GEOPHYSICAL INVESTIGATION FOR HYDROCARBON POTENTIAL OVER PART OF UPPER BENUE TROUGH (ADAMAWA BASIN) NORTHEAST, NIGERIA USING AEROMAGNETIC DATA

ABSTRACT

This study present the results of the analysis and interpretation of aeromagnetic data over part of Adamawa trough with the aim of investigating the hydrocarbon potential of the study area. The study area covers an area of 12,000 km2 located between latitude 8.50oN and 9.50oN and longitudes 11.50oE and 12.50oE. This study adopted both qualitative analysis and quantitative analysis to delineate the deep seated anomalies and estimate the sedimentary thickness for hydrocarbon maturation and accumulation respectively. Upward continuation and analytic signal techniques were used to delineate the deep seated structures using the total magnetic intensity map while three depth estimating techniques were employed to determine the thickness of sediments in the study area; Source parameter imaging, Euler deconvolution and spectral method. The analytic signal shows two major region; regions whose amplitude responses are high which are mainly basement intrusions with varying degree of deformations at the south-eastern to the central part the study area; and regions whose amplitude are low, which depicts regions with relatively good sedimentation at the northern part of the study area. The residual map was subjected to the depth estimating techniques and the results obtained corroborate; the SPI, Euler Deconvolution and Spectral method shows a thick sedimentation of 4.42 km, 4.20 km and 4.17 km at the north-eastern part of the study area respectively. The SPI, Euler deconvolution and the Spectral method reveal shallow depth of 0.06 km at the southeast, 0.10 km at the southeast and 0.42 km at the southwest part of the study area respectively. The maximum sedimentary thickness of above 4 km obtained in this study at the north-eastern part of the study area which corresponds to Numal is sufficient enough for hydrocarbon maturation and accumulation. The study area was found to have a good prospect for hydrocarbon exploration.

CHAPTER ONE

1.0       INTRODUCTION

1.1       Background of the Study

The substantial part of Nigeria source of income (80%) is acquired from hydrocarbon (oil and gas) profit whereupon more than 180 million teeming populaces depend (Adewumi et al., 2017). As the hydrocarbon potential of the prolific Niger delta becomes depleted or exhausted in the nearest future due to continuous exploitation and inherent crises in the existing oil producing region (Niger delta) which had led to undue reduction in oil production for export and domestic use, it is of necessity to shift attention to other inland sedimentary basins such as the study area (Adamawa Basin) in Nigeria for possible hydrocarbon exploitation.

The exploration for solid minerals, oil and gas across the Benue trough and its adjoining areas has increased tremendously with the aim of increasing the nation’s hydrocarbon reserve and alternative means of generating more revenues.  The interest in sedimentary basins arises from the fact that they are the usual potential host rocks for oil and gas and most solid minerals. The discovery and harnessing of hydrocarbon in the study area will expand Nigeria’s oil and gas holdings, boost quantities of the hydrocarbon potentials of the Nigerian inland basins, in this way, help the economy of Nigeria and also provide many new employments to lessen unemployment rate currently facing Nigeria (Adewumi et al., 2017).

The present  study attempts  to  use the qualitative and  quantitative interpretation  of aeromagnetic data over part of Adamawa Basin to examine the subsurface structures of the study area for possible hydrocarbon maturation and accumulation within the area using some advanced enhancement techniques.

Magnetic anomaly maps generally reflect variations in the Earth’s magnetic field resulting from the underlying rocks’ magnetic properties (e.g. magnetic susceptibilities). Though most rock-forming minerals have negligible to very low magnetic susceptibility and therefore are essentially non-magnetic, certain types of rocks contain enough magnetic minerals (especially magnetite) to generate recognisable magnetic anomalies. Sedimentary rocks generally have the lowest magnetic susceptibility, whereas metamorphic and basic igneous rocks have the highest magnetic susceptibilities (Dobrin and Savit, 1988; Telford et al., 1998; Kearey et al., 2002). Magnetic anomalies are caused by magnetic minerals contained in rocks; such anomalies are usually associated with underlying basement (igneous and/or metamorphic) rocks or by igneous bodies within sedimentary successions such as intrusive plugs, dykes, sills, lava flows and volcanic rocks (Gunn, 1997), as well as due to cultural iron contamination and heterogenic alterations in sedimentary rocks possibly caused by hydrocarbon migration (Costanzo- Alvarez et al., 2000; Aldana et al., 2003).

The magnetic method is useful whenever the object of investigation has a contrast in the magnetic susceptibility or remanence that can be detected by the magnetometer.  This oldest of the geophysical exploration method is used for:

i.         Location and definition of the extent of a Sedimentary Basin

ii.        Depth determination to the basement rock (Magnetic source)

iii.        Thickness of the sediment present which can be used to assess whether the sediment is enough to warrant exploration of petroleum.

iv.        Local relief of the basement surface which may produce the structural relief in the overlying sediments.

v.         Basement lineation

Analysing the local geology based on the deviations found in the earth magnetic field is the main aim of the magnetic survey of the earth subsurface. The magnetic surveying method uses the Earth magnetic anomalies ensuing from the magnetic properties of the underlying rocks to investigate subsurface geology. This anomaly mostly occurs as a result of the differences in the magnetic susceptibility of the rocks underlying subsurface.

Magnetic survey can be performed on Land, at Sea and in the air (as in the data to be used in this study), when performed in air an aeromagnetic study is undertaken and as such a magnetometer is towed behind an aircraft. The technique is widely employed and the speed of operation of airborne (Aeromagnetic) survey makes the method very attractive in the search for minerals and hydrocarbon explorations.

Aeromagnetic surveys have become common in hydrocarbon exploration in recent years. The development of more accurate magnetometer, aircraft positioning and data processing have led to the acquisition of high-resolution aeromagnetic surveys. Therefore, this work is an attempt to characterize geological structures and sedimentary basins in the Adamawa Trough and some adjoining areas by the use of potential field data.

Aeromagnetic data have long been used by the petroleum industry to map geological structures in, and to estimate depth to magnetic basement (Steenland, 1965). The computer processing and interpretation of the Aeromagnetic data led to anomaly estimation of depth to basement (Spector and Grant, 1970). The aeromagnetic survey is widely applied to the exploration for minerals and geothermal resources. This method was re-discovered by the petroleum industry in the early 1990s. It has been demonstrated in presentations, the literature and other forums that high resolution aeromagnetic data provide  valuable  data  to  solve  petroleum  exploration  problems.  High  resolution aeromagnetic data for petroleum exploration are commonly defined as data collected at a flight line spacing of 800 metres or less, at flight-heights of 150 metres or less, at 15 metres or less sample spacing along the flight lines and at better than 0. I nT accuracy.

1.2       Statement of the Research Problem

Hydrocarbon is a major source of revenue in Nigeria. The substantial part of Nigeria’s source of income (80%) is acquired from hydrocarbon (oil and gas) profit whereupon more than 180 million teeming populace depends on upon. The depleting nature of hydrocarbon potential of the prolific Niger Delta region stages a major concern and the activities of the militancy in the Niger delta calls for the need to delve into other inland sedimentary basins in Nigeria such as the Adamawa trough presumed to be rich in hydrocarbon. This study therefore attempts to analyses and interpret the high-resolution aeromagnetic data over the study area to assess its hydrocarbon potential.

1.3       Aim and Objectives of the Study

The aim of this study is to analyse and interpret aeromagnetic data of part of Upper

Benue trough (Adamawa Basin) for possible hydrocarbon accumulation. The objectives are to;

i.      produce the composite map of the study area and interprets its total magnetic intensity (TMI) map

ii.      delineate subsurface structures (long wavelength anomaly) in the study area using upward continuation filter and analytic signal.

iii.      determine the sedimentary thickness using SPI, Euler deconvolution and spectral depth analysis

iv.      delineate areas of possible hydrocarbon potential in the study area.

1.4        Justification of the Study

Benue trough is largely referred to as failed rift valley (Cratchley et al., 1984 and Nwogbo, 1997) and so it is expected that the region should be a major depositional basin and therefore a good site for hydrocarbon maturation and accumulation.

With the availability of  high-resolution aeromagnetic data, this study will properly contribute to the knowledge of the sedimentary thickness, which is one of the factors for possible accumulation of hydrocarbon within the study area.

1.5       Scope of Study

This work is limited to the analysis and geophysical interpretation of the four aeromagnetic data acquired for this research and to speculate on possible hydrocarbon potential within the area. This aeromagnetic study of Adamawa Trough is limited to the information that can be derived from the potential field.

1.6        The Location of the Study Area

This study covers an area of approximately 12,100 km2  in the north-eastern part of Nigeria. The area (Figure 1.0) is bounded by latitudes 8.5 000’N to 9.5000’N and longitudes 11.5000′E to 12.5000’E. The study area is located Adamawa Basin in the North Eastern part of Nigeria and covering Dong, Numal, Monkin and Jada.

The area has two distinct seasons; dry and wet. The dry season usually last from November to February and the rainy seasons last from March to October. Wet season starts normally from early April till ending of October, on average of about 210 days. While the dry season last from early November to March ending, on average of about 150 days.  The  first  half  of  the  dry season  is  called  harmattan  period  (with  night-time temperatures drop of as low as 11 °C) while the average monthly temperature for the dry season range from 21–25 °C. The vegetation of the study area falls within the southern Guinea Savanna zone. The vegetation type is an open forest of tree savanna (mostly economic trees) where clusters of trees stands amongst grass and shrubs (Plateau and Nassarawa Geographical Information System).

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