CHAPTER ONE

INTRODUCTION

1.1 Introduction

Airborne geophysics is the study of the physical properties of the earth to learn more about its

composition and structure from an airborne platform usually an aeroplane or a helicopter.

The airborne geophysical surveys aid in the search for petroleum and in general airborne

surveys are use at the early stages of exploration to cover large areas rapidly (reconnaissance

survey) and also areas with social economic political barrier or environmentally hazardous

(Murphy 2007) in order to help define targets for more costly and time-consuming

exploration activities such as seismic and drilling. In order to achieve this measurements are

made at or near the earth surface to obtain data that when analysed are used to reveal the

internal structures of the earth (rocks and minerals). Thus Geophysics seeks to look for

contrasts in rock properties associated with uncommon specific structures which can contain

the minerals being sought for (Telford et al. 1990; Lowrie 2007). These specific minerals

only show up as anomalies in the measured and recorded data and are interpreted in terms of

variations in their physical properties. The sedimentary formation has shown great potentials

for natural resources observed from extensive geophysical investigations.

In exploration/prospecting for oil and gas in the Niger delta various geophysical surveys

(gravity magnetic and seismic) have been carried out across the area (Eletta and Udensi

2012). In the light of the above we decided to combine two geophysical methods namely;

aerogravity and aeromagnetic methods to ascertain how each method can complement the

other for better subsurface mapping of structures in the Niger Delta which has not been done

before. This will yield better results as the limitation/disadvantages of one will be

compensated by the other and vice versa. It is necessary to use integrated method because of

the presence of detectable and significant contrasts in physical properties of the subsurface

lOMoARcPSD|6699325

2

structure. It is equally necessary to use integrated method because the magnetic and gravity

methods offer innovative techniques for exploring the subsurface structure and mapping of

lithology to infer the presence of detectable and significant contrasts in physical properties of

the subsurface (Agunleti and Salua 2015; Olasehinde 2009; Obiora et al. 2015).

Magnetic and gravity data can be used in many ways to solve different exploration problems

depending on the geologic setting and rock parameters (Okiwelu et al. 2013). Magnetic and

gravity are not tools to map structures; but the data can be analyzed to provide insight to

elements of petroleum exploration and production (Johnson 1998). Gravity and magnetic

data provide a low cost way to screen large areas as well as construct important alternative

models to delineate subsurface structures and reach a better understanding of the geology.

The density contrasts presented by the juxtaposition of sediments with shales and salt make

detailed gravity modeling in this region a valuable exercise. The magnetic data provide

insight into mapping basement surfaces and delineating shallower volcanic and in some cases

shale or salt diapers.

Aeromagnetic survey maps the variation of the geomagnetic field which occurs due to

changes in the percentage of magnetite in the rock and reflects the variations in the

distribution and type of magnetic minerals below the earth surface and measure variations in

basement susceptibility (Sunmonu and Alagbe 2014). According to Aryamanesh (2009) the

applications of aeromagnetic play an important role in tracing lithological contacts and to

recognize the structures like faults lineaments dykes and layered complexes. The

aerogravity method has found numerous applications in engineering and environmental

studies including locating voids and karst features buried stream valleys water table and the

determination of soil layer thickness. The success of the gravity method depends on the

different earth materials having different bulk densities (mass) that produced variations in the

measured gravitational field. The gravity method has good depth penetration compared to

lOMoARcPSD|6699325

3

ground penetration radar high frequency electromagnetic and Dc-resistivity techniques and is

not affected by high conductivity values of near-surface clay rich soils (Mickus 2004).

1.2 Location and Geology of the Niger Delta

The Niger Delta Basin is situated in the Gulf of Guinea (Tuttle et al. 1999) it is one of the

most prolific hydrocarbon basins in the world. The Niger Delta has an area of 300000km

2

sediment thickness of over 10000km and sediment volume of 500000km

3

(Okiwelu et al.

2013). The study area (Figure 1.1) lies between latitudes 3

o

30' – 4

o

00'N longitude 6

o

00' –

7

o

00'E (offshore) and latitude 4

o

00' – 4

o

30'N longitude 6

o

30' – 7

o

00'E (onshore). The Niger

Delta sediments are divided into three distinct units of Eocene to Recent ages that form major

transgressive and regressive cycles. The Niger Delta generally displays three vertical

lithostratigraphic subdivisions: an upper delta top facies; a middle delta front lithofacies; and

a lower pro-delta lithofacies. These lithostratigraphic (Figure 1.2) units correspond

respectively with the continental sands of Benin Formation (Oligocencene-Recent) the

alternating sand/shale paralic of Agbada Formation (Eocene-Recent) and the marine prodelta

shales of Akata Formation (Paleocene-Recent). The sands and sandstones of Agbada

formation are the main hydrocarbon reservoirs. The shape and internal structure of the Niger

Delta are also controlled by fracture zones along oceanic crust. The Niger delta sits at the

southern end of Benue trough corresponding to a failed arm of rift triple junctions (Lehner

and De Ruiter 1977).

lOMoARcPSD|6699325

4

Figure 1.1: Map of Nigeria showing the study area (Obaje 2009)

Study area

lOMoARcPSD|6699325

5

Figure 1.2: Stratigraphy of the Niger delta (Obaje 2009).

1.3 Purpose of the Study

The aim of this study is to employ aerogravity and aeromagnetic data to investigate parts of

Niger Delta. This will help to:

i. Determine the depth to basement (sedimentary thickness).

ii. Delineate the basement topography.

iii. Infer the structural trends and structures within the area from the residual magnetic

intensity and Bouguer gravity maps.

iv. Determine the density contrasts magnetic susceptibility and types of mineralization

prevalent in the area.

v. Estimate the Curie point depth thermal gradient and heat flow of the study area.

lOMoARcPSD|6699325

6