ABSTRACT
and bottom hole temperatures from wells in the Eastern Niger Delta suggests that two leg dogleg geothermal characterize the geothermal gradients pattern of the Central and the Coastal Swamp in contrast to the single gradient patterns seen in the Shallow Offshore.
In the shallow/continental sections in the Niger Delta, geothermal gradients vary between 10 – 18 o C/Km , increasing to about 24 o C/Km seawards. In the deeper (marine/parallic) section, geothermal gradients vary between 18 – 45 oC/Km.
The average geothermal gradient for the various depobelts is 19 oC/Km for the Central Swamp, 17oC/Km for the Coastal Swamp and 20oC/Km for the Shallow Offshore. Geothermal gradients in the Eastern Niger delta increase eastwards, northwards and seawards from the Coastal Swamp.
Vertically, thermal gradients in the Niger Delta show a continuous and non-linear relationship with depth, increasing with diminishing sand percentages.
As sand percentages decrease eastwards and seawards, thermal gradient increases. Thermal conductivies also decreases with depth from about
2.3 W/mK in the continental sands to 1.56 W/mK in the parallic and continuous shaly sections.
Isothermals constructed at three depth levels: 1000m, 2000m, and 3000m shows that depressed temperatures occur in the western and north central parts and elevated temperatures in the eastern and northern parts of the study area, respectively.
Heat flow computed from 1 – D modelling software and calibrated against BHT and reservoir temperatures suggests heat flow variations in the Niger Delta to range from 29 – 55 mW/m2 (0.69 – 1.31 HFU) with an average value of 42.5 mW/m2 (1.00 HFU).
Lower heat flows (< 40 mW/m2) occur in the western and north central parts of the parts of the study area, and is likely to be influenced by high sedimentation rates.
Higher heat flows (40 – 55 mW/m2) occur in the eastern and northwestern parts of the study area. Radiogenic heat production from crustal rocks and shale’s may account for the heat flow in the east. Hydrothermal convection is likely to have elevated the heat flow in the northwest.
The hydrocarbon maturity modelling results show vast differences in timing and levels of kerogen transformation into petroleum.
Result suggests that the potential source rocks (Paleocene, Eocene, Oligocene and partially the Lower Miocene) have attained maturity status to generate hydrocarbons. The depth to the onset of the oil window decreases from the west to the east and to the northwest.
TABLE OF CONTENTS
Title Page i
Certification ii
Dedication iii
Acknowledgements iv
Abstract vi
Table of Contents vii
List of Figures xi
List of Tables xiv
1.0 INTRODUCTION 1
1.1 General Introduction 1
1.2 Location of the Study Area 2
1.3 Statement of the Problem 2
1.4 Scope of Previous Studies 6
1.5 Purpose and Scope of Present Research 8
2.0 GEOLOGIC AND STRUCTURAL SETTING 10
2.1 Background Geological Information 10
2.1.1 Lithostratigraphy of the Niger Delta 10
2.1.1.1 The Akata Formation 13
2.1.1.2 The Agbada Formation 13
2.1.1.3 The Benin Formation 14
2.1.2 Depositional Belts 14
2.2 Regional Structural Setting 15
2.2.1 Structural Evolution of the Niger Delta 17
2.2.2 Structural Patterns of the Niger Delta 18
2.3 Source Rocks of the Niger Delta 22
2.4 Hydrocarbon Properties in the Niger Delta 23
3.0 BACKGROUND ON THERMAL STUDIES 25
3.1 Thermal Studies 25
3.2 Heat Transfer Mechanisms 27
3.3 Determination of Static Formation or Virgin Rock Temperatures 28
3.4 Geothermal Gradients and Heat Flow Determinations 32
3.5 Thermal Conductivity Estimation 34
3.6 Transformation of Organic Matter into Hydrocarbon 37
3.7 Time and Temperature: the kinetics of maturation 41
3.8 Thermal Maturity Modelling 41
3.7.1 Burial History Analysis 42
3.7.2 Thermal History 42
3.7.3 Heat Flow Estimation 43
3.7.4 Geochemical Parameters 43
4.0 DATA ANALYSIS 46
4.1 Basic Data Used 46
4.1.1 Collection and Analysis 46
4.1.2 Analytical Software’s 46
4.2 Temperature Data 46
4.2.1 Temperature Corrections 47
4.2.2 Temperature Scales and Conversion factors 47
4.2.3 Determination of Geothermal Gradients 49
4.2.3.1 Mean Annual Surface temperature 49
4.2.3.2 Methodology 51
4.2.4 Temperature and Geothermal Gradient Mapping 51
4.3 Sand and Shale Percentages 52
4.3.1 Method of Determination 52
4.3.2 Sand Percentage Mapping 52
4.4 Thermal Maturity Modelling 52
4.4.1 Burial History Analysis 53
4.4.1.1 Model Construction 53
4.4.1.2 Input Parameters 53
4.4.2 Thermal History 54
4.4.3 Paleobathymetry 54
4.4.4 Heat Flow 59
4.4.5 Calibration Parameters 59
4.4.6 Petroleum Geochemistry 61
4.4.6.1 Organic Matter Content and Quality 61
4.4.7 Thermal Conductivity variations in the Niger Delta 64
4.4.8 Sedimentation Rates in the Niger Delta 68
5.0 RESULTS AND INTERPRETATION 70
5.1 Geothermal Gradients 70
5.1.1 Geothermal Gradients Variation in the Shallow (Continental) section 70
5.1.2 Geothermal Gradients Variation in the Deeper (Marine / Parallic) Section 71
5.1.3 Average Geothermal Gradients Variation 71
5.2 Subsurface Temperature Variations in the Niger Delta 81
5.3 Temperature Fields 81
5.3.1 Temperature Fields at 1000m depth 82
5.3.2 Temperature Fields at 2000m depth 82
5.3.3 Temperature Fields at 3000m depth 82
5.3.4 Isothermal Maps 92
5.4 Sand Percentage Variations in the Niger Delta 96
5.5 Heat Flow Variations in the Coastal Swamp, Central Swamp and Shallow Offshore 101
5.6 Burial History and Hydrocarbon Maturation Modelling 102
5.6.1 Thermal modelling of Obigbo-1 well (Central Swamp) 104
5.6.2 Thermal modelling of Akaso – 4 well (Coastal Swamp) 108
5.6.3 Thermal modelling of Opobo South – 4 well (Coastal Swamp) 111
5.6.4 Thermal modelling of Kappa – 1 well (Shallow Offshore) 115
5.7 Maturity and Hydrocarbon Generation 118
5.71 Paleocene source rocks 118
5.72 Eocene source rocks 121
5.73 Oligocene source rocks 123
5.74 Miocene source rocks 125
6.0 DISCUSSION OF RESULTS AND CONCLUSION 127
6.1 Geothermal Gradients and Subsurface temperature variations in the Niger Delta 127
6.2 Factors affecting Geothermal Anomalies and Heat Flow variations in the Eas
tern Niger Delta 128
6.3 Burial History and Hydrocarbon Maturation Modelling 134
6.3.1 Source rocks 134
6.3.1.1 Paleocene Source rocks 134
6.3.1.2 Eocene Source rocks 136
6.3.1.3 Oligocene Source rocks 136
6.3.1.4 Miocene Source rocks 137
6.4 Implications of Results 137
6.5 Summary, Conclusion and Recommendations 138
REFERENCES 140
INTRODUCTION
A good knowledge of the geothermal gradients, subsurface temperature distribution and heat flow regime is invaluable in understanding the thermal maturation patterns of sediments as well as in unravelling the past thermal regimes in an area.
The maturation of disseminated sedimentary organic matter into petroleum and its conversion to oil and natural gas is usually controlled by the temperature history of the sedimentary basin.
It is well established that the thermal history of a sedimentary basin is closely related to the mechanisms of basin formation (Sleep, 1971; McKenzie, 1978) and the development of suitable environments for the maturation of hydrocarbons.
The implication for petroleum exploration is that the present day temperature field contributes to the probability of occurrence of economic hydrocarbon reserves.
It is therefore appropriate to assess carefully the present thermal regime in the Niger Delta basin, where hydrocarbon exploration has been going on since the late 1950s.
The database from which geothermal gradients, subsurface temperature and heat flow variations were estimated came from about seventy wells in the Niger Delta.
REFERENCES
Akpabio, I.O., Ejedawe J.E., Ebeniro J.O. and. Uko, E.D, 2003. Geothermal gradients in the Niger Delta basin from continuous temperature logs. Global Journal of Pure and Applied Sciences. v. 9 (2), p. 265 – 272.
Allen, P.A., and Allen, J.R. 1990. Basin Analysis – Principles and Applications. Blackwell Scientific Publications, London, 449 p.
Anderson, R.N., Cathles L.M. III, and Nelson H.R. Jr., 1991, “Data Cube” depicting fluid flow history in Gulf area Coast sediments: Oil & Gas Journal, v. 89, November 4, 1991, p. 60 – 65.
Andrews-Speed, C.P., Oxburgh, E.R. and Cooper, B.A. 1984. Temperatures and depth- dependent heat flow in the western North Sea. American Association of Petroleum Geologists Bulletin,v. 68, p. 1764 – 1781.
Avbovbo, A.A., 1978, Geothermal gradients in the southern Nigerian basin: Bulletin Canadian Petroleum Geology., v. 26, 2, p. 268 – 274.
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