north-central parts of the study area. The concentrations of equivalent uranium and thorium range from 1.1 to 6.9 ppm and 4.7 to 25.7 ppm respectively. A closer look at these composite maps revealed that, zones with high, low and very low concentrations show correlations with each other. This depicts that the two radioactive elements have similar distribution patterns in the area of study. Also, very low concentration of the three radio elements where observed more at NNW parts of the area around Doma, Gidan Rai, Amaku, Lafia, Agyaragu and Ogbabu.

Quantitative interpretations

Results of the statistical analysis of the three radioelements over the seven (7) lithological units found in the study area are summarized in Table 1. The lithological units includes; Nkporo, Agwu, Lafia-Wukari, Eze-Aku (Ess), Eze-Aku (Esh), Asu-River and Bima formations (Figure 2). Coefficient of variability (CV%) of the three radioelements showed a normal distributions as represented by coefficient of variability values less than 100%. This shows exhibition of normal distribution from the three radioactive elements (Figure 3, Figure 4 and Figure 5). The comparative units for eUD%, KD% and DRAD were plotted after separations were carried out and also the statistical parameters obtained were calculated as maximum, minimum mean (X), standard deviation (𝛿), and (X+3𝛿) for each exposed lithlogic units. This gives clarifications or illustrations for typical crossover anomalies over the anticipated petroleum depositions (Table 2). The conservative estimation of the statistical parameters is based on the samples derived from the population. The DRAD arithmetic mean and the three standard deviations (X+3δ) reaches 35.6, 70.1, 5.90, 1.40, 38.2, 29.4 and 5.7 over the Lafia-Wukari, Nkporo, Agwu, Eze-Aku (1&2), Bima Formation and Asu-River group, respectively. DRAD, KD% and eUD% anomaly maps of the (Figure 6, Figure 7 and Figure 8) showed the residual DRAD anomaly zones and the KD% and eUD% contents all over the study area. These maps reflect distinctive anomalies that may be indicators of probable petroleum accumulation spots. According to the scholarly work of Saunders, et al. [8], petroleum accumulations are determined by positive DRAD and negative KD. These positive and negative zones are clearly revealed on the DRAD and KD anomaly maps (Figure 6 and Figure 7). These anomalous values are shown as positive and negative numbers. The areas of possible petroleum accumulation are inferred at the top north eastern corners, northwestern and southern parts of the area of study (Figure 6 and Figure 7), around Agima, Odugbeho Mbacha, Egwo, Okpeje Uaboju-Egu, Aondo-Aka, Ogbabu, Kwara, Doma, Ayaragu, Otobi, Otukpo, Taraku, Moi Igbo, Ndeel, Ito, Obi, Kardoko, Lafia, Kanje, Adudu, Maga and Giringwe.

Conclusion

Aero radiometric data analysis and interpretations using the simplified thorium normalization technique has shown applicability as a guide in reconnaissance hydrocarbon prospecting by delineating areas with probable potential for hydrocarbon accumulations over parts of the Central Benue trough, Nigeria. The statistical analysis applications on the three radio element variables depicted relative low readings of CV% suggesting high degree of homogeneity, and a normal distribution exhibited over each lithological unit. The positive and negative values recorded in the DRAD and KD% maps suggest favourable areas for petroleum accumulation. These indications led to the delineation of probable zones for hydrocarbon exploratory within the area of study. It can be deduced from the study that, the preliminary knowledge/information gotten from the thorium normalization technique can serve as guide for hydrocarbon exploration in the study area.

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