Aeromagnetic method of exploration is famed for its suitability for locating buried magnetic ore bodies because of their magnetic susceptibility. This method has been used in the early stage of petroleum exploration to determine depth and major structures of crystalline Basement rocks underlying the sedimentary basin. In this study, high resolution aeromagnetic data were used to ascertain the viability for hosting hydrocarbon potentials of the study area which forms part of the Illummeden Basin (also known locally as the Sokoto embayment) of West Africa. This was largely carried out through Spectra analysis to determine sediment thickness. The results of the analysis of the aeromagnetic data show that, deeper magnetic source ranges from 0.41 km to 2.69 km, shallow magnetic sources from 0.17 km to 0.97 km. Areas with shallow sediment thickness could not allow the thermal maturation of the sediments, since temperature increase with depth and a depth of two kilometers and above has a temperature range of 60 °C and above. Areas with sediment thickness of 1.5 km and above were delineated and considered as sub-basins and hence potential areas for hydrocarbon exploration.
The present work determines the potentials of the study area for hydrocarbon, based on the high resolution digital aeromagnetic data over Sokoto basin, indicating that the basin is characterized by shallow and deeper sediments thickness. Areas with shallow sediments thickness could not allow the thermal maturation of the sediments since temperature increase with depth and a depth of two kilometer and above has a temperature range of 60˚C and above [
Most economic minerals, oil, gas, and groundwater lie concealed beneath the earth surface, thus hidden from direct view. The presence and magnitude of these resources can only be ascertained by geophysical investigations of the subsurface geologic structures in the area. If the area under investigation has no previous geological information and the primary aim of the study is to search for hydrocarbon deposits; the first question that must be answered, is whether the sedimentary basin is large enough and thick enough to justify any further investigations [
The aim of a magnetic survey is to investigate subsurface geology on the basis of magnetic anomalies in the Earth’s magnetic field resulting from the magnetic properties of the underlying rocks [
In this study, high resolution aeromagnetic data were used to ascertain the viability of the area for hydrocarbon. The possible occurrence of minerals, oil and gas in commercial quantities in the Sokoto basin has been a subject of controversy due to very scanty prospectivity data. Apart from the Niger Delta basin whose current production of petroleum is derived, Nigeria is blessed with other numerous Sedimentary basins comprising the Anambra, Bida, Sokoto, Borno (Chad) and Dahomey basins as well as the Benue trough which is made up of Lower, Middle and upper Benue troughs; these basins have structural and stratigraphical similarities with contiguous intracratonic rifted basins of Niger Republic, Chad Republic, Sudan, Uganda, Tanzania and Kenya where commercial oil accumulations have been discovered and currently been exploited [
The aim of this work is to analyze and interpret the high resolution aeromagnetic data over the study area: to determine the thickness of the sediments through Spectral analysis. The work is aimed at acquainting future researchers with the knowledge of the area and to shed more light on sediment thickness which is crucial to hydrocarbon generation in the study area.
The study area lies between longitudes 4˚00'E and 6˚00'E and latitudes 11˚30'N and 13˚30'N (
varying from 250 to 400 m above sea level. The plain is occasionally interrupted by low Mesas. A low Escarpment known as the “Dange Scarp” is the most prominent feature in the basin. The area to the east of the escarpment consists of mainly an undulating sandy plain, which extends south-westwards to the outcropping basement complex [
The Sokoto basin falls within the hottest parts of Nigeria, belonging to the Sahel region of Africa. Temperatures are generally extreme, with average daily minimum of 16˚C, during cool months of January and December, and the hottest months of April to June with an average maximum of 38˚C and minimum of 24˚C. Throughout the year average minimum temperature is 36˚C and average daily minimum is 21˚C. Rainfall is generally low with mean annual rainfall ranging from 600 mm to 1000 mm across the Basin. Much of the rain falls between the months of May to September, while the dry months are October to April [
The geology of Sokoto Basin is very well documented by several authors such as [
The sediments of the Sokoto basin were accumulated during different phases of deposition. Overlying the Pre-Cambrian Basement unconformably, is the Illo and Gundumi Formations which are made up of grits and clays, constitute the Pre-Maastrichtian “Continental Intercalaire” of West Africa. They are overlain unconformably by the Maastrichtian Rima Group, consisting of Mudstones and friable Sandstones of Taloka and Wurno Formations separated by the fossiliferious, calcareous and shaley Dukamaje Formation. The Dange and the Gamba Formations which are mainly shales are separated by the calcareous Kalambaina Formation, which all constitute the Paleocene Sokoto Group. The overlying continental Gwandu Formation forms the Eocene Continental Terminal [
The high resolution aeromagnetic data that was used for the study consist of sixteen (16) sheets of aeromagnetic maps of total field intensity in half degree sheets obtained from Nigerian Geological Survey Agency (NGSA) [
Data was processed using various filters on the Aeromagnetic data which revealed certain features that aid the interpretation. Thus the digitized data was imported into the computer to produce the total Magnetic intensity maps. The Total Magnetic data was further subjected to Polynomial filtering to obtain both the Regional and the Residual maps. Spectral analysis was used to determine average sediment thickness. All these were carried out using the Oasis montaj, Math lab, and ArcGIS software.
In polynomial fitting the regional is matched with mathematical Polynomial of low order to expose the residual features as random errors, and the treatment is based on statistical theory. The observed data are used to compute, usually by least square method, the mathematically described surface given the closet fit to the magnetic field that can be obtained within a specified degree of detail. This surface is considered to be the regional field and the residual is the difference between the magnetic field values thus determined [
Z = A x + B y + C (1)
The next stage of complexity is the representation of a second order polynomial where,
Z = A x 2 + B y 2 + C x y D x + E y + F (2)
The residual magnetic field of the study area was produced by subtracting the regional field from the total magnetic field using the Polynomial fitting method. The computer program aero-super map was used to generate the coordinates of the total intensity field data values. This super data file, for all the magnetic values was used for production of composite aeromagnetic map of the study area using Oasis Montaj software version 7.0.1 [
There are different methods that are used for depth calculation, such as the inversion methods based on Parker’s forwarding calculation technique [
In this work, the characteristics of the residual magnetic field was studied using statistical spectral method. This was done by first transforming the data from space to the frequency domain and then analyzing their frequency characteristics. In the general case, the radial spectrum may be conveniently approximated by straight line segments, the slopes of which relate to depths of the possible layers, [
The logarithm of the energy values versus frequency on a linear scale was plotted and the linear segments located the truncation effect (or Gibbs phenomenon). Three or two linear segments could be seen from the graphs. The first points on the frequency scale was ignored because the low frequency components in the energy spectrum are generated from the deepest layers whose locations are most likely in errors, each linear segment groups points due to anomalies caused by bodies occurring within a particular depth. If z is the mean depth of the layer, the depth factor for this ensemble of anomalies is exp(−2zk). Thus the logarithmic plot of the radial spectrum would give a straight line whose slope is −2z.
The mean depth of the burial ensemble is thus given as
Z = − m 2 (3)
where (m) is the slope of the best fitting straight line. Equation (3) can be applied directly if the frequency unit is in radian per kilometer. If however, the frequency unit is in circle per kilometer, the corresponding relationship can be expressed as
Z = − m 4 π (4)
In this study the aeromagnetic data set was divided into a block of 7.5' × 7.5' (13.73 km × 13.73 km) data points totaling 252 blocks excluding the Niger Republic part, which was subjected to Fast Fourier transformation (FFT) to compute the power spectrum of the magnetic data using Oasis montaj software.
The total magnetic intensity map (
into three main sections: The northern part is characterized by low magnetic intensity values indicated by dark-light-blue-green-colour, while the eastern, western as well as the southern parts of the study area are characterized by low magnetic intensity values having dark-light-blue-green-colour dominating the area. The south western and south eastern parts of the study area are dominated by high magnetic intensity values, with pockets of reddish-pink colours disseminated in the northern part. Yellowish-orange-colours accompany the reddish-pink-colours depicting medium magnetic intensity values. The lowest total magnetic intensity value of the study area is 481.4 nT and highest value of 633.9 nT (
The residual magnetic map (
2.5 nT to 11.8 nT and occur along the red colour anomalies, dominating the entire map. Green colour anomalies are found in the northern, southeastern, northeastern and south-western parts, with pockets of small occurrences in the south-south which varies from −17.2 nT to 0.8 nT. Blue colour anomalies range from −81.2 nT to −19.9 nT and are the most dominant, occurring in almost every part of the map. The residual magnetic map was divided into 252 blocks which were subjected into Fourier Transform, to obtain the radially average power spectrum (
The logarithm of spectral energies was plotted against obtained frequencies for the various blocks (
Blocks | Deeper Sources (D1) | Shallow Sources (D2) |
---|---|---|
Block 1 | 1.48 | 0.17 |
Block 2 | 1.27 | 0.38 |
Block 3 | 1.38 | 0.21 |
Block 4 | 1.38 | 0.23 |
Block 5 | 2.03 | 0.30 |
Block 6 | 1.98 | 0.33 |
Block 7 | 1.51 | 0.31 |
Block 8 | 1.48 | 0.39 |
Block 9 | 1.13 | 0.19 |
Block 10 | 2.09 | 0.71 |
Block 11 | 1.32 | 0.32 |
Block 12 | 1.30 | 0.38 |
Block 13 | 1.24 | 0.30 |
Block 14 | 1.52 | 0.72 |
Block 15 | 1.27 | 0.45 |
Block 16 | 1.35 | 0.55 |
Block 17 | 1.95 | 0.77 |
Block 18 | 1.92 | 0.97 |
Block19 | 1.31 | 0.54 |
Block 20 | 1.24 | 0.36 |
Block 21 | 1.06 | |
Block 22 | 1.38 | 0.50 |
Block 23 | 1.23 | 0.24 |
Block 24 | 1.56 | 0.59 |
Block 25 | 1.11 | 0.35 |
Block 26 | 1.73 | 0.66 |
Block 27 | 1.35 | 0.44 |
Block 28 | 1.28 | 0.30 |
Block 29 | 1.16 | 0.46 |
Block 30 | 1.40 | 0.65 |
Block 31 | 1.28 | 0.40 |
Block 32 | 1.74 | 0.66 |
Block 33 | 1.39 | 0.53 |
Block 34 | 1.36 | 0.54 |
Block 35 | 1.43 | 0.52 |
Block 36 | 1.83 | 0.88 |
---|---|---|
Block 37 | 1.09 | 0.46 |
Block 38 | 1.39 | 0.28 |
Block 39 | 1.20 | 0.35 |
Block 40 | 1.19 | 0.27 |
Block 41 | 1.31 | 0.52 |
Block 42 | 1.34 | 0.78 |
Block 43 | 1.38 | 0.78 |
Block 44 | 1.52 | 0.80 |
Block 45 | 1.68 | 0.71 |
Block 46 | 1.41 | 0.42 |
Block 47 | 1.17 | 0.46 |
Block 48 | 1.28 | 0.46 |
Block 49 | 1.37 | 0.50 |
Block 50 | 2.69 | 0.71 |
Block 51 | 1.43 | 0.82 |
Block 52 | 1.47 | 0.57 |
Block 53 | 1.67 | 0.74 |
Block 54 | 1.07 | 0.19 |
Block 55 | 1.20 | 0.27 |
Block 56 | 1.16 | 0.25 |
Block 57 | 1.20 | 0.46 |
Block 58 | 1.24 | 0.70 |
Block 59 | 1.11 | 0.58 |
Block 60 | 1.72 | 0.60 |
Block 61 | 1.64 | 0.75 |
Block 62 | 1.79 | 0.73 |
Block 63 | 1.21 | 0.26 |
Block 64 | 1.08 | 0.56 |
Block 65 | 1.15 | 0.51 |
Block 66 | 1.34 | 0.59 |
Block 67 | 1.60 | 0.66 |
Block 68 | 1.19 | 0.34 |
Block 69 | 1.58 | 0.56 |
Block 70 | 1.43 | 0.61 |
Block 71 | 1.07 | 0.29 |
Block 72 | 1.27 | 0.67 |
Block 73 | 1.39 | 0.49 |
---|---|---|
Block 74 | 0.89 | 0.50 |
Block 75 | 1.10 | 0.67 |
Block 76 | 1.12 | 0.57 |
Block 77 | 1.41 | 0.60 |
Block 78 | 1.30 | 0.36 |
Block 79 | 1.26 | 0.61 |
Block 80 | 1.14 | 0.36 |
Block 81 | 1.38 | 0.61 |
Block 82 | 1.27 | 0.26 |
Block 83 | 1.74 | 0.64 |
Block 84 | 2.02 | 0.92 |
Block 85 | 1.71 | 0.71 |
Block 86 | 1.22 | 0.34 |
Block 87 | 1.76 | 0.84 |
Block 88 | 1.93 | 0.85 |
Block 89 | 2.41 | 0.90 |
Block 90 | 0.85 | |
Block 91 | 1.40 | 0.71 |
Block 92 | 2.26 | 0.72 |
Block 93 | 1.36 | 0.65 |
Block 94 | 1.37 | 0.74 |
Block 95 | 1.12 | 0.49 |
Block 96 | 1.07 | 0.35 |
Block 97 | 1.58 | 0.85 |
Block 98 | 1.14 | 0.61 |
Block 99 | 1.29 | 0.32 |
Block 100 | 1.79 | 0.88 |
Block 101 | 1.58 | 0.24 |
Block 102 | 1.34 | 0.69 |
Block 103 | 1.13 | 0.72 |
Block 104 | 1.27 | 0.58 |
Block 105 | 1.46 | 0.71 |
Block 106 | 1.44 | 0.72 |
Block 107 | 1.11 | 0.70 |
Block 108 | 1.20 | 0.60 |
Block 109 | 1.17 | 0.44 |
Block 110 | 1.35 | 0.77 |
Block 111 | 1.17 | 0.48 |
---|---|---|
Block 112 | 1.28 | 0.48 |
Block 113 | 1.20 | 0.39 |
Block 114 | 1.24 | 0.50 |
Block 115 | 1.15 | 0.72 |
Block 116 | 1.67 | 0.78 |
Block 117 | 1.63 | 0.70 |
Block 118 | 1.61 | 0.76 |
Block 119 | 1.26 | 0.35 |
Block 120 | 1.16 | 0.50 |
Block 121 | 1.42 | 0.58 |
Block 122 | 0.95 | 0.50 |
Block 123 | 1.16 | 0.59 |
Block 124 | 0.76 | 0.41 |
Block 125 | 1.84 | 0.82 |
Block 126 | 1.36 | 0.37 |
Block 127 | 1.22 | 0.50 |
Block 128 | 1.34 | 0.52 |
Block 129 | 1.37 | 0.60 |
Block 130 | 1.30 | 0.57 |
Block 131 | 1.14 | 0.51 |
Block 132 | 0.17 | 0.25 |
Block 133 | 1.60 | 0.90 |
Block 134 | 1.23 | 0.43 |
Block 135 | 1.29 | 0.49 |
Block 136 | 1.21 | 0.59 |
Block 137 | 0.88 | 0.38 |
Block 138 | 1.27 | 0.62 |
Block 139 | 0.67 | |
Block 140 | 0.64 | |
Block 141 | 1.25 | 0.41 |
Block 142 | 2.05 | 0.72 |
Block 143 | 1.22 | 0.62 |
Block 144 | 1.14 | 0.68 |
Block 145 | 0.99 | 0.36 |
Block 146 | 1.13 | 0.55 |
Block 147 | 0.98 | 0.37 |
Block 148 | 0.72 | |
---|---|---|
Block 149 | 0.75 | |
Block 150 | 0.96 | 0.55 |
Block 151 | 0.85 | |
Block 152 | 1.02 | 0.51 |
Block 153 | 0.90 | 0.42 |
Block 154 | 0.99 | 0.47 |
Block 155 | 0.52 | |
Block 156 | 1.37 | 0.46 |
Block 157 | 0.79 | |
Block 158 | 1.72 | 0.56 |
Block 159 | 1.02 | 0.47 |
Block 160 | 0.73 | |
Block 161 | 0.77 | |
Block 162 | 1.52 | 0.23 |
Block 163 | 1.29 | 0.64 |
Block 164 | 1.00 | 0.53 |
Block 165 | 1.09 | 0.53 |
Block 166 | 1.41 | 0.59 |
Block 167 | 0.79 | 0.31 |
Block 168 | 1.08 | 0.56 |
Block 169 | 0.65 | 0.39 |
Block 170 | 0.54 | |
Block 171 | 0.89 | 0.40 |
Block 172 | 0.86 | 0.41 |
Block 173 | 0.83 | 0.45 |
Block 174 | 1.37 | 0.69 |
Block 175 | 1.16 | 0.65 |
Block 176 | 1.19 | 0.56 |
Block 177 | 1.67 | 0.56 |
Block 178 | 1.21 | 0.60 |
Block 179 | 1.23 | 0.59 |
Block 180 | 1.18 | 0.38 |
Block 181 | 1.04 | 0.34 |
Block 182 | 1.19 | 0.51 |
Block 183 | 1.81 | 0.44 |
---|---|---|
Block 184 | 0.62 | 0.33 |
Block 185 | 0.56 | |
Block 186 | 0.65 | 0.42 |
Block 187 | 0.49 | |
Block 188 | 0.51 | |
Block 189 | 1.06 | 0.44 |
Block 190 | 1.59 | 0.61 |
Block 191 | 1.15 | 0.49 |
Block 192 | 0.71 | |
Block 193 | 1.44 | 0.61 |
Block 194 | 1.21 | 0.59 |
Block 195 | 0.72 | |
Block 196 | 0.63 | |
Block 197 | 0.91 | 0.35 |
Block 198 | 1.26 | 0.66 |
Block 199 | 0.90 | 0.39 |
Block 200 | 0.77 | 0.49 |
Block 201 | 0.58 | |
Block 202 | 0.81 | 0.35 |
Block 203 | 1.03 | 0.46 |
Block 204 | 0.51 | |
Block 205 | 1.01 | 0.28 |
Block 206 | 1.18 | 0.56 |
Block 207 | 1.54 | 0.54 |
Block 208 | 1.70 | 0.38 |
Block 209 | 1.43 | 0.57 |
Block 210 | 0.64 | |
Block 211 | 1.12 | 0.5 |
Block 212 | 1.02 | 0.48 |
Block 213 | 1.70 | 0.32 |
Block 214 | 1.56 | 0.51 |
Block 215 | 0.64 | |
Block 216 | 0.62 | |
Block 217 | 0.52 |
Block 218 | 0.79 | 0.39 |
---|---|---|
Block 219 | 1.91 | 0.34 |
Block 220 | 0.87 | 0.45 |
Block 221 | 1.32 | 0.67 |
Block 222 | 1.13 | 0.48 |
Block 223 | 2.05 | 0.57 |
Block 224 | 2.60 | 0.60 |
Block 225 | 0.62 | 0.33 |
Block 226 | 0.52 | |
Block 227 | 0.57 | |
Block 228 | 0.62 | |
Block 229 | 1.04 | 0.41 |
Block 230 | 1.05 | 0.47 |
Block 231 | 0.56 | |
Block 232 | 0.41 | |
Block 233 | 0.58 | |
Block 234 | 1.19 | 0.45 |
Block 235 | 1.05 | 0.42 |
Block 236 | 0.81 | 0.48 |
Block 237 | 1.23 | 0.62 |
Block 238 | 1.12 | 0.46 |
Block 239 | 1.27 | 0.43 |
Block 240 | 1.63 | 0.45 |
Block 241 | 0.84 | 0.30 |
Block 242 | 0.44 | |
Block 243 | 0.49 | |
Block 244 | 0.85 | 0.41 |
Block 245 | 0.56 | |
Block 246 | 0.48 | |
Block 247 | 1.42 | 0.48 |
Block 248 | 1.02 | 0.43 |
Block 249 | 0.87 | 0.40 |
Block 250 | 1.43 | 0.46 |
Block 251 | 0.93 | 0.40 |
Block 252 | 0.54 |
From the spectral analysis (
The major sub-basins identified include: sub-basin 1, with an approximate width of 43 km and length of 71 km; sub-basin 2, with an approximate width of 29 km and length of 31 km; sub-basin 3, with an approximate width of 20 km and length of 41 km; sub-basin 4, approximately 32 km in width and 39 km in length; sub-basin 5, with an approximate width of 19 km and length of 45 km; sub-basin 8, with an approximate width of 7 km and length of 40 km.
The result of the contoured deeper magnetic source depth (D1) was super imposed on the geologic map (
The following conclusions can be drawn from this study:
1) Spectral analysis indicates that the basin is characterized by shallow and deeper sediments thickness.
2) Areas with shallow thickness of sediments, could not allow thermal maturation of the sediments, since temperature increase with depth. Depth of 2 km and above has a temperature range of 60˚C and above. The oil window according to [
The study area if given much attention, will add to the economic growth of Nigeria based on the hydrocarbon potentials revealed by this study. Therefore the use of other geophysical methods such as seismic and well logging is highly useful.
The author is grateful to the Nigerian Geological Survey Agency (NGSA), for releasing the High Resolution Digital Aeromagnetic data at a subsidized rate and Adamawa State University, Mubi Nigeria, for granting the author a study fellowship.
Kamureyina, E. (2019) Determination of Hydrocarbon Potentials Using High Resolution Aeromagnetic Data over Sokoto Basin, Northwestern Nigeria. International Journal of Geosciences, 10, 419-438. https://doi.org/10.4236/ijg.2019.104024