ANALYSIS OF AEROMAGNETIC DATA ACROSS PARTS OF LOWER BENUE TROUGH AND NIGER DELTA, NIGERIA

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ABSTRACT

 

Aeromagnetic data obtained from the Nigerian Geological Survey Agency, Abuja were used to carry out this research. The study area is at the coordinates of 5°00′00′′ - 6°02′00′′ N and 7°00′00′′ - 8°01′00′′ E covering a distance of about 12, 100 km2. The study area belongs to two geologic basins which include the Benue Trough and Niger Delta. These basins are known for their economic importance. The study aims at using a computer program to spectrally analyse and compute the sedimentary thickness of the area and correlate the geological and the enhanced aeromagnetic maps for solid minerals deposit. Also, to estimate the hydrocarbon window, Curie point depth, geothermal gradient and heat flow within the study area. The obtained data were processed and enhanced using applicable techniques. Two depth sources were revealed; using Geosoft’s Oasis montaj version 7.1; these are sedimentary thickness of 7.9 km for deeper sources and 0.42 km for shallower sources. The Curie point estimated in the study area ranges from 7.1 km to 12.5 km with an average of 10.1 km. Qualitative analysis of the study area shows that it is structurally controlled, giving room to mineralization; and the shallow source depth of 0.42 km in the study could be attributed to intrusion and mineralization. Shallow depths encourage mineralization, and the study area is endowed with solid minerals (Fig. 2). Geothermal gradient and heat flow estimated in the study averaged at 59.7°C/km and 149.3 mWm-2 respectively.  The high geothermal gradient in the area is as a result of the rifting and metamorphic activities in the host basins. The sedimentary thickness (7.9 km deep) and geothermal gradient averaging 59.7°C/km are testaments to possible hydrocarbon accumulation in the area.




TABLE OF CONTENTS

Title page                                                                                            i

Declaration                                                                                          ii

Certification                                                                                        iii

Dedication                                                                                           iv

Acknowledgements                                                                              v

Table of Contents                                                                                vi

List of Tables’

List of Figures                                                                                     

Abstract                                                                                             

 

CHAPTER 1:         INTRODUCTION                                                   1

1.1     Overview                                                                                   1

1.2     Location of the Study Area                                                          2

1.3     Climate of the Study Area                                                            2

1.4     Physiography of the Study Area                                                   3

1.5     Geology of the Study Area                                                           5

1.5.1  General geology                                                                         5

1.5.2  Local geology                                                                             7

1.6     Statement of the Problem                                                           12

1.7     Aim and Objectives                                                                     12

1.8     Scope of the Study                                                                     12     

1.9     Justification of the Study                                                             13

1.10   Advantages and Disadvantages of Airborne Magnetic                     13     

 

CHAPTER 2:         LITERATURE REVIEW                                          14

2.1     Overview                                                                                   14

2.2     Basic Theory of the Magnetic Method                                            15

2.2.1  The nature of the geomagnetic field                                             15

2.2.1.1 The main field                                                                          15

2.2.1.2 The external field                                                                      17

2.2.1.3 Variations in the main field                                                        18

2.2.2  Magnetization of rocks                                                                18

2.2.3  Magnetic moment                                                                       18

2.2.4  Intensity of magnetization                                                           19

2.2.5  Magnetic susceptibility                                                                19

2.2.6  Magnetic induction                                                                     20     

2.3     Definition of Techniques                                                              21

2.3.1  Reduction to magnetic equator                                                    21

2.3.2 Shuttle Radar Topography Mission (SRTM) elevation data               23

2.3.3  Source parameter imaging (SPI)                                                   23

2.3.4  Analytic signal                                                                            24

2.3.5  Derivatives                                                                                24

2.3.6  Vertical derivatives                                                                     25

2.3.7  Spectral analysis                                                                         26

2.3.8  Curie point depth                                                                       27

2.3.9  Geothermal gradient                                                                   29

2.4     Previous Aeromagnetic Study of the Area                                      31

CHAPTER 3:         MATERIALS AND METHODS                                 37

3.1     Data Acquisition and Description                                                  37

3.2     Instrumentation                                                                         50

3.2.1  The proton magnetometer                                                           50

3.3     Methodology of the Study                                                           51

 

CHAPTER 4:         RESULTS AND DISCUSSION                                56

4.1     Qualitative Interpretation                                                            56

4.1.1  The TMI map and results                                                             56

4.1.2  The RTE map and results                                                             58

4.1.3  The SPI map and results                                                              59

4.1.4  First vertical derivative map of depth slice 1                                  63

4.1.5  The Analytic signal map of depth slices 1 and 2 and results             64

4.2     Quantitative Interpretation                                                          73

4.2.1  Estimation of Sedimentary thickness by means of spectral analysis  73

4.2.2  Estimation of Curie point depth, geothermal grad. and heat flow     75

4.3     Discussion                                                                                 82         

4.4     Application of these findings to the search for Solid Minerals          84

CHAPTER 5:         SUMMARY,CONCLUSION AND RECOMENDATIONS 85

5.1     Summary                                                                                   85

5.2     Conclusion                                                                                 86     

5.3     Recommendations                                                                      88

          References                                                                                 89






 

LIST OF TABLES

 

Table                                                                                       Page

1.1.    Summary of Lithology, Structure and Topography                10

3.1.    Some of the acquired data                                                  43

4.1.    Calculated SPI values in the study area                                59

4.2.    Spectral depth estimation of the study area                         74

4.3a.  Estimated depth to the top boundary                                  80

4.3b.  Estimated depth to centroid                                               81

4.3c.   Estimated Curie point depth                                     ­81

4.4.    Estimated geothermal gradient and heat flow                      82







 

LIST OF FIGURES

 

Figure                                                                                                    Page                                                                              

1.           Location map of the study area (adapted from Google 2015 map)           2

2.          

9

Geology map of the study area (After Nigeria’s geology map of mineral resources by NGSA)                                                     

3.          

14

The Earth’s dipolar magnetic field and the inclination of field at

varying latitudes.                                                           

4.           The magnetic elements                                                                       16     

5.           Geothermal gradient of the Earth’s interior (after Wikipedia, 2015)                   30

6.           The TMI map of Okigwe area                                                               38     

7.           The TMI map of Afikpo area                                                                39

8.           The TMI map of Aba area                                                                    40                                            

9.           The TMI map of Ikot-Ekpene area                                                        41

10.        The TMI of the study area                                                                   42

11.        The Shuttle Radar Topography Mission (SRTM) map of the study area (measured in metres)                                                                                             49                    

12.        A typical flight pattern of the survey                                                    50

13.     TMI map of the study area at interval of 20 nT min = -100.6 nT and max = 207.2 nT                                                                                                    65     

14.     RTE map of the study area contoured at interval of 20 nT min = -98.5 and max = 197.7                                                                                                66     

15.     Source Parameter Imaging (SPI) map of the study area                          67

16.     First vertical derivative map of depth slice 1                                          68

17.     First vertical derivative of slice 2                                                          69                                

18.     Lineament trend of the study area                                                        70

19.     Analytic signal map of depth slice 1                                                      71

20.     Analytic signal map of depth slice 2                                                    72

21.     2D radially averaged power spectrum of the study area                          74

22a.   Chart for block 1                                                                               76

22b.   Chart for block 2                                                                               76

22c.    Chart for block 3                                                                                77                                                                                                                                                                                                                                                                                                                                               

22d.   Chart for block 4                                                                                77

22e.   Chart for block 5                                                                               78

22f.    Chart for block 6                                                                               78

22g.   Chart for block 7                                                                                79

22h.   Chart for block 8                                                                               79

22i.    Chart for block 9                                                                               80

 


 

 


 

 

CHAPTER 1

INTRODUCTION

 

1.1        OVERVIEW

Geophysical exploration methods represent a primary tool for investigation of the subsurface and are applicable to a wide range of problems.

 

The particular geophysical method, precisely used in this work is the magnetic method, precisely the aeromagnetic method in which measurements of the Earth’s magnetic field are made using magnetometers towed behind an aircraft. The aircraft typically flies in a grid-like pattern with height and line spacing determining the resolution of the data (Taufiq et al., 2014)

 

The results of interpretation made on aeromagnetic data are very useful in oil and mining industries. In oil exploration they are mainly used to determine the thickness of the non-magnetic sedimentary section overlaying a magnetic basement. The assumption usually made is that the structure of the sediments is controlled by the basement topography and in this case variations in the depth of the basement can suggest possible location of oil traps. In mining exploration the prime use is as an aid to geological mapping. It is particularly useful for these two reasons. Firstly, not all rocks are magnetic and is therefore usually possible to follow the outlines of magnetic structures over considerable distances without too much confusion from neighbouring anomalies. Secondly, many of the minerals contain an appreciable quantity of magnetite.

 

In this chapter, the geology, climate and physiography of the study area which lies in southern Nigeria will be discussed.


Figure 1:       Location map of the study area (adapted from Google 2015 map)

 

1.2        LOCATION OF THE STUDY AREA

 

The study area falls within the Lower Benue Trough and Niger Delta, which are sedimentary basins. The geographical coordinates of the study area are 500′N – 600′N. The study area covers an area of 12,100 Km2. The major towns are Okigwe, Afikpo, Aba and Ikot Ekpene (Figure 1).

= Major road

= Minor road

 

1.3        CLIMATE OF THE STUDY AREA

Nigeria may be divided into four climate regions as follows (Iloeje., 1981).

i.              the subequatorial South

ii.            the tropical hinterland

iii.           the tropical continental north

iv.           the high plateau

 

The study area falls in the sub – equatorial south. The subequatorial south extends from the coast to roughly 130 to 160 kilometres inland. The annual rainfall is between 2000 and 4000 mm (Jalon Consultants., 1980; Amos-Uhegbu et al., 2013). The total rainfall increases from 150 centimetres in the west to more than 300 centimetres in the Delta area. Relative humidity is normally over 90% in the early morning, but falls to between 60% and 80% in the afternoon. Mean annual temperatures register over 29 C (Amos-Uhegbu et al., 2013).

 

1.4     PHYSIOGRAPHY OF THE STUDY AREA

The physiography of the study area can broadly be described in terms of three distinct parts namely; northern part; central part and southern part. In the northern part is located such major town as Afikpo. Okigwe is the major town in the central part and Umuahia is located in the southern part.

 

The northern part can be described in terms of two physiographic units (Ahamefula., 2012). These units are:

i.             the north–east and western low lands which have some isolated ridges, and

ii.            the high lands of NW–SE trend that consists of long and scattered ridges. These ridges are continuous in some areas and isolated in other places.

 

The northern fringes of the area have the highest elevation of about 120 m above sea level. These ridges are prominent in villages such as Amankwu, Ndi Uduma and Asaga. Observation shows that in some places low lands or valleys occur in between ridges.

 

The central part of the study area, where Okigwe is located, is dominated by two striking features. Respectively, these are the Udi – Okigwe – Arochukwu and Awka – Umuchu – Umuduru sedimentary cuestas (Uma., 1986). These cuestas form regional watersheds standing at about 200m above the surrounding plains, the Udi – Okigwe – Arochukwu cuesta separates the west – flowing headstreams of the Imo River and the east flowing headstreams of the Cross River. The Udi – Okigwe – Arochukwu cuesta is more imposing than the smaller, less continous Awka – Umuchu – Umuduru cuesta which separates the west and north – west flowing streams of Marnu and Orashi rivers from those of the Imo River that flow eastwards.

In the area between these two major topographical features are undulating terrains comprising dissected ridges and numerous isolated hills. Springs and effluent seepage can be found at the foot of some of these hills and ridges. Most of the effluent seepage are emphemeral and some represent through flow of rainfall events; however a few of them are perennial, but a much reduced discharge during the dry season (Uma., 1986).   

The southern part of the basin, where Umuahia can be found, is characterized by a gentle slope of the ground towards the south. The ridges trend generally in the north – south direction and are separated by valleys mostly those of the Imo River and its major tributaries, which are 40 to 60 m below the surrounding landscape. Some of the valleys are dry. Towards the most southern end of the study area, which contains the major town of Aba, the ridges give way to a nearly flat topography.

 

1.5     GEOLOGY OF THE STUDY AREA

1.5.1  General geology

The study area lies in two major tectonic features in southern – eastern Nigerian. These are the Niger Delta complex and the Lower Benue trough.

 

The north easterly trending Benue Trough is located at a zone of weakness existing between the West African and Congo cratons since the late Precambrian to early Palaeozoic eras. It is one of the marginal basins that developed during the cretaceous following the opening of the South Atlantic Ocean in the Mesozoic.

 

The tectonic activity that gave rise to the structural trough occurred in Albian times, that is during the early cretaceous period (Murat., 1972). The stratigraphic sections of the trough are composed primarily of a hierarchy of sedimentary rocks deposited during major transgressive – regressive cycles as a result of epeirogenic movements. The trough was filled in three phases as a result of these movements. 

 

The first transgressive – regressive cycle extended from mid – Albian to the end of the Cenomanian (Murat., 1972) and resulted in the disposition of the Asu River Group. The second cycle which comprises the Lower Turonian (Murat., 1972) resulted in the deposition of the Cross – River Group. The deposits of the third cycle (upper Turonian – lower Santonian) have been largely eroded as a result of folding and uplift. The deposits are mainly marine shale. The tectonic event which resulted in this folding and uplift took place from the upper Santonian to probably lower campanian (Murat., 1972) and was characterized by compresional movements along established NE–SW trend. These movements gave rise to the Abakiliki anticlinorium and the Afikpo Syncline. Simultaneously, there was subsidence of the Anambra platform to the West of Abakiliki anticlinorium. The Anambra basin is the result of this subsidence.

 

In addition to the sediments the Benue Trough is characterized by Tertiary to Recent Volcanic rocks of basic and intermediate composition.

 

Most of the study area is situated on the Benue Trough. The remaining part is situated on the Niger Delta Basin. The Niger Delta Basin is a Tertiary structure formed during the Eocene and filled with Tertiary sediments of enormous thickness.


 

1.5.2  Local geology

Figure 2 shows the geology of the study area. It reveals the various formations in the area which has been summarised in table 1.1 in terms of lithology, structure, and topography.

Some of the Formations in the study area are Ajalli Formation, Mamu Formation, Nkporo Formation etc.

 

Ajalli Formation consists of false bedded sandstone. The formation is slightly diachronous, ranging from Middle to late Maastrichtian from south to north. Cross bedding is the dominant sedimentary structure of the formation. It is associated with reactivation surfaces, mud drapes, tidal bundles, backflow ripple channels cut and fills, lateral accretion surfaces, as well as Skolithos and Ophiomorpha ichnogenera (Ladipo et al., 1992). The topography of Ajali Formation shows plateau in the north, cuesta in the south and escarpment.

 

Mamu Formation succeeds the Upper Campanian and is made up of sands, gravels, shales with some limestones in the south and coal seams in the central to the upper parts of the basin. The thickness of the Formation varies across the Lower Benue Trough. The topography shows lower slopes.

 

The basal facies of the late Cretaceous sedimentary cycle in the Anambra Basin (of the Lower Benue Trough) is the Nkporo Shale which indicates a late Campanian age. In general this Formation is exposed at Leru 72 km Enugu – Port Harcourt Express way and has been described as a coarsening upward deltaic sequence of shales and interbedded sands and shales (Ladipo et al., 1992). The lithology is mainly Shales with lenticular sand bodies. Nkporo Shale is moderately dip to west and south in terms of structure and its lithology shows lower slopes.