SUBSURFACE INVESTIGATION OF PARTS OF BENUE TROUGH AND BORNU BASIN, NORTHEAST NIGERIA, USING AEROMAGNETIC DATA

ABSTRACT

The geophysical investigation of the subsurface structures of parts of Benue Trough and Bornu Basin, northeast Nigeria, using aeromagnetic data was carried out in this study. The area under investigation is bounded by latitude 9.50 N to 12.00N and longitude 9.50 E to 12.00  E. It is covered by 25 aeromagnetic maps. The aeromagnetic maps were digitized  on  a  3  km  by  3  km  grid  and  later  compiled  to  produce  a  combined aeromagnetic data file of the study area. The 3 km spacing interval imposed a Nyquist frequency of 0.167 km-1  while the data file generated comprised of 7921 data points. The data obtained were subjected to filtering process so as to obtain residual data necessary for interpretation. The residual data were subjected to  structural analysis using Centre for Exploration Targeting (CET) grid plug-ins, 2D subsurface modelling and depth analysis using spectral analysis and source parameter imaging. The filtering process separated the deep seated (long wavelength) anomaly from the shallow (short wavelength) anomaly. The low magnetic values tend towards northern portion of the study area. The magnetic (structural) trends in the study area were east-west, northeast- southwest, and northwest-southeast trends with the dominating structural trend in the area as northwest-southeast. There were various magnetic lineaments in the study area. These lineaments were suggested to be extension of landward Oceanic fracture zones. The results of CET structural analysis showed that the two basins (Bornu and Benue Trough) had similar structural relationship with more structural  activities in Bornu Basin, the basement complex region and the volcanic areas at the eastern part. It also showed that structural similarities exist between the surface geology and the surface lineament map of the area. The maximum sedimentary thickness obtained with spectral analysis was 3.72 km. This occurred in the central part around Gombe and south of Damaturu and Bulkachuwa. The result of the Source Parameter Imaging (SPI) has its maximum sedimentary thickness of about 5.0 km around Gombe, Ako Gombe, Bulkachuwa and Damaturu areas. The result of 2D modelling showed that the sedimentary thicknesses ranged from 0.0 km to a maximum of 4.60 km. The maximum sedimentary thicknesses were found around Gombe, Ako Gombe, Bulkachuwa and Damaturu areas, with a value of about 3.00 km to 4.60 km. The maximum sedimentary thicknesses obtained, which range between 3.72 km to about 4.60 km are adequate for the hosting of hydrocarbons. The minimum sedimentary thickness delineated by these methods could be found around Bauchi axis in the basement complex region, Kaltungo and the volcanic areas in the eastern part of the survey area. The results of these analytical methods were all in agreement. The boundary between the Bornu Basin and the Upper Benue Trough was successfully delineated through trend analysis of the total magnetic intensity, upward continuation filter and 2D modelling of the subsurface structures. The end results of these methods showed that Upper Benue Trough was separated from the Bornu Basin at about latitude 11.00  N. This area corresponds to ―Dumbulwa-Bage High‖. However, the subsurface lithology obtained from 2D modelling of the residual field showed the presence of two lithological units. The sedimentary rock unit underlined by the basement rock consists of shales, sandstones, limestones, siltstones, clay and non-marine facies, while the Basement rock units were composed of pegmatite, granite gneiss and migmatites.

CHAPTER ONE

1.0        INTRODUCTION

1.1       Background to the Study

1.1.1    Field of Applied Geophysics

The science of Geophysics applies the principles of physics to the study of the Earth. Geophysical investigations of the interior of the Earth involve taking measurements at or near the Earth‘s surface that are influenced by the internal distribution of physical properties. Analysis of these measurements can reveal how the physical properties of the Earth‘s interior vary vertically and laterally. By working at different scales, geophysical methods may be applied to a wide range of investigations from studies of the entire Earth to exploration of a localised region of the upper crust for engineering or other purposes (Kearey, Brooks and Hill, 2004).

The cardinal point in applied geophysics is to add a third dimension to geological maps. The trained eye of the field geologist is replaced with the scientific instrument whose function is to detect changes in the physical properties of the rocks which lie concealed beneath the surface of the earth. Thus, geophysics involves the study of those parts of the  earth  hidden  from  direct  view  by  measuring  their  physical  properties  with appropriate instruments, usually on the surface. It also includes interpretation of the measurements to obtain useful information on the structure and composition of the concealed zones.

The initial step in the application of geophysics to the search for minerals probably was taken in 1843 when von Wrede pointed out that the magnetic theodolite used by Lamont to measure variations in the earth‘s magnetic field, might also be employed to discover magnetic ore bodies (Telford, Geldart, Sherrif and Keys, 2001). The continued increase in the demand for metals of all kinds and the enormous increase in the use of oil and natural  gas  during  the  past  few  decades  have  led  to  the  development  of  many geophysical techniques of ever-increasing sensitivity for the detection and mapping of unseen deposits and structures. Advances have been especially rapid during the past three decades or so, because of the development of new electronic devices for field equipment and the wide spread application of the digital computer in the interpretation of geophysical data.

Geophysical surveying provides a relatively rapid and cost effective means of deriving aerially distributed information on subsurface geology. In the exploration for subsurface resources, the geophysical methods are capable of detecting and delineating local features of potential interest. Since the majority of mineral deposits are beneath the surface, their detection depends upon those characteristics which differentiate them from the surrounding media. Methods based upon variations in the elastic properties of rocks have been developed for determining structures associated with oil and gas, such as faults, anticlines and synclines, though these are often thousands of meters below the surface. The variations in the electrical conductivity and natural currents in the earth, the rates of decay in the artificial potential differences introduced into the ground, local changes in gravity, magnetism and radioactivity- all provide information to the geophysicist about the nature of the structures below the surface, thus permitting him to determine the most favourable places for locating the mineral deposits he seeks.

Geophysical techniques can therefore only detect a discontinuity, that is, where one region differs sufficiently from another in some property. Applied geophysics in the search for minerals, oil and gas may be divided into the following general methods of exploration: gravity, magnetic, electrical, electromagnetic, seismic, radioactivity, well logging  and  miscellaneous  chemical,  thermal  and  other  methods.  The  choice  of technique or techniques to locate a certain mineral depends upon the nature of the mineral and the surrounding rocks.

1.1.2    Geophysical Methods

There are broad divisions of geophysical survey methods from those that make use of natural  fields  of  the  Earth  and  to  those  that  require  the  input  into  the  ground  of artificially generated energy. The natural field methods utilize the gravitational, magnetic, electrical and electromagnetic fields of the Earth, searching for local perturbations in these naturally occurring fields that may be caused by concealed geological features of economic or other interest. Artificial source methods involve the generation of local electrical or electromagnetic fields that may be used analogously to natural fields, or, in the most important single group of geophysical surveying methods, the generation of seismic waves whose propagation velocities and transmission paths through the subsurface are mapped to provide information on the distribution of geological boundaries at depth (Kearey, Brooks and Hill, 2004).

Generally, natural field methods can provide information on Earth properties to significantly greater depths and are logistically simpler to carry out than artificial source methods. The latter, however, are capable of producing a more detailed and better resolved picture of the subsurface geology.

Several geophysical surveying methods can be used at sea or in the air. The higher capital and operating costs associated with marine or airborne work are offset by the increased speed of operation and the benefit of being able to survey areas where ground access is difficult or impossible. A wide range of geophysical surveying methods exists, for each  of which  there is  an  operative  physical property to  which  the method  is sensitive. The type of physical property to which a method responds clearly determines

its range of applications. Thus, for example, the magnetic method is very suitable for locating buried magnetic ore bodies because of their magnetic susceptibility. Similarly, seismic  or  electrical  methods  are  suitable  for  the  location  of  a  buried  water  table because,  saturated  rock  may be  distinguished  from  dry rock  by its  higher  seismic velocity and higher electrical conductivity (Kearey, Brooks and Hill, 2004).

1.2       Magnetic Method

The magnetic and gravity methods have much in common, but magnetic interpretation is generally more complex and variations in the magnetic field are more erratic and localised. This is partly due to the difference between the dipolar magnetic field and the monopolar gravity field, partly due to the variable directions of the magnetic field, whereas the gravity field is always in the vertical direction, and partly due to the time- dependence of the magnetic field, whereas the gravity field is time-invariant (ignoring small  tidal  variations).  A  gravity  map  is  usually dominated  by  regional  effects;  a magnetic map generally shows a multitude of local anomalies.

The aim of a magnetic survey is therefore to investigate subsurface geology on the basis of anomalies in the Earth‘s magnetic field resulting from the magnetic properties of the underlying rocks. Although most rock-forming minerals are effectively non-magnetic, certain rock types contain sufficient magnetic minerals to produce significant magnetic anomalies. Similarly, man-made ferrous objects also generate magnetic anomalies.

The aeromagnetic method of geophysical surveying has been established for more than five decades as a powerful method in mining and petroleum exploration (Reford and Sumner, 1964).  Many important  discoveries  can  be either directly or  indirectly be credited to aeromagnetic method. The data collection method is also very fast and cheaper than other methods.

1.2.1    The Aeromagnetic Survey

The study of the earth‘s magnetism is the oldest branch of the subject of geophysics. Sir William Gilbert (1540-1603) made the first scientific investigation of the terrestrial magnetism when he showed that the earth‘s magnetic field was equivalent to that of a permanent magnet, lying in a general north-south direction, near the earth‘s rotational axis  (Telford,  Geldart,  Sherrif  and  Keys,  2001).  In  1843,  Von  Wrede  first  used variations in the magnetic field to locate deposits of magnetic ore. The publication of the Examination of Iron Ore Deposits by Magnetic Measurements by Thalen in 1879 marked the beginning of applied geophysics (Telford, Geldart, Sherrif and Keys, 2001).

The  cardinal  point  of  a  magnetic  survey is  to  find  the  distribution  of  magnetized material whose magnetic field is given on a plane surface. Magnetic prospecting thus involves the measurement of variations in the earth‘s magnetic field. It is a natural source method in which local variations introduced by magnetic properties of rock near the surface causes minute changes in the main field. Determination of the structure and nature of the magnetized material is therefore an inverse problem of potential field theory. That is, the source is determined from its potential (Grant and West, 1965).

Magnetic exploration is carried out on land, at sea and air. Until the middle 1940s, all magnetic exploration was carried out on the ground using field methods similar to those in gravity surveys. Today, virtually all magnetic survey for oil is done from the air or ships. This is also true for most reconnaissance surveys for minerals. The speed, economy and convenience of aeromagnetic surveys are the main factors for this trend. Aeromagnetic interpretation is the drawing of inferences about the geology and ore potential of a given region from aeromagnetic survey data.

Aeromagnetic surveys simply map the distribution of magnetic minerals in the earth‘s crust. The major magnetic minerals are magnetite, titanhematite, maghemite, pyrotite, and native iron ore Fe-Ni-Co alloys. These minerals give rise to magnetic anomalies, either because of their abnormally large magnetic susceptibilities or because they have high remanent magnetization.

Of the magnetic minerals that occur in nature, magnetite is the most abundant. On a global  basis,  the  others  can  probably  be  ignored.  Thus  aeromagnetic  surveys,  in particular terms, map the magnetite in the rocks below the aircraft. While aeromagnetic surveys are extensively used as  reconnaissance tools, there has been  an increasing recognition  of their value for evaluating  prospective areas  by virtue  of the unique information they provide. Sharma (1987) outlined the roles of aeromagnetic survey as follows:

i.          Delineation of volcano-sedimentary belts under sand or other recent cover, or in strongly metamorphosed terrains when recent lithologies are otherwise unrecognizable.

ii.         Recognition and interpretation of faulting, shearing and fracturing not only as potential hosts for a variety of minerals, but also an indirect guide to epigenetic, stress related mineralization in the surrounding rocks.

iii.        Identification  and  delineation  of  post-tectonic  intrusive.  Typical  of  such targets are zoned syenite or carbonatite complexes, kimberlites, tin-bearing granites and mafic intrusions.

iv.       Direct detection of deposits of certain iron ores.

v.         Identification  of  environments  favourable  for  groundwater  exploitation including fracture systems in crystalline rocks and bedrock aquifers under alluvial covers.

vi.        In prospecting for oil, aeromagnetic data can give information from which one can determine depths to basement rocks and thus locate and define the extent of sedimentary basins. Sedimentary rocks however exert such a small magnetic effect compared with igneous rocks that virtually all variations in magnetic intensity measurable at the surface result from topographic or lithologic changes associated with the basement or from igneous intrusions (Dobrin, 1976).

1.3       Statement of the Problem

The Benue Trough is generally known to contain numerous mafic and felsic intrusives, sub- basinal structures with a bright prospect for hydrocarbon accumulation. Benue Trough is also close to the Niger Delta, where most of the hydrocarbon mining activities in the country are taking place. Due to the location of the basin and the expected basement depth predicted by previous researches, has drawn attention to the area due to the possibility of hydrocarbon potential of the Trough. This work will explore the possibility of hydrocarbon potentialities of the Upper Benue Trough and southern Bornu Basin in other to expand the petroleum base of the Country (Nigeria).

1.4       Justification of the Study

The use of modern digital processing tools (conversion of aeromagnetic data from space domain  (wavelength) to  frequency domain  (wave-number)  for proper  handling  and better interpretation) on the aeromagnetic data of the study area is expected to shed more light in unfolding the historical evolution and tectonic settings of the survey area. These digital tools would be combined in such a manner to investigate and interpret the aeromagnetic data covering the survey area and thus the emerging picture is expected to yield better resolution than ever known. The following are the outlines of the objectives that justify this research work:

1.   The origin and tectonic evolution of the Benue Trough and Bornu Basin have been the subject of several suggestions. Most of the suggestions are based on qualitative interpretation of data and inferences from regional surveys carried out in the adjacent Benue Trough. While working on the central part of Nigeria, Ajakaiye,  Hall,  Ashiekaa  and  Udensi  (1991)  and  Ajakaiye,  Hall,  Millar, Verhejen, Award and Ojo (1986) noted that magnetic features in the area are mainly magnetic lineaments with definite characteristics which exist within the Nigerian  landmass.  The  lineaments  they  noted  coincided  with  the  major structural  trends  such  as  the  Benue  Trough  in  Nigeria  and  fractures  in  the oceanic  crust  of West  Africa.  They stated  that  onshore lineaments  in  West Africa are extensions of St. Paul, Romanche, Chain and Charcot fracture zones in the Mid-Atlantic Ocean.

The lineaments are believed to be part of the major zones of weakness in the crust that predate the opening of the Atlantic Ocean and were reactivated in the early stages of continental drift. Three of these fracture zones, St. Paul, Romanche and Chain are believed to pass through the study area (Latitude 9.50 –

12.00 and Longitude 9.50 – 12.00). By reason of the detailed nature of the present

study, qualitative and quantitative analysis of magnetic trends will be made. Suggestions will be put forward in respect of the nature of the magnetic lineaments and their relationship with the St. Paul, Romanche and Chain fracture zones of the Mid- Atlantic.

2.   The boundary delineation and mapping of the geological structures within the study area have mostly been done by geologists (Ojo and Pinna, 1982; Whiteman, 1982; Dessauvagie, 1974; Benkhelil (1988 and 1989) and Zaborski,

1998 among others). Detailed geophysical examination has not been carried out

to a large extent as compared to geological survey in the study area which would have correlated the geological results/findings. The present work is expected to achieve this through the use of CET grid analysis.

3.   The Benue Trough is of interest due to its large area of coverage, it forms a regional structure which is exposed from the northern frame of the Niger Delta and runs northeast-wards for about l000 km to underneath Lake Chad, where it terminates. For this reason, researches conducted within the area are usually restricted to a section of the basin either the lower, upper or the middle Benue basin. The location of the basin and the expected basement depth predicted by previous researches drawn attention to the area due to the possibility of hydrocarbon potential of the Trough. The Benue Basin is close to the Niger Delta, where most of the hydrocarbon exploration activities in the country are taking place. The average depth estimate by Likkasson, Ajayi and Shemang (2005) was 4.6 km while 5.6 km was obtained by Onwuemesi (1995). These had shown that the depth can be within the range of expected depth for the curie temperature expected for thermal saturation for crude oil. The present work would probably confirm this.

4.   The dynamism observed in geophysical data interpretation due to advent of the computer, had opened up a lot of room for more research even within areas that had been investigated. Today we have more efficient analyzing software which can reveal features that might have escaped the sensitivity of previous tools employed. Thus re-evaluating previous studies with more advanced techniques may be useful.

1.5       Aim and Objectives of the Present Study

This study is expected to generate and upgrade the existing geophysical knowledge of the survey area that had been previously made available by other researchers. The primary aim of this study is to produce information (database) that could be a guide to both magnetic mineralisation and hydrocarbon potentialities of the study area and to also serve as a guide to future geological mapping of the study area.

1.5.1    Objectives of the Study

The objectives of this study include:

a)  to carry out a surface mapping of the study area and qualitatively analyse the trend to determine prominent structural features of the area;

b)    to enhance location  of  prominent  deep  seated  (long  wavelength) anomalies within the study area through the use of upward continuation filter control on the total magnetic intensity data;

c)  to delineate the two basins and determine their structural relationship through the use Centre for Exploration Targeting (CET) Grid Analysis;

d)  to estimate and analyse depth to magnetic source bodies through spectral depth analysis and Source Parameter Imaging (SPI);

e)  to  carry  out  detail  modelling  of  the  residual  magnetic  field  anomalies  to determine the subsurface structures of the study area, particularly the thicknesses of the two basins (thickness of the sedimentary rocks); and

f)   to determine the economic potentials of the study area in terms of hydrocarbon potentials.

1.6       The Study Area (Location and Extent)

The Benue Trough is commonly subdivided into three main domains corresponding to both geological and geomorphologic partitions. These include the Southern (Lower) Benue Trough which covers areas within the south-eastern Nigeria, the central (Middle) Benue Trough which covers Gboko, Makurdi and Lafia areas and the Northern (Upper) Benue Trough that covers Gombe-Yola areas.

The present study area covers extensively the Upper Benue Trough, the southern part of the Chad Basin (also known as lower Bornu Basin), the Keri – Keri Formation around Gombe and part of the Younger Granites region of the Jos and Bauchi areas. This area is situated at the North-Eastern part of Nigeria (Figure 1.1). The area is bounded by Latitude 9.50N to 12.00N and Longitude 9.50E to 12.00E. The physiological features recognized in the area are the river Benue and its tributaries like river Gongola, Amumma, Rowai, Ruhu, Misau and so on.

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