Interpretation of High Resolution Aeromagnetic Data for Hydrocarbon Exploration in Bornu Basin, Northeastern, Nigeria

Hydrocarbon exploration in Bornu basin, Northeastern Nigeria commenced due to the discovery of gigantic hydrocarbon reserves in extended basins in neighboring countries. This study was carried out to map the Mungono and Marte parts of the Bornu basin for geologic structures that could guide in exploring new hydrocarbon fields using aeromagnetic method. Magnetic Intensity grids and their derivatives were used for mapping these structures while depths of magnetic sources (basement) and corresponding sedimentary thickness were estimated using Euler deconvolution and Source Parameter Imaging (SPI) methods. Anomalies show characteristic high positive polarity in the south in contrast to low magnetic signature of northern domain. Features such as faults, folds and intrusive rock bodies were identified as geological structures that could serve as hydrocarbon entrapment. Lineament analysis shows that fractures generally trend NE-SW following the dominant rift system of Bornu basin and Benue Trough. Three basement depressions with thick sediments (more than 3000 m) were also delineated which should be investigated for further petroleum exploration. The outcome of this study would help in delineating promising areas for detail hydrocarbon prospecting in the area.


Introduction
Petroleum exploration and production activities in Nigeria have over the years focused on the Niger Delta province, which is the source of the hydrocarbon produced in the country. Recently, the research for hydrocarbon has been extended to Inland petroleum basins to improve the nation's reserve. Bornu Basin is one of the mostly explored inland basins in Nigeria with the early speculation of commercial quantities of hydrocarbon [Okosun, 1995;Anakwuba and Chinwuko, 2012]. Although this exploration started long ago, results are insufficient to confidently reach conclusion about the hydrocarbon potential in the basin. Aggressive exploration in Bornu Basin commenced more recently due to the discovery of hydrocarbon deposits in commercial quantities in adjacent (Doba, Doseo, Bongor, LogoreBirni and Termit-Agadem) basins in Nigeria's neighboring countries (Chad, Niger and Sudan) with similar structural settings [Nwankwo and Ekine, 2009;Anakwuba et al., 2011;Dieokuma et al., 2013]. New drilling and exploration results have also confirmed that the basin have possible source rocks, reservoir rocks and migration/structural pathway [Dieokuma et al., 2013]. However, the amount of in situ information has not really improved during the time due to security issues. Thus, a more remote method of exploration such as airborne method is required to continue the research and gather useful information about the basin.
Highlighted technical issues in the search for hydrocarbon in the basin are typically related to sediment distribution and variation in the sediment thickness across the basin, structural uncertainty as well as thermal maturation. A regional study is required to focus on basin structure (basin geometry, basement topography, fractures, anticlinal and synclinal structures), and sediment thickness variation across the basin especially around the promising area (Lake chad region). This will be helpful in explaining the results obtained from existing wells and also guide in systematic hydrocarbon exploration.
Geophysical methods have been used previously to investigate sedimentary basin in order to determine the hydrocarbon potential of such basin. These methods provide useful information regarding the basement rocks, geologic fabrics and structural features that could have impact on the overlying sedimentary units [Osinowo et al., 2014;Obiora et al., 2015]. Moreover, magnetic method has been remarkably useful as a result of the relatively high magnetic susceptibility contrasts between basement rocks and sedimentary rocks, which usually play an obscure role on the magnetic signals emerging from the underlying basement rocks [Dobrin and Savit, 1988;Kearey et al., 2002;Osinowo et al., 2014]. Aeromagnetic survey is a geophysical method that has been applied for modern geological mapping. Moreover, its operating principle is similar to that of the magnetic survey but covers larger areas of the Earth's surface mostly as regional reconnaissance. The main objective of aeromagnetic survey is the investigation of the subsurface geology based on anomalous variation of Earth's magnetic field linked to properties of the subsurface rocks [Kearey et al., 2002;Onuba et al. 2008].
Several studies have been done with the use of airborne geophysical survey to delineate the structural patterns of the rocks in Nigeria [e.g., Ajakaiye 1981;Adeniyi, 1985;Yusuf, 1990;Alagbe and Sunmonu, 2014;Anakwuba and Chinwuko, 2012;Aderoju et al., 2016;Oha et al., 2016]. Anakwuba and Chinwuko [2012] also employed aeromagnetic data to evaluate the hydrocarbon deposits in some parts of Chad Basin, Nigeria. The authors identified both shallow and deep depth sources with average depth of 1.5 km and 3.8 km respectively having a thick sedimentary coverage with thickness ranging from 1 km to about 6 km. They also identified three major faults delineating potential areas for exploration of hydrocarbon. Osinowo et al. [2014] employed aeromagnetic data to map Siluko area of the transition zone in the southwestern Nigeria. They identified some structures and thick sedimentary deposits as possible factors responsible for the deposition of tar sand and bitumen/oil shows in the area. Oha et al. [2016] used high resolution aeromagnetic data in parts of the southern Benue Trough for the estimation of the depth of magnetic sources and mapping of the structural features. Their study shows that southern Benue Trough have high potential for large accumulation of base ore mineralization and less suitable for hydrocarbon exploration. The reason attributed to the outcome of their study is the identification of abundant intrusive bodies in the study area. Aderoju et al. [2016] investigated the potential for hydrocarbon in some parts of Chad Basin using high resolution aeromagnetic data. They identified thick sedimentary formation, faults, fractures and magnetic aureoles that could be indications of high prospect for hydrocarbon exploration. Bello et al. [2017] used qualitative and quantitative interpretations of aeromagnetic data in the onshore area of Niger Delta to estimate the depth to basement and to delineate possible areas that could be suitable for hydrocarbon exploration. In their study, they identified thick sedimentary fills, structural lineaments (faults and fractures), and magnetic intrusions. They concluded in their study that the identified features in the study area may suggest possible areas for hydrocarbon exploration.
Therefore, this study was carried out using aeromagnetic data to add value to the understanding of the subsurface geology of the Bornu Basin. This is expected to give valuable information about the the basin structures and its hydrocarbon potential. This study is focused on basin geometry and features, basement topography and fill thickness variation across the study area. The results obtained from this study will help in delineating promising areas for detail hydrocarbon prospecting.

Description of the study area
The study area is in the northeastern part of Nigeria ( Figure 1). It is within the Marte Local Government area of Borno State, Nigeria, on the western coast of Lake Chad. It covers area of about 7,200 km 2 between longitudes 13° 30' 00'' E and 14° 00' 00'' E and latitudes 12° 00' 00'' N and 13° 00' 00'' N. It can be easily accessed through a road about 60 km North of Bama and through another road which is about 20 km South of Baga in Maiduguri area ( Figure 1).

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Aeromagnetic data for hydrocarbon exploration

Regional Geologic Framework of Chad Basin
Chad basin is the largest intracratonic basin in North-central Africa, and it is characterized by Cretaceous and Tertiary rift covering a very wide land mass area from parts of Algeria, Niger, Chad, Southern Sudan, to northern parts of Cameroon and Nigeria [Matheis, 1975;Avbovbo et al., 1986]. Figure 2 is a regional map showing the location of Chad and neighboring basins.
The breakup of West Africa and South America was accompanied by rifting in the Early Cretaceous and this led to the formation of the basins, thereby creating depositional sites for the continental and marine rocks that were deposited in the basin [Fairhead and Green, 1989;Genik, 1992Genik, , 1993. These continental rocks were deposited in the northwest to southeast trending Western African Rift System, to a thickness of 2,000 to 5,500 m [Genik, 1993]. Early Cretaceous is marked by the deposition of fluvial and lacustrine rocks in the basin [Genik, 1992;1993]. A regional sag event in the Late Cretaceous (Cenomanian to Maastrichtian) formed a broad basin leading to a marine transgression where the shallow marine to marginal marine and coastal plain rocks accumulated. During the Late Cretaceous to Oligocene, thick fluvial and lacustrine rocks were deposited which occurred during the last rifting phase in the Chad Basin [Genik, 1993]. The total petroleum system includes Cretaceous and Tertiary lacustrine and marine source rocks, Cretaceous and Tertiary clastic reservoirs, shale seals, and traps that are mostly structural. Hydrocarbons were generated from Cretaceous and Tertiary lacustrine and marine source rocks and most likely began in the Late Cretaceous. The generated hydrocarbons migrated into Cretaceous and Tertiary reservoirs. Hydrocarbon traps are generally structural and include tilted faulted blocks, drape anticlines, reverse-faulted structures and rollover folds. Some inversion features are recognized [Biswas, 2016;Biswas and Acharya, 2016].
The Nigerian region of Chad basin is known as Bornu Basin which constitutes about 10% of the Chad basin.
From the Nigeria context, the separation of the African and South American lithospheric plates and the formation of rift system resulted into subsidence that led to the development of the Benue Trough and the Bornu Basin. Bornu Basin is known to be a broad sediment-filed depression stranding Northeastern Nigeria and adjourning parts of the Chad Republic.
The stratigraphy of the Bornu Basin has been previously described by various researchers [Avbovbo et al., 1986;Obaje, 2009;Nwakwo and Ekine, 2009;Odebode, 2010;Olabode et al., 2015]. Chad Formation consist of mudstone and traces of sandstone, muddy sandstone and claystone. The sequence of the Formation consists of massive and gritty clays, loose to uncemented sands and silts. These Formations underlain by Fika Shale which is dark grey to black in color and its average thickness is about 430 m. Below the Fika Shale are the Gongila and Bima Formations having average thickness of 320 m and 3 km respectively (Odebode 2010).Bornu Basin is mostly covered by the sediments of the Chad Formation that are Pleistocene to Pliocene age. It consists mostly of massive and gritty clays, silts and loose to uncemented sands (Figure 1). Older sediments are not exposed in the study area but have been penetrated by wells drilled in the area. Most structures in the basin are results of tectonic activities that occurred during Late Cretaceous which reshaped the basin.

Materials and methods
The total magnetic intensity (

Qualitative Interpretation
The study area is within low latitude region where magnetic remanence complicates interpretation as reducing the TMI grid to pole and/or equator produced strong artifacts that are not recognized in the original field. These effects are caused by extremely low magnetic declination (near zero). Analytic signal (AS) was alternatively computed to locate the edges of the major magnetic sources since it is almost independent of magnetization direction. Analytic signal (AS) is given as the square root of the sum of the squares of the derivatives in the x, y and z directions as seen in equation 1. (1) where / , / and / are the first derivatives of the total magnetic field.
High pass filtering technique were used to output low frequency anomalies. In this process, high frequency anomalies were cut off and events reflecting local geology of the study area are enhanced. A cut off wavelength of 100 m was observed to be considerable with the geology of the area. First vertical derivative helped to enhance shallow sources as well as magnetic lineaments. Horizontal (x and y) derivatives were also computed with respect to x and y azimuths to enhance location of geological contacts [Phillips, 1999]. Tilt derivative filtering was also carried out to enhance weak magnetic anomalies overshadowed by strong structures.

Quantitative Interpretation
Two depth estimation techniques, Euler (located) deconvolution [Reid et al., 1990] and Source Parameter Imaging (SPI) [Thurston and Smith, 1997;Fairhead et al., 2004] were used to model the basement topography and subsequently give variation in thickness of basin across the study area.

Source Parameter Imaging (SPI)
SPI technique uses the principle of complex analytic signal to compute source parameters from gridded magnetic data. This method adopts first and second order derivatives and thus susceptible to noise and interference effects [Nabighian et al,, 2005]. SPI grids are used to estimate the edge locations, depths and dips. For vertical contacts, the poles of the local wave number define the inverse of depth (equation 2).
where Kmax is the peak value of the local wave number K over the step source given in equation 3.

Euler Deconvolution
Euler deconvolution was used to estimate magnetic source distance and depth. This approach calculates solutions to conventional Euler equation [Reid et al., 1990] assuming there is no remnant magnetization. Euler deconvolution uses the total magnetic field, first horizontal derivative in x, y and vertical derivative in z-direction.
The method makes use of a structural index (S.I.) to compute depth estimates for a variety of geologic structures such as faults, magnetic contacts, dykes, and sills by varying the S.I. locations and depths (x 0 , y 0 , z 0 ) of any sources are calculated using equation 5: where ƒ is the observed field at location (x, y, z) and B is the base level of the field (regional value at the point (x, y, z)) and S.I. is the structural index or degree of homogeneity.

Geological Structure Mapping
Total Magnetic Intensity (TMI) grid and its derivatives were used to interpret the aeromagnetic data for identification of geological structures in the area. TMI of the area ranges from -124.7 to 256.8 nT; high intensities dominate the southern part whereas relatively low intensities cover the entire north. These magnetic patterns reflect the general geology of the area. The magnetic highs that characterized the south may be attributed to intrusive, buried bodies, basement horsts or relatively shallow crystalline basement. Notable magnetic features identified in the TMI grid ( Figure 4a) include an elongated NE-SW trending positive anomaly located west of Marte, and a round shaped negative anomaly around Munguno axis. Other prominent features are ovoid shaped magnetic lows and cover Marte, Dikwa and east of Munguno. The location and edges of these magnetic sources are further delineated using analytic signal grid since the operation is independent of magnetic remanence. There is no remarkable change in the positions of the anomalies seen in both TMI and the analytic signal grids as indicated in Figure 4a & b. However, the analytic signal grid shows that the magnetic bodies around Dikwa and Marte axis form an elongated NE-SW trending mega-structure that extend from southwestern to the northeastern part of the study area as indicated in Figure 4b. This body might represent series of undulating folds in the sedimentary fill.
Derivatives and high pass filtering techniques were carried out to give more detailed information related to the structural geology of near-surface (sedimentary fill and underlying basement). These grids enhanced features that are not clearly discernible in the Analytic signal and TMI grids. The vertical and tilt derivatives grids show that the major structural trends of the area are generally NE-SW, NW-SE and E-W. A more detail structural outline of the magnetic feature around Dikwa and Marte are displayed in the high pass and derivatives grids (Figures 5 and 6). These operations enhanced some sausage shape (curvilinear) features around the magnetic lows observed in the TMI grid. The patterns and the shapes of these features show that they might represent undulating folds which can serve as structural traps in the area. The maps were grey shaded to enhance these lineaments and produce the lineament map of the area as shown in Figure 7. The southern part typified by magnetic highs also corresponds to region with high fracture density with fractures generally trending NE-SW and NW-SE; these likely represent dikes, sills, faults, joints etc.

Basement Topography
Euler deconvolution result was used to generate the 3D basement topography of the area as shown in Figure 9.

Conclusions
In this study we identified regions with thick sedimentary deposits which could be featured by the presence of high oil and gas reservoirs. The average thickness of sedimentary formation obtained from SPI and Euler methods undulating feature that extend from Dikwa to the northeastern part was identified within the sediment. Although the bodies have the characteristics of folds (anticlines or syncline in sediment) which could also serve as good structural trap, the geological description of the magnetic event cannot be clearly ascertained solely from magnetic interpretation as no well has been drilled around it. These identified structures (faults, intrusive and fold) should be further explored using seismic method. This study has shown that the study area has high prospect for hydrocarbon occurrence due to the availability of huge pile of sedimentary fills and presence of petroleum entrapping structures (faults, fold, intrusive). It was also shown that the subsurface modelling of the basin using magnetic method is affected by presence of intrusive within the sedimentary rocks, therefore additional data using other geophysical method is required to reduce the uncertainty in interpreting magnetic anomalies.

Acknowledgements
The authors thank College of Petroleum Engineering and Geosciences, KFUPM, Dhahran Saudi Arabia for its continuous support in provision of valuable software. We also thank the Nigerian Geological Survey Agency for the provision of the aeromagnetic data used in this study.

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Aeromagnetic data for hydrocarbon exploration