Basement Architecture in Part of the Niger Delta from ... · Basement Architecture in Part of the Niger Delta from Aeromagnetic Data and its Implication for Hydrocarbon Prospectivity

The Pacific Journal of Science and Technology –512– http://www.akamaiuniversity.us/PJST.htm Volume 14. Number 2. November 2013 (Fall)

Basement Architecture in Part of the Niger Delta from Aeromagnetic Data and its Implication for Hydrocarbon Prospectivity.

Sunday Oladele, M.Sc.1* and Samuel B. Ojo, Ph.D.2

1Department of Geosciences, University of Lagos, Lagos, Nigeria.

2Obafemi Awolowo University, Ile-Ife, Nigeria.

E-mail: [email protected]*

ABSTRACT Aeromagnetic mapping of Niger delta basement has been carried out. The control of deep-seated basement on sedimentation and subsequent influence on hydrocarbon accumulation necessitates a good understanding of the basement architecture in hydrocarbon exploration. The objective of this study is therefore to delineate the basement structural framework that may assist in defining areas of the sedimentary section of promising prospects for hydrocarbon exploration. To achieve this goal, the aeromagnetic data was subjected to various geophysical enhancement techniques which include tilt depth, Euler deconvolution, Analytic Signal, derivatives and 2-D modeling. The result indicated NE-SW, NW-SE, and E-W tectonic trends that affected the area. The NE-SW structures are truncated by the NW-SE trending faults which suggests an older age of the NE-SW structures. Producing targets tend to be rooted by basement highs and depth to magnetic basement attained 15.1km.

(Keywords: basement architecture, aeromagnetic, Niger delta, hydrocarbon prospecting, geophysics)

INTRODUCTION The use of aeromagnetic data in the petroleum industry can be traced back to World War II when the method was first developed (McIntyre, 1980). Aeromagnetic data were employed in hydrocarbon exploration for regional reconnaissance survey because a large area can be covered quickly and is useful for providing first-hand information about the basement. Hydrocarbon trapping structures within the sedimentary sections are sometimes controlled by topographic and structural features on the basement surface which can be satisfactorily

captured or imaged by aeromagnetic data. Most exploration works in the Niger delta have been concentrated on the seismic survey of Tertiary deltaic hydrocarbon bearing sequence with little or no attention paid to the underlying basement structures. The knowledge of basement framework has been known to exert significant control on the structural disposition and petroleum system of the overlying sedimentary section. The study area (Figure 1) is located in the Niger Delta between Latitudes 4.00N – 5.50N and Longitudes 5.50E – 7.00E covering an area of 27,722 km2. The objectives of this study are to image the basement depth, architecture, dip, and strike, using magnetic attributes and inversion techniques which include continuation, spectral analysis, derivatives, analytical signal, Euler deconvolution and 2-D magnetic modeling. Geology of the Study Area The Niger Delta is situated in the apex of the Gulf of Guinea on the West Coast of Central Africa. The stratigraphic and structural disposition of the Niger Delta has been documented by several workers (e.g. Reijers, 1996). The Niger Delta was formed by the buildup of sediments over a crustal tract developed by rift-faulting during the Precambrian with outlines controlled by deep seated faults associated with rifting (Weber and Daukoru, 1975). Rifting diminished in the late Cretaceous and gravity tectonism became the primary deformational process after the rifting phase in the Niger Delta. The Delta started as two different depocenters in the Bende-Ameki area, east of the Delta and in the Anambra Shelf, West of the delta in the mid to late Eocene (Hospers, 1965). These two depocenters later formed a single deltaic sedimentary basin in the late Miocene to date.


Figure 1: Index Map of the Niger Delta and Offshore Nigeria (after Doust and Omatsola, 1990) Showing

the Location of the Study Area. The Tertiary section of the Niger Delta is divided into three Formations representing prograding depositional facies that are distinguished on the basis of sand-shale ratios (Michele et al., 1999). The basal Paleocene to recent Akata Formation is composed of thick pro-delta marine shales and clays with some turbidites and sand bodies. The typically overpressure Akata formations, is the potential source rock and is estimated to be up to 7000 m thick (Short and Stauble, 1967). Overlying the Akata formation is the Eocene to Recent Agbada Formation which consists of paralic sequence of interbedded siliclastic sand and shales with thickness of up to 4500 m (Weber and Daukoru, 1975). The sandy part constitutes the main hydrocarbon reservoir and the shale, the cap rock. These two formations were finally capped by the Benin formation, which is essentially a continental alluvial sand of the Late Eocene to Recent age. The formation is up to 2000 m thick (Avbovbo, 1978). The delta is characterized by syn-sedimentary growth faults and clay diapirs that are the result of gravity tectonism.

MATERIALS AND METHODS The aeromagnetic data was made available by Geological Survey of Nigeria Agency (GSNA) on a scale of 1:100,000 in half degree sheets. The survey was carried out along a set of parallel flight lines oriented in NE-SW direction at 2 km interval. The tie lines were flown perpendicular to the flight lines at 20 km interval. A terrain clearance of 762 m (2500 ft) above sea level was adopted. The processing was initiated by digitizing nine analog aeromagnetic maps using the method described by Kangolo (1996). The data was then transformed to an equally spaced 1 km / 1 km grid using the minimum curvature method (Webring, 1981). The gridded sheets were digitally merged into a composite aeromagnetic map which preserved the sanctity of the original maps. In order to significantly improve the correspondence of the observed magnetic anomalies to the causative geology, the composite Total Magnetic Intensity (TMI) was reduced to the equator (RTE-TMI) (Figure 2).

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Figure 2: Reduced to the Equator Map of the Total Intensity Map of the Study Area (A-A’ and B-B’ are the profiles modeled).

Depth to the magnetic basement was determined using the tilt depth method (Salem et al, 2007). The tilt angle is based on the ratio of first order vertical and horizontal derivatives. The zero contours delineate the spatial location of the edges of the magnetic sources whilst the depth to the source is the physical distance between the zero contours and either +450 or the -450 contour or their average. The depths so computed were plotted against their respective coordinates and gridded to produce depth to basement map of the area. Euler deconvolution of the RTE-TMI map was also accomplished to further image the basement structures. The method is a semi-automatic technique which requires no prior information about the source magnetization direction and is

insensitive to magnetic inclination, declination and presence of remanence. The Euler method was applied using the structural index of zero, indicative of contact model. The RTE – TMI was upward continued to 3 km to accentuate basement effect before the application of the horizontal derivatives. Deep seated fractures zones were mapped from the resulting map. Because the second vertical derivative allows clearer imaging of the causative structures, faults that vertically offset the basement anomalies became mappable. Dip-slip faults were mapped along trend lines where anomalies abruptly lose amplitude. The faults are generally mapped to be dipping to the direction.


The analytic signal (AS) of the upward continued data was computed in order to compute the magnetization distribution, hence the basement lithology. The analytic signal of magnetic anomaly is defined as the square root of the squared sum of the vertical and two orthogonal horizontal derivatives of magnetic field (MacLeod at al, 1993). A lithologic map was produced from the AS map. Lithologies were identified on the basis of linear continuity, magnetization level and general character of the map. The faults and fracture zones identified on derivative maps were then superimposed on the lithology map to produce the structural map of the magnetic basement of the area. Two profiles from the residual map were modeled to further image the basement architecture. RESULTS AND DISCUSSION Depth to Magnetic Basement The depth to basement map (Figure 3) shows the spatial variation of depth to the basement in the area. Maximum depth is observed in the western area where the magnetic basement attains 15.1 km below the surface. The magnetic basement rises to 5 km north of the deepest area before sloping down to 14 km northwest of the study area. The value of 3.8 km in the central area indicates the shallowest basement area. A basement ridge trending NNE -SSW straddles the entire area of investigation. The eastern region of the basement surface is generally deep and is characterized by a NE-SW trending trough. This configuration is typical of horst and graben structure. Euler Deconvolution The 3-D Euler deconvolution (Figure 4) shows the derived source location (circles) indicating the depth values. The tight clustering solutions were generated using a structural index (S.I) of zero; which is representative of contacts and faults. The lineaments may be lithological contacts, fractures or faults. Furthermore, the solutions reveal depth estimates to the top of the sources as ranging from near-surface to greater than 14 km. The alignment of solutions at all depths is an indication that the basement lineaments

propagate into the sedimentary section above the basement. Thus, this implies that there is geological connection between the basement lineaments and the development of structures within the sedimentary section in the study area. Four basement low labeled A, B, C, and D distributed in the eastern, western, northern and southeastern sides of the study area. A circular feature X of shallow solution is observed in the central area and may be a polygonal basement block uplifted to a shallow depth. A linear trend of shallow sources can be traced from this circular structure southwards. This suggests a shallow basement ridge. There are three main trends having good clustering. They are NE-SW, NW-SE and E-W trends. The combination of these trends segmented the area into a series of complex fault block pattern when observed in plan view. Three pronounced arcuate pattern solutions labeled P, Q and R, are obvious from Figure 4. Dip of Contacts Local dip of contacts delineated from second vertical derivative map is displayed in Figure 5. Nine dip slip faults (A-G and FZ1-FZ2) were mapped in the area under investigation. A northwest dipping fault FZ2 bisects the study area into two halves in the NE-SW direction. The vertical displacement of the fault blocks gives rise to the observed horst and graben basement morphology. Fracture Zones The horizontal derivative map of the upward continued data (Figure 6) map reveals parallel to sub-parallel NE-SW trending fracture zones (FZ1–FZ4) deep seated in the basement underlying the study area. Two of these linear structural trends correspond to the locations and direction of the Benue and Charcot fracture zones, which were formed during the opening of the mid- Atlantic Ocean in the late Jurassic to early Cretaceous time (Haack et al., 2000) and they are usually the location of giant petroleum fields.


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Figure 3: Depth to Basement Map Computed using Tilt Depth Method,

Figure 4: Solutions of the Euler Deconvolution using Structural Index of 0. The solutions emphasize deeper solutions at the expense of shallow sources. A,B, C and D are local sub-basins and P, Q and R

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Figure 5: Local Dips of Faults (A-G and FZ1-FZ2) Delineated from Second Vertical Derivative Map of the

Study Area. Figure 6: Correlation of Mapped Fractured Zones FZ1 – FZ4 with Map of Tectonic Setting of the Niger Delta Geological Framework of the Niger Delta (After Doust and Omatsola, 1990 and Lauferts, 1998).

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Analytic Signal The analytic signal map (Figure 7) manifests variation in the magnetization of the magnetic sources in the study area. The area is divided into zones of high and low magnetization. The regions of highs (0.0023 to 0.0051 nT/km) labeled A-F generally show NE-SW trends, except for B and E which are north-south, and are interpreted as faulted basement blocks having high magnetization. The low anomaly texture (0.0008 to 0.0023 nT/km) show dual trends. Blocks G and J assume NW-SE while other low magnetization anomalies show NE-SW trends.

Basement Map Figure 8 shows the interpretation of the aeromagnetic map. The map based interpretation emphasizes magnetic lineaments that indicate fracture zones or faults in some places and magnetic domains. At least, two cycles of tectonic events are identified in the area from the cross-cutting relationship. The NE-SW structures are truncated by the NW-SE trending faults; this suggests an older age of the NE-SW fracture zones.

Figure 7: Analytical Signal Color Map of the Study Area showing Basement Blocks of Different Magnetizations.


Figure 8: Constructed Basement Map of the Study Area. FZ1 to FZ4 are deep-seated fracture zones. F1-

F1’ and F2-F2’ are strike slip faults. 2-D Models The 2-D NW-SE geological model (Figure 9) represents a vertical cross-section across the central region (profile A-A’). Five blocks resulting from tectonic history of the area were modeled from this profile. The resulting fault blocks pattern gives the horst and graben structure underlying this section of the study area. The modeled basement blocks show magnetic susceptibility ranging from 0.0017 to 0.0033 cgs. Wells-1 and 5 located on this profile show association with basement high. The magnetization model constructed to reproduce the magnetic anomaly along profile B-B’ (Figure 10) closely matched the observed values and reflect the structural and lithological complexity of the modeled profile. Well-3 is also associated with the basement high.

CONCLUSION The basement of part of the Niger delta and its relationship to hydrocarbon prospectivity has been investigated using aeromagnetic data. The study provided information about the magnetic basement architecture and its implication for hydrocarbon exploration with the help of analytic signal, Euler deconvolution, horizontal and vertical derivatives. Basement lineaments influenced the location of intra-sedimentary structures and the producing targets in the area tend to be rooted by basement highs. Since risk reduction is of paramount importance in hydrocarbon exploration, this study therefore made a strong case for the inclusion of basement studies in the analysis and risk assessment of prospective target in the study area. Modern explorer should therefore seek to understand the basement framework

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Figure 10: Geological Model along Profile B – B’. Top – Plan View; Middle – Magnetic Anomaly; Bottom

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and its relationship to a producing target even when the basement structure is known to be beyond drillable depths. One key reason is that structural analysis of the basement can advance the understanding of the overlying structures and petroleum system for an area. REFERENCES 1. Avbovbo, A.A. 1978. “Tertiary Lithostratigraphy of

Niger Delta”. American AAPG Bulletin. 62:295-306.

2. Doust, H. and E. Omatsola. 1990. “Niger Delta”. In Divergent Passive Margin Basins. J.D. Edwards and P.A Santogrossi (eds). American Association of Petroleum Geologists. Memoir. 48: 201-238.

3. Haack, R.C., P. Sundararaman, J.O. Diedjomahor, H. Xiao, N.J. Grant, E.D. May, and K. Kelsch. 2000. “Niger Delta Petroleum Systems, Nigeria”. In: M.R. Mello and B.J. Katz (eds.). Petroleum Systems of South Atlantic Margins. AAPG Memoir. 73:213 - 231.

4. Hospers, J. 1965. “Gravity Field Structure of the Niger Delta, Nigeria, West Africa”. American Association of Petroleum Geologists Bulletin. 76:407- 422.

5. Kangkolo, R. 1996. “A Detailed Interpretation of the Aeromagnetic Field over the Mamfe Basin of the Nigeria and Cameroon”. Ph.D. Thesis, Ahmadu Bello University: Zaria, Nigeria. 64 – 66.

6. Lauferts, H. 1998. “Deep Offshore West Niger Delta Slope, Nigeria - Scale and Geometries in Seismic and Outcrop Indicating Mechanisms for Deposition”. Extended Abstracts. AAPG International Conference and Exhibition. 18 - 19.

7. MacLeod, I.N, K. Jones, and T.F. Dai. 1993. “3-D Analytic Signal in the Interpretation of the Total Magnetic Field Data at Low Magnetic Latitudes”. Exploration Geophysics. 24:679 - 688.

8. McIntyre, J.I. 1980. “Geological Significance of Magnetic Patterns related to Magnetite in Equatorial Guinea, Africa”. USGS World Energy. 57pp.

9. Michele, L.W.T., R.C. Ronald, and E.B. Micheal. 1999. “The Niger Delta Petroleum System: Niger Delta Province, Nigeria, Cameroon and Equatorial Guinea, Africa”. USGS World Energy. 57pp.

10. Reijers, T.J.A. 1996. Selected Chapters on Geology. A Case Study of the Niger Delta. Shell Petroleum Development Company.113- 114

11. Salem A., W. Simon, D. Fairhead, D. Ravat, and R. Smith. 2007. “Tilt Depth Method: A Simple Depth Estimation Method using First Order - Order Magnetic Derivatives”. The Leading Edge. 26(12).

12. Short, C. and A.J. Stauble. 1967. “Outline of Geology of Niger Delta”. American Association of Petroleum Geologist Bulletin. 51:761-799.

13. Weber, K.J. and E. Daukoru. 1975. Petroleum Geology of the Niger Delta, Tokyo. 9th World Petroleum Congress Proceedings. 2:209 - 211.

14. Webring, M.W. 1981. “MNIC: A Gridding Program Based on Minimum Curvature”. U.S. Geological Survey open file report 81 – 1224, 41pp.

ABOUT THE AUTHORS Sunday Oladele, is a Lecturer at the Department of Geosciences, University of Lagos. He holds degrees in Geology and Applied Geophysics. Samuel B. Ojo, is the Shell Professor of Geophysics in the Department of Geology, Obafemi Awolowo University, Ile-Ife. He is an author of several reputable publications. SUGGESTED CITATION Oladele, S. and S.B. Ojo. 2013. “Basement Architecture in Part of the Niger Delta from Aeromagnetic Data and its Implication for Hydrocarbon Prospectivity”. Pacific Journal of Science and Technology. 14(2):512-521.

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Basement Architecture in Part of the Niger Delta from ... · Basement Architecture in Part of the Niger Delta from Aeromagnetic Data and its Implication for Hydrocarbon Prospectivity