Determination of Structural and Geometrical Parameters of the Kribi-Campo Sedimentary Sub-Basin Using Gravity Data

In order to produce a more detailed structural and geometrical information, and determine sediments thickness along the Kribi-Campo sub-basin, statistical spectral analysis and horizontal gradient analysis of residual anomalies coupled with the Euler deconvolution approach were applied on the gravity data in the area. The results obtained from the 2D spectral analysis on anomaly grids gave a depth to the basement rocks of the basin from 0.60 km to 3.93 km. This represents the thickness of the sedimentary formations overlying the basement. The interpretation of the spectral analysis results indicated that the potential hydrocarbon field areas are situated between Kribi and Lolabe and at Campo given that those areas have the highest sedimentary thicknesses values. From the analysis of the horizontal gradient, deep faults mainly striking SW-NE have been traced and a structural map of the area has been produced. By applying the Euler deconvolution method to the gravity data, information about the depth and trend of the main subsurface structures have been obtained.


Introduction
The Kribi-Campo basin is located at the northern edge of the South Atlantic rift

Geological and Tectonic Settings
The geological formations of Kribi in South Cameroon belong to four major lithological and structural units [5], the Ntem Archean unit; the Nyong Unit; the Neo-proterozoic cover and sedimentary formations. The study area is situated at northern edge of the Congo craton, at the transition zone between this mega-structure and the Mobile zone. The most relevant geological formations that can help to better describe this area are mainly the lower Nyong Unit formations and the sedimentary formations. The lower Nyong Unit is made up of ancient Archean rocks of the Ntem basement that underwent Eburnean orogenesis, their formation resulted from the collision between the Congo craton and the São Francisco [6] [7]. This unit was subject to high degree Tectonometamorphism dated ± 2050 Ma associated to the charnokites formations dated ±250 Ma confirming the hypothesis that the Nyong Unit is a remobilized portion of the Archean Ntem complex [5]. The rocks in this area are mainly schists, gneisses that have been intruded by granidiorites and sedimentary formations rocks such as limestones and sandstone [8] [9].
In general, the region has a complex and uneven tectonic structure. This tectonic seems to have given rise to a vertical movement of the basement with subsidence to the North and uplift to the South [10] [11]. This basement movement must have provoked irregularities in the formations at depth, giving rise to faults, horsts and grabens characteristic of the boundary between the Congo Craton and the Pan-African folds belt [12]. The main faults in the region consist of the

Data Used
Two dataset have been combined to carry out this work: the existing ORSTOM data and the newly collected ones. In Figure 2, we present the distribution of the gravity data alongside with the altitude variation in the study area. The ORSTOM data (collected during the ORSTOM survey in 1968) [13] represented by red triangles have been combined with the new gravity data (collected by the team of geophysicist of the University of Yaoundé 1 in March 2015) represented by the blue squares. The new data constitute over 223 points measured along the basin area with a spacing of 0.5 to 1 km. These data were collected using the Lacoste-Romberg G-823 gravity-meter. The irregularity in the data spacing is due to the inaccessibility of some sites given that the study area is found in the dense

Spectral Analysis
This method is carried out through 2D Fast Fourier Transform which transforms gravity data from the space domain to the wavenumber domain to estimate the depths of the structures responsible for the measured anomaly. It has been used extensively by many authors, namely [14] [15] [16]. The finite discrete Fourier transform is given by the equation: where ( ) b x represents the discrete N data array of gravity data obtained by sampling a continuous profile at evenly spaced intervals x ∆ . i is the complex operator, ω = 2πk is the spatial frequency and k = λ -1 is the wavenumber in the x direction.
The expression of the Bouguer Slab Effect is then given by the equation: where E is the power spectrum of ( ) B k . When the square of the Fourier amplitude spectrum is plotted against the radial frequency, the slope of the relationship between the wave number of the gravity field and the logarithmic power spectrum provide information about the depths to basement of the anomaly sources.

Euler Deconvolution
The Euler Method is a technique generally used to locate the apparent depth to the gravity or magnetic anomaly source. Considering a degree of homogeneity, the gravity or magnetic field is related to its gradient component in order to trace the surface of the ground contact. The degree of homogeneity is expressed by the structural index which defines the measure of the fall-off rate of the field with distance from the source. The Euler homogeneity equation is given as: where (x 0 , y 0 , z 0 ) is the position of the magnetic or gravity source whose total field (T) is detected at (x, y, z,). B is the regional gravity or magnetic field. N is the measure of the fall-off rate of the gravity field and may be interpreted as the structural index (SI). This value needs to be chosen according to a prior knowledge of the source geometry.
The Euler depth appear wherever there are lithological discontinuities in the geological formations. They represent the structural and/or stratigraphic changes of various geological formations [3].

The Horizontal Gradient Method
According to [17], the horizontal gradient performed at different heights of the anomaly observation allows for the location of discontinuities and the determination of their dip.
The horizontal gradient is an operation that measures the rate of change of a potential field in the x and y directions [18] in order to image subsurface structures. However, the total horizontal gradient magnitude (HGM) is preferred for its simplicity. The HGM operator is defined by the relation below: where G is the Bouguer gravity field.
The horizontal gradient method is used to locate the boundaries of density contrast from gravity data. These results mark the top edges of gravity or density boundaries. Thus, the maximum value of the horizontal gradient anomalies is placed on top of the sources edges. However, offsets occur when edges are not vertical or when several anomalies are close together. The biggest advantage of the horizontal gradient method is its low sensitivity to the noise in the data, be-  [4].
The works of [18] [19] showed that the maxima of the horizontal gradient of gravity anomalies help in locating contacts associated with abrupt changes in density, which are interpreted either as faults, geological contacts or intrusions.
Faults are expressed by a quasi-linear disposition of at least three maxima and horizontal limits of intrusive bodies are shown by quasi-circular disposition of many maxima [20].

Gravity Data Analysis
The gravity anomaly maps generally superpose the effects of deep, shallow, local and extended gravity contrasts. The effects of a local or shallow structure are often hidden in the signatures of regional structures. We carried out regional-residual separation using the polynomial fitting method with the aim of isolating the anomalies caused by deep and extended sources (long-wavelength anomalies) from those caused by local and shallow density contrasts (short wavelength anomalies). The residual field is obtained by estimating the regional gravity field and removing it from the observed field which is the Bouguer anomaly ( Figure 3). In effect, the order of the regional field n is assimilated to a polynomial of n degree. When n is small, the regional anomaly possesses values which are relatively more different from those of the Bouguer anomaly. In this case, the thickness of the part of the crust causing the corresponding residual anomalies is relatively large. This thickness decreases when n increases. In fact,  [20].
In this work, we have used a polynomial of degree '1', for spectral analysis, Horizontal gradient and Euler deconvolution, so as to have a better chance of locating the major contacts.
In Figure 2, we present the distribution of the gravity data alongside with the altitude variation in the study area. The ORSTOM data (collected during the ORSTOM survey in 1968) [21] represented Ma'an and the nearly circular anomaly to the east of Bipindi (A11). [21] suggested that the nearly circular anomaly of Bipindi was caused by a low density intrusive block having a density contrast of −0.095 g/cm 3 .

Estimation of the Thickness of the Basin
We applied a 2D spectral analysis on grids centered on positive anomalies in the basin situated on the western area of the map (A1, A2, A3, A4, A5, A6, and A7), which enabled us to determine the depths. The power spectrum has been   Table 1. These results are therefore important for the selection of new exploration areas.

Horizontal Gradient Method
We used the Oasis montaj 8.0 software to calculate the amplitude of the horizontal gradient of the residual data of the study area ( Figure 6). We can clearly observe on this map the regions with high gradient amplitude indicating high density variation between contacts. The two major lineament that are interpreted from this map are striking in the direction NW-SE from Kribi right   necessarily vertical and relatively deep or produced by several boundaries.

Euler Solutions
For the Euler method, the following parameters have been used to compute the

Structural Map of the Basin
The combination of the above described results, namely the spectral analysis, horizontal gradient and Euler solutions coupled with the results published by [2] have enabled us to propose a structural map of the Kribi-Campo sedimentary   basin ( Figure 8). This map shows quasi-linear contacts (numbered 1 to 12) which can describe faults and quasi-linear contacts (denoted C 1 , C 2 and C 3 ) corresponding to horizontal limits of intrusive bodies.

Discussion
The results presented in the above sections are in accordance with the fact that  giving the same approximate directions to major lineaments in the region and (2) three circular contacts C 1 , C 3 and C 3 representing rocks intrusions amongst which one had been characterized and modelled in the works of [2]. This study and/or hydrocarbon resources. This suggestion is supported by the fact that the presence of oil and gas in a basin might be due to two factors: in-situ generation and migration of fluids into the basin [22]. The subsurface pressure which is a function of the sediment thickness (i.e., the sediment weight) is one of the environmental conditions needed for oil and gas formation in a basin. The understanding of the fluid flow formation in the region could be elucidated by Euler solutions,the structural map and gradient maps analysis.

Conclusion
The aim of this study was to provide new insights on the structural setting and the geometrical characteristics of the Kribi-Campo basin. We used the polynomial fitting method to carry out the separation of the residual and regional components of the gravity field. We observed that the positive residual