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© 2018 Discovery Publication. All Rights Reserved. www.discoveryjournals.org OPEN ACCESS
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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;
Email address: [email protected]
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, 74-83
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 44.815mWm-2 while the geothermal
gradients vary from 13.460CKm-1 to 33.660CKm-1 with an average of 23.540CKm-1. The geothermal gradient and heat flow values are
REPORT Vol. 14, 2018
Science ISSN 2278–5485 EISSN 2278–5477
DISCOVERY
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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 +20.00 to + 60.00mgal. The total magnetic field intensity
data ranges from -72.50 to +47.20nT. 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
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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]
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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;
Tz = mZ + To (1)
where
Tz = well bore temperature in oC at depth ZKm
To = mean surface temperature in oC
m = geothermal gradient in oC /Km
The surface (ambient) temperature for the Niger delta is assumed to be 27oC [7, 26]. The heat flow was computed using the
Fourier one dimensional
dTQ K mK
dz (2)
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 -40.0 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.
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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 -72.5 x 10-6 (blue) to +47.2 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]
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.
A
B
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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]
B
B
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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 13.460Ckm-1 to 33.660Ckm-1 with an average of 23.540Ckm-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
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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 12.20 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 17.00Ckm-1), low heat flow (
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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 (200Ckm-
1 to 33.660Ckm-1) and heat flow (35.00 mWm-2 to 70.31mWm-2) is associated with positive gravity values (20.0 to +60.0 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.
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