An integrated geothermal, gravity and aeromagnetic study for possible structural feature analysis of the Eastern Niger Delta sedimentary basin

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1 Vol. 14, 2018 ISSN EISSN Science DISCOVERY An integrated geothermal, gravity and aeromagnetic study for possible structural feature analysis of the Eastern Niger Delta sedimentary basin Emujakporue GO, Ekine AS Department of physics, University of Port Harcourt, Choba, Rivers State, Nigeria Correponding Author: Department of physics, University of Port Harcourt, Choba, Rivers State, Nigeria; address: Article History Received: 10 July 2018 Accepted: 27 August 2018 Published: August 2018 Citation Emujakporue GO, Ekine AS. An integrated geothermal, gravity and aeromagnetic study for possible structural feature analysis of the Eastern Niger Delta sedimentary basin. Discovery Science, 2018, 14, Publication License This work is licensed under a Creative Commons Attribution 4.0 International License. General Note Article is recommended to print as color version in recycled paper. Save Trees, Save Nature. ABSTRACT A correlative interpretation of the geothermal, gravity and magnetic data of the eastern Niger Delta sedimentary basin has been carried out. The heat flow values range from 19.32mWm -2 to 70.31mWm -2 with an average of mWm -2 while the geothermal gradients vary from CKm -1 to CKm -1 with an average of CKm -1. The geothermal gradient and heat flow values are Page74

2 low in the central and southeast regions and high in the southwest and seaward regions of the study area. The Bouguer gravity value in the continental ranges between 0 to -40.0mgal with the minimum located in the center of the subaerial. The free-air anomaly data of the offshore region shows positive values ranging from to mgal. The total magnetic field intensity data ranges from to nT. The aeromagnetic map shows high magnetic field strength intensity in the southwest and marine regions while the center is characterized by minimum (low) magnetic values. A combinatorial comparison of the methods shows that the central region, characterized with low geothermal gradient/heat flow values, is also associated with negative (low) gravity and minimum magnetic values. This region in the onshore may be associated with thick sediment, low density sediment or uncompensated down warp of the earth crust in the subsurface. The marine region with high geothermal gradient, heat flow, high (positive) gravity and magnetic intensity is characterized with rising basement of high density, low sedimentary thickness, presence of Charcot and Chain faults zones in the southwest, and the transition from continental to oceanic crust beneath the Niger delta. The contour lines of the magnetic basement depth show that the average depth to basement varies from 6.10 to 12.20km. This sedimentary thickness in the area is ideal for hydrocarbon accumulation. The study also shows that the area has graben and horsts, rollover structures and growth faults and shale diapers. The thermal and structural analysis show that such a situation is favourable for hydrocarbon generation and entrapment. The identified faults and structures in the area are probable migratory routes for hydrocarbon. Keywords: Niger delta; Geophysical methods; Integration; Charcot fault; anomaly. 1. INTRODUCTION The subject of integrated geophysical surveys has received considerable attention in the technical literatures over the past 40 years. The choice of the method for a geophysical survey is guided by a number of considerations such as the object of the survey, the geology and topography of the area to be investigated and type of information sought about the subsurface. The last factor is of fundamental importance [1, 2]. The geophysical investigations of the study area involved analysis of aeromagnetic, gravity and geothermal data. A geological analysis of this model provided evidences for the mechanisms that led to the present interpretation. The objectives of this work are to delineate depth to basement, structural and thermal variations of the subsurface and their implication for hydrocarbon maturation and migration. The accurate prediction of subsurface geothermal data is very important for sedimentary basin modeling, analysis of crustal tectonics, hydrocarbon maturity, generation and migration [3, 4]. Knowledge of subsurface temperature distribution is valuable in understanding the geologic and geophysical processes in sedimentary basin. Geothermal data is one of the primary factors controlling hydrocarbon generation and migration [5, 6, 7]. Gravity and magnetic methods of exploration are inexpensive and alternative geophysical techniques used for delineating subsurface structures for better understanding of the subsurface geology. Initially, gravity and magnetic methods are used for mapping basement and basin edges [8]. Recently, they have been used for modeling prospect targets in hydrocarbon exploration. Magnetic data are also used for mapping basement surfaces and for delineating volcanic intrusion, salt and shale intrusion. The magnetic method is applicable in basin analysis because of its response to the differences in the basement and overlying sediment susceptibility. Gravity method corresponds to the density contrast between geological bodies in the subsurface. Traditionally, both methods are used for regional, large-scale tectonic evaluation and understanding of a basin. These data, in conjunction with satellite altimeter derived gravity, are very useful for the study of sedimentary basin. The geology of the Niger Delta is only known through the numerous subsurface data acquired during oil prospecting activities. Few of these data have been published but the history and structures of the Niger Delta are relatively well known [9, 10, 11, 12, 13]. The origin and evolution of the Niger Delta cannot be fully explained without a correct understanding of its tectonic framework and history. Summary of the geology of the Niger Delta The study area is located within the Niger Delta sedimentary basin (Figure 1). The location of wells used for the study is shown in the Map. The Niger Delta is the youngest sedimentary basin within the Benue Trough system. Its development started after the Eocene tectonic phase [9, 11]. Up to 12km of deltaic and shallow marine sediments have been accumulated in the basin. The Niger and Benue Rivers are the main supplier of sediments in the basin. Three lithostratigraphic units are distinguishable in the Tertiary Niger Delta (Figure 2). The basal Akata Formation, which is predominantly marine predator shale is overlain by the paralic sand/shale sequence of the Agbada Formation. The topmost section is the continental upper deltaic plain sands the Benin Formation. Virtually all the hydrocarbon accumulations in the Niger Delta occur in the sands and sandstones of the Agbada Formation where they are Page75

3 trapped by rollover anticlines related to growth faults and simple rollover structures development [9, 15, 16]. The multiple growth faults are associated with antithetic faults and collapsed crystal structures. Figure 1 Base Map of the Niger Delta showing the well locations Figure 2 Stratigraphic columns showing the three formations of the Niger Delta. Modified from [14, 11] Page76

4 2. MATERIALS AND METHODS Three geophysical methods have been adopted for this study. These methods are Gravity, Magnetic, and Geothermal. In this study, the gravity data of the study area had two original sources. The first gravity data was modified from computed free-air and Bouguer gravity map of the Niger Delta [17, 18]. In the offshore region, the data is a satellite altimeter derived free-air gravity anomaly map of the area. The free air gravity data is of high resolution and it image the bathymetry and near subsurface structures in the sea. The onshore region is the Bouguer gravity anomaly map. The second set of the gravity data was modified from Hosper s bouguer gravity map of the Niger Delta [19, 20, 21, and 22]. In this second gravity data, the free-air gravity anomaly map seawards of the continental shelf and slope was based on data obtained during Walda and Atlantis 11 cruises only. The magnetic data used in this work was obtained from two sources. The first one was the processed aeromagnetic data obtained by the Nigeria Geological Survey Agency, NGSA. The magnetic data was acquired by Fugro Airborne service in 2009 [23, 24, 25]. It is of high resolution with a terrain clearance of 100 m and line spacing of 500 m. The Total Magnetic Intensity (TMI) map used was produced using the Oasis Montaj geophysical software [23, 25]. The second set of total magnetic intensity map was obtained from [17, 18]. The geothermal study in this research was carried out with the temperature data obtained from some wells in the study area. The geothermal gradient of the study area was calculated from available bottom hole temperatures of 19 petroleum wells. The geothermal gradient at depth Z is calculated assuming a linear relation of temperature and depth given in equation; where T z = well bore temperature in o C at depth ZKm T o = mean surface temperature in o C m = geothermal gradient in o C /Km T z = mz + T o (1) The surface (ambient) temperature for the Niger delta is assumed to be 27 o C [7, 26]. The heat flow was computed using the Fourier one dimensional dt Q K mk (2) dz where K = thermal conductivity Q = heat flow m = dt dz = geothermal gradient The thermal conductivity of the sand and shale lithologies were computed [10] and substituted into equation 2. The results of the three methods were interpreted independently and then integrated in order to gain more insight into the geology of the area. 3. RESULTS AND DISCUSSION The structure of the Niger Delta has been extensively discussed by several authors which include [9, 27, 7, 28]. The free-air and bouguer gravity maps used are shown in Fig. 3. The Bouguer and free-air gravity values range between -40 to +60 mgal. The onshore part is characterized by a broad negative bouguer anomaly ranging from 0.0 to mgal (blue colour) in the central region which is roughly oriented northwest. Positive free-air anomalies ranging from 0.0 to +60mgal (red colour) was observed in the continental shelf and offshore regions. The negative anomaly in the subaerial may be related to the effect of thick and low density sediment, and downwarp of the earth's crust. The positive anomaly in the offshore part of the offshore may be attributed to basement rise at depth, low sedimentary thickness and the transition from continental to oceanic crust beneath the Niger delta. The dark lineaments in the southwest region with positive free air gravity represent the Charcot and chain faults zones in the oceanic ridge. These faults are surrounded by a trough filled with sediment. The gravity anomaly increases from the continental shelf toward the continental slope. Page77

5 B Figure 3 The free air (offshore) and Bouguer (onshore) gravity field of the Niger Delta region. (A) After [17, 18]; (B) Modified after [29, 22] The total aeromagnetic intensity maps of the study area are shown in Figs. 4A and 4B.The aeromagnetic total intensity values range from x 10-6 (blue) to x 10-6 (red and pink colours) tesla. The aeromagnetic maps show high magnetic field intensity value in the offshore region while the center region in the onshore is characterized by minimum (negative) magnetic values (blue colour). Figure 4A Total Magnetic Intensity Map of the Study Area [17] A The dominant long magnetic wavelength anomalies on the map may be attributed to the deep seated basement under the basin. The map is characterized with magnetic highs and lows which are paired together. The magnetic highs are on the northern side of the magnetic lows. The most common trend in the map is in the NE-SW direction. This trend is related to the Pan African trend and corresponding to the trend Niger Delta [25, 30, 31, 27]. The most pronounced trend in the magnetic map is in the northeast-southwest. The study revealed that the subsurface is characterized by northeast-southwest lineaments or fracture zones. Some of these faults correspond to the chain fracture and Charcot fault zones in the offshore area (the arrows in Figs. 3 and 4). These faults extend to the onshore area toward the Benue trough in the east. Page78

6 B Figure 4B Total Magnetic Intensity Map of the Study Area [after 25] The basement configuration of the Niger Delta The basement configuration map of the Niger delta generated from magnetic data shows various basement blocks which are mainly having northeast-southwest and northwest-southeast trends in the tectonic framework (Figure 5). These two trends may be as a result of the position of the Niger delta during the opening of the southern Atlantic. The northeast-southwest basement trends may be attributed to extensions in the African continent of the Charcot and Chain oceanic fracture zones while the northwest-southeast trends may be due to block faulting which occurred along the edge of the African continent during the early stage of divergence. The contour lines of the magnetic basement depth show that the basement is deep in the central continental region where the magnetic and bouguer gravity values are minimum and shallow in the sea region where the magnetic and gravity values are high. The depth to basement within the central region of the onshore ranges between 9000 and12, 000 metres. On the other hand, the depth to basement in the sea region ranges between 6100 to 8000 metres. Figure 5 Basement configuration of Niger Delta Sedimentary Basin based on Magnetic data [31] Page79

7 Figure 6 Geothermal Gradient Map of the Study Area Figure 7 Heat Flow Map of the Study Area The results of the geothermal studies are presented in the form of geothermal gradients and heat flow maps in Figs. 6 and 7 respectively. The geothermal gradients vary from Ckm -1 to Ckm -1 with an average of Ckm -1. The gradients are lowest in the central and southeast regions respectively. The highest geothermal gradient occurs in the southwest region. The regional heat flow varies from 19.32mWm -2 to 70.31mWm -2 with an average of 44.82mWm -2. Heat flow is lowest in the central region. The maximum heat flow occurs in the southwest and northern regions. The high heat flow values in the southwest coincide Page80

8 with the position of the Chain and Charcot faults zones. The computed heat flow values are comparable with those of other continental margins of the world. The spatial heat flow variation can also be related to the structures in the subsurface. The low heat flow zones correspond to area with thick sediment probably with high sandstone contents. Comparison of the results from the three geophysical methods shows that the central region in the onshore with low (minimum) geothermal gradient and heat flow values correlates with negative gravity and minimum total magnetic field strength values. The offshore region where the geothermal gradients and heat flow are high are associated with positive gravity values and high magnetic intensity. The southwest region with high values is attributed to the Chain fractures and Charcot fault zones, which are the major structural features observed in the study area. The seaward region of the study area with high geothermal gradient and heat flow values may be characterized by an earlier inception of hydrocarbon generation than the continental center with minimum geothermal gradient. Owing to the fact that the sedimentary sequence becomes progressively younger from north to south, the sediments of the offshore belt could have been exposed to heat effect for a shorter period of geological time than those of the northwest. Consequently, the maturity per unit depth in the sea region would be less than those of the onshore. The magnetic basement configuration map of the study area shows that the average thickness of the sediments varies from 6.10 to km. This sedimentary thickness is favourable for hydrocarbon generation and accumulation. The geothermal gradient and heat flow values are suitable for hydrocarbon maturity and generation. The Chain and Charcot faults trending in the NE-SW extends to the onshore area. These fractures and fault zones are associated with high magnetic value, positive free air gravity anomaly and high heat flow. The Charcot, chain faults and the associated fractures are possible path migration for hydrocarbon. From the gravity and magnetic data, it may be inferred that the basement is associated with horst and graben. A seismic section and its model for part of offshore Niger delta are shown in Figures 8 and 9 respectively. The seismic section and model show that the basement is associated with horst and graben [32, 33, 34] while the upper region is associated with different structural styles. The seismic data interpretation also show the shallow structures such as counter regional faults rollover structures and growth faults and shale diapers in the sediment [35, 36]. This study has help in understanding the extensional structures, grabens, shale diapirs and regional faults in the study area. The graben and diapiric structures can be compared to the low and highs in the total magnetic field. Figure 8 Seismic profiles across offshore Niger delta showing different structural belts after Shell Deepwater Services Regional Study Team, 2002 (After 32). Figure 9 Model of Niger delta [after 32]. 4. CONCLUSION The gravity, magnetic and thermal studies of the eastern Niger Delta have been carried out to delineate the subsurface structures. The results reveal that the center region in the onshore has low geothermal gradient (13.0 to Ckm -1 ), low heat flow (<27mWm - 2 ), minimum magnetic field intensity and negative bouguer gravity anomalies (0.0mgal to -40mgal). This regions is interpreted to be Page81

9 associated with high sedimentary thickness and high sand contents of the continental environment. The geothermal gradient and heat flow values increase towards the marine environment of the study area. The sea region with high geothermal gradient (20 0 Ckm - 1 to Ckm -1 ) and heat flow (35.00 mwm -2 to 70.31mWm -2 ) is associated with positive gravity values (20.0 to mgal) and high magnetic intensity. The results obtained from the three techniques were correlated with the seismic data from the area. The Chain fracture and Charcot fault zones in the southwest of the study area are associated with high geothermal gradient, heat flow, positive free-air gravity and high magnetic intensity values. They are possible paths for hydrocarbon migration. Acknowledgement The authors wish to acknowledge Department of Petroleum Resources and Shell Petroleum Development Company, Nigeria for making the data available for the geothermal analysis. We also thank the Ali and Fairhead for making the aeromagnetic map available in the public domain. REFERENCE 1. Parasnis D. S. Principles of Applied Geophysics. 4th edition, Chapman and Hall, London, Telford, W. M., Geldart, I. P., Sheriff, R. E. and Keys R. E. Applied Geophysics; Cambridge Unw. Press, Cambridge, London Tissot, B. P., Band, J. F. and Espillie J. E. Principal Factors controlling the timing of petroleum generation. In facts and principles of world petroleum occurrences-canadian Sco. Petroleum Geologists. Memoir 6, 1980: Wapples, D. W. Time and temperature in petroleum formation: Application of Lopatin method of petroleum exploration. AAPG. Bull. 1980: 64: Ejedawe, J. E. Patterns of incidence of oil reserves in Niger Delta Basin: American Association of Petroleum Geologists, 1981: 65: Ejedawe, J. E, Coker, S. J. L., Lambert-Aikhionbare, D. O., Alofe, K. B., and Adoh, F.O. Evolution of oil-generative window and oil and gas occurrence in Tertiary Niger Delta Basin: American Association of Petroleum Geologists, 1984: 68: Uko, E. D. Thermal modeling of the Northern Niger delta. Unpublished PhD Thesis, University of Science and Technology, Port Harcourt Jacques, J. M., Parsons, M. E., Price, A. D. and Schwartz, D. M. Improving geologic understanding with gravity and magnetic data: Examples from Gabon, Nigeria and the Gulf of Mexico. First break, 2003: Ekweozor, C. M. and Daukoru, E. M. Northern delta depobelt portion of the Akata Agbada (1) petroleum system, Niger Delta, Nigeria, In, Magoon, L.B., and Dow, W.G., eds. The petroleum system from source to Trap, AAPG. Memoir 60: Tulso, (1994) Emujakporue, G. O. Subsidence and geothermal history in the eastern Niger delta with implications for hydrocarbons. Unpublished PhD. Thesis U.P.H Doust, H. and Omatsola, E. M. The Niger delta in Divergent/passive margin basins, ed., J. D. Edwards and P. A. Sentugross, AAPG. Memoirs 45, 1990: Stacher, P. Present understanding of the Niger delta hydrocarbon habitat, In, Oti, M.N. and Posstma, G.eds. Geology of Deltas: FRotterdam A. A., Baklkema, 1995: Ezekiel, J. C., Onu, N. N., Akaolisa, C. Z. and Opara, A. I. Preliminary interpretation of gravity mapping over the Njaba sub-basin of southeastern Nigeria: An implication to petroleum potential. Jour of Geology and Mining Res. 2013: 5 (3): Shannon, P. M. and Naylor, N. Petroleum basin studies; London, Graham and Trotman Limited, 1989: Kulke, H. Nigeria, in, Kulke, H., ed., Regional Petroleum Geology of the World. Part II: Africa, America, Australia and Antarctica: Berlin, Gebrüder Borntraeger, 1995: Klett, T. R., Ahlbrandt, T. S. Schmoker, J. W. and Dolton, J. L. Ranking of the world s oil and gas provinces by known petroleum volumes: U.S. Geological Survey Open-file Report , 1997): CD-ROM. 17. Fairhead, J. D., Williams, S. E., Fletcher, K. M. U., Green, C. M. and Vincent, K. Trident a new satellite gravity model for the oceans EAGE Amsterdam, extended abstract st EAGE Conference & Exhibition - Amsterdam. The Netherlands, Fairhead, J. D., Green, C. M, Masterton, S. M. and Guiraud, R. The role that plate tectonics, inferred stress changes and stratigraphic unconformities have on the evolution of the West and Central African Rift System and the Atlantic continental margins. Tectonophysics, 2013: 594: Hospers, J. Gravity field and structure of the Niger Delta, Nigeria, West Africa. Geol. Soc. America Bull., 1965: 76: Hospers, J. The geology of the Niger Delta area, in the geology of the east Atlantic continental margin, Great Britain, Inst. Geol. Sci. Rept. 1971: 70/16: Mascle, J. Submarine Niger Delta: Structural Framework. J. Min. Geo. 1976:13 (1): Okereke, C. S. A gravity study of the lithospheric structure beneath the West Africa rift system in Nigeria and Cameroon, PhD Thesis, University of Leeds, UK, 1984 Page82

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