An integrated geothermal, gravity and aeromagnetic study for...

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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

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This work is licensed under a Creative Commons Attribution 4.0 International License.

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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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