ABSTRACT
Reservoir quality assessment and characterization of sandstone units was carried out across the Afikpo area of the Southern Benue Trough. The study involved field investigations and laboratory studies/analyses. Field samples were subjected to Grain Size Analysis (GSA), Sand Equivalent test and Methylene Blue test. Studies revealed that the sandstones are friable, cross-bedded and show coarsening upward motif. Results from lithofacies analysis indicate seven (7) lithofacies deposited in a low to high energy environment. They are (1) Dark Gray Shale facies (Micaceous Dark Gray Shale Facies and Fossiliferous Dark Gray Shale Facies), (2) Bioturbated Sandstone Facies, (3) Wave Rippled Sandstone Facies, (4) Cross Stratified Sandstone Facies, (5) Horizontal/Laminated Sandstone Facies (6) Heterolithic Sandstone Facies, and (7) Pebbly/Conglomeritic Sandstone Facies. The Micaceous Dark Gray Shale Facies belongs to the Eze-Aku Group while other liithofacies are of the Nkporo Group. Granulometric analysis of sand samples indicates sediments that are poorly to well sorted, generally positively to very positively skewed and mesokurtic to leptokurtic. Results from bivariate plots indicates river sand deposition while multivariate plots indicate the sediments are fluvial and deposited in a shallow marine environment. Out of 18 locations sampled, 13 locations had Sand Equivalent values ranging between 90% – 99%, 4 locations had values between 80% – 89% and one location had 77%. These results indicate that a high number of the samples contained little clay because the higher the Sand Equivalent values, the cleaner the sands. The Methylene BlueF test revealed that the clays contained in the samples are not susceptible to moisture (do not swell) as they had Methylene BlueF values ranges of 1.7g/kg and 3.3g/kg. These values do not exceed the Methylene BlueF limit which is 10g/kg. Permeability results range from 35.07mD to 4112.56mD indicating moderate to excellent reservoir while porosity values indicate poor to good with value range of 7.0 – 15.7%. Regression analyses showed a good correlation between porosity and permeability with R2 (coefficient of correlation) being 74.6%. There was a weak correlation between Sand Equivalent and Methylene BlueF Tests with R2 being 39.5%. Thus, combining results derived from the various analyses, sandstones within the study area possess good reservoir qualities especially to house gas hydrocarbon.
TABLE OF CONTENTS
Title Page I
Approval Page II
Dedication III
Acknowledgement IV
Abstract V
Table of Contents VI
List of Figures X
List of Tables XIII
CHAPTER ONE: INTRODUCTION 1
1.1 Background Information 1
1.2 Location and Accessibility of the Study Area 3
1.3 Study Objectives 3
1.4 Scope and Methodology 5
1.4.1 Preliminary Study 5
1.4.2 Detailed Mapping 6
1.4.3 Laboratory Analyses 6
1.5 Literature Review 7
CHAPTER TWO: GEOLOGICAL SETTING 9
2.1 Regional Geographic Setting 9
2.1.1 Topography 9
2.1.2 Drainage 9
2.1.3 Climate 13
2.1.4 Vegetation 18
2.2 Regional Stratigraphic Setting 21
2.2.1 Tectonic Setting 21
2.2.2 Stratigraphic Setting 24
CHAPTER THREE: FACIES DESCRIPTION 29
3.1 Rock Facies 29
3.2 Dark Gray Shale Facies 33
3.2.1 Micaceous Dark Gray Shale Facies 33
3.2.1.1 Interpretation 35
3.2.2 Fossiliferous Dark Gray Shale Facies 35
3.2.2.1 Interpretation 35
3.3 Bioturbated Sandstone Facies 36
3.3.1 Interpretation 36
3.4 Wave Rippled Sandstone Facies 36
3.4.1 Interpretation 39
3.5 Trough Cross Stratified Sandstone Facies 39
3.5.1 Interpretation 39
3.6 Herring-bone Cross Stratified Sandstone Facies 39
3.6.1 Interpretation 41
3.7 Tabular Cross Stratified Sandstone Facies 41
3.7.1 Interpretation 41
3.8 Horizontal/Laminated Sandstone Facies 41
3.8.1 Interpretation 41
3.9 Heterolithic Facies 41
3.9.1 Interpretation 45
3.10 Pebbly/Conglomeritic Sandstone Facies 45
3.10.1 Interpretation 45
3.11 Lithofacies Correlation 64
3.12 Lithofacies Association 66
3.13 Depositional Model 68
CHAPTER FOUR: SEDIMENTOLOGICAL ANALYSIS 69
4.1 Introduction 69
4.2 To prepare Methylene Blue 70
4.3 Laboratory Tests 70
4.3.1 Samples Tested 70
4.3.2 Sand Equivalent Test 70
4.3.2.1 Sand Equivalent Test Apparatus 70
4.3.2.2 Procedure 72
4.3.2.3 Calculations 74
4.3.2.4 Precautions 74
4.3.3 Methylene Blue Test (Clay Index Test) 75
4.3.3.1 Apparatus 76
4.3.3.2 Procedure 76
4.3.3.3 Precautions 78
4.4 Sieve Analysis 82
4.4.1 Procedure 82
CHAPTER FIVE: RESULTS AND DISCUSSION 88
5.1 Results from Sand EquivalentMB Test 88
5.2 Results from Methylene Blue Test 94
5.3 Correlation between the Sand Equivalent and Methylene Blue values 94
5.4 Results from Sieve Analysis 101
5.4.1 Univariete Plots 101
5.4.2 Bivariate Plots 101
5.4.3 Multivariate Plots 101
5.5 Estimating Permeability Based on Grain Size 107
5.5.1 Krumbein and Monk’s Equation 109
5.5.2 Granular Parameter 109
5.6 Porosity Determination 111
5.7 Regression Analysis 115
5.8 Integrating Analyses Carried Out to Assess and Characterize the Sandstone Units within the Study Area 115
CHAPTER SIX: SUMMARY AND CONCLUSION 118
REFERENCES 120
APPENDIX 124
LIST OF FIGURES
Fig. 1: Map of Nigeria Showing the Study Area (Modified from Google earth 2013) 2
Fig. 2: Accessibility Map of the Study Area 4
Fig. 3: Topographic Map and Cross-section of the Study Area 10
Fig. 4: Elevation Maps of the Study Area 11
Fig. 5: Drainage Map of the Study Area 12
Fig. 6: Eastern States of Nigeria: Climatic Regions 14
Fig. 7: Eastern States of Nigeria: Rainfall 16
Fig. 8: Eastern States of Nigeria: Relative Humidity 17
Fig. 9: Eastern states of Nigeria: Mean Annual Temperature 19
Fig. 10: Eastern States of Nigeria: Vegetation Types 20
Fig. 11: Tectonic Map of Southeastern Nigeria from Albian to Lower Santonian 23
Fig. 12: Tectonic Map of Southeastern Nigeria during the Campanian to Eocene 26
Fig. 13: Geologic Map of Southern Nigeria Showing the Study Area 27
Fig. 14: Outcrop Map of the Study Area 30
Fig. 15: Dark Gray Shale Facies 34
Fig. 16: Bioturbated Sandstone Facies 37
Fig. 17: Wave Rippled Sandstone Facies 40
Fig. 18: Cross Stratified Sandstone Facies 42
Fig. 19: Horizontal/Laminated Sandstone Facies 43
Fig. 20: Heterolithic Facies 44
Fig. 21: Channels 47
Fig. 22: Lithofacies Map of the Study Area 48
Fig. 23: Patterns and symbols used on graphic sedimentary logs in this work. 49
Fig. 24: Graphic Sedimentary Log of Location UI/01 50
Fig. 25: Graphic Sedimentary Log of Location UI/02 50
Fig. 26: Graphic Sedimentary Log of Location UI/03 51
Fig. 27: Graphic Sedimentary Log of Location UI/04 51
Fig. 28: Graphic Sedimentary Log of Location UI/05 52
Fig. 29: Graphic Sedimentary Log of Location UI/06 52
Fig. 30: Graphic Sedimentary Log of Location UI/07 53
Fig. 31: Graphic Sedimentary Log of Location UI/08 53
Fig. 32: Graphic Sedimentary Log of Location UI/10 54
Fig. 33: Graphic Sedimentary Log of Location UI/11 54
Fig. 34: Graphic Sedimentary Log of Location UI/12 55
Fig. 35: Graphic Sedimentary Log of Location UI/13 55
Fig. 36: Graphic Sedimentary Log of Location UI/14 56
Fig. 37: Graphic Sedimentary Log of Location UI/15 56
Fig. 38: Graphic Sedimentary Log of Location UI/16 57
Fig. 39: Graphic Sedimentary Log of Location UI/18 57
Fig. 40: Graphic Sedimentary Log of Location UI/19 58
Fig. 41: Graphic Sedimentary Log of Location UI/20 58
Fig. 42: Graphic Sedimentary Log of Location UI/21 59
Fig. 43: Graphic Sedimentary Log of Location UI/22 59
Fig. 44: Graphic Sedimentary Log of Location UI/23 60
Fig. 45: Graphic Sedimentary Log of Location UI/24 61
Fig. 46: Graphic Sedimentary Log of Location UI/25 62
Fig. 47: Statigraphic Correlation Panel of the Study Area 65
Fig. 48: (a) Sand Equivalent Test Apparatus 71
(b) Samples for Sand Equivalent Test (120g of Sieve Samples Passing the 0/2mm Fraction). 71
Fig. 49: Sand Equivalent Test Experiment 73
Fig. 50: Sand Equivalent Test Experiment 74
Fig. 51: (a) Methylene Blue Test Apparatus 77
(b) Samples for Methylene Blue Test (30g of Sieve Samples Passing the 0/0.125mm Fraction). 77
Fig. 52: Methylene Blue Test Experiment 79
Fig. 53: Methylene Blue Test Experiment 80
Fig. 54: Spot Tests for End-point of Methylene Blue 81
Fig. 55: Cumulative Curve of Location UI/03 83
Fig. 56: Cumulative Curve of Location UI/06 83
Fig. 57: Cumulative Curve of Location UI/07 84
Fig. 58: Cumulative Curve of Location UI/08 84
Fig. 59: Cumulative Curve of Location UI/13 85
Fig. 60: Cumulative Curve of Location UI/16 85
Fig. 61: Cumulative Curve of Location UI/17 86
Fig. 62: Cumulative Curve of Location UI/18 86
Fig. 63: Cumulative Curve of Location UI/19 87
Fig. 64: Cumulative Curve of Location UI/20 87
Fig. 65: Control Scheme Diagram 89
Fig. 66: Plot of Sand Equivalent and Methylene Blue Results 99
Fig. 67: Column Chart Showing the Distribution of Mean, Sorting, Skewness and Kurtosis 105
Fig. 68: (a) Plots of Mean Diameter Against Deviation (Sorting) 106
(b) Plots of Skewness Against Standard Deviation (Sorting) 106
Fig. 69: Effect of Grain Size on Permeability and Porosity 112
Fig. 70: Linear Plots of (a) Permeability (mD) against Porosity (%) 116
(b) Sand Equivalent Test (%) against Methylene Blue Test (g/kg) 116
LIST OF TABLES
Table 1: Regional Stratigraphic Sequence of South Eastern part of Nigeria 28
Table 2: Outcrop locations and their details 31
Table 3: (a) Range of wavelength (L), height (H) and ripple index for wind, wave-formed and current ripples 38
(b)Ripple height (H), ripple length (L) and ripple index measurements 38
Table 4: Summary of the lithofacies and their characteristics within the study area 63
Table 5a: Sand Equivalent TestMB Result 90
5b: Sand Equivalent Test MB Result in Range 92
Table 6: Average values of the Sand Equivalent Test MB 93
Table 7: Methylene Blue Test result 95
Table 8: Average values of the Methylene Blue Test 98
Table 9: Average values of the Sand Equivalent Test MB and Methylene Blue TestF 100
Table 10a: Measured percentiles from the cumulative frequency curve of the different locations 102
10b: Interpretations using the measured percentiles from the cumulative frequency curve of the different locations 102
Table 11: Summary of distribution of mean, sorting, skewness and kurtosis 104
Table 12: Discriminations between beach versus shallow marine, and shallow marine versus fluvial-deltaic environments 108
Table 13a: Estimating permeability based on grain size 110
13b: Terms applied to permeability values 110
Table 14a Estimating porosity using mean and permeability values 3
14b Terms applied to porosity values 113
Table 15: Calculated reservoir parameters of selected sandstone units in the study area 114
CHAPTER ONE
INTRODUCTION
1.1 BACKGROUND INFORMATION
The study area is located in the Southern Benue Trough (Fig. 1), between latitudes 5°49′N and 5°54′N, and longitudes 7°54′E and 8°00′E. Benue Trough is an intra-continental rift basin characterized by tectonic and magmatic activities that occurred during Cretaceous times. The Benue Trough was affected by Santonian tectonic activity which deformed the “Benue Trough” and inverted the main depocenter of the Abakaliki Trough and subsequently created the Anambra and Afikpo Basins to the north-west and south-east respectively (Murat, 1972; Benkhelil and Guiraud, 1980; Benkhelil, 2001). The Santonian tectonics differentiated the sedimentary successions into pre and post Santonian packages. The post-Santonian successions are Campanian – Maastrichtian in age (Reyment, 1965); they occur both in the Anambra and Afikpo Basins respectively. In the Afikpo Basin, the Campanian – Maastrichtian succession comprises the Nkporo, Mamu, Ajali and Nsukka Formations.
Sandstones within the Afikpo area occur as ridges which consist of sands that are occasionally pebbly with few heterolithic beds as well as a lot of clays/fines. These ridges form northeast-southwest-trending topographic prominences while the shales underlie the swales i.e. the depressed areas. The sandstone ridges are dry and often barren of vegetation, while the swales are swamps. The ridges show extensive and deep weathering and laterization, such that exposures of fresh rock are available only along new roadcuts, ditches/gullies, quarries and some stream channels. The ridges are asymmetrical, with their gentler, coarser flanks facing the southeast and east, while the steeper flanks face the west.
1.2 LOCATION AND ACCESSIBILITY OF THE STUDY AREA
The study area lies within the Afikpo Basin and covers about 44km2. It is limited by latitudes 5°49′N and 5°54′N, and longitudes 7°54′E and 8°00′E. The area is bounded in the north by Ibii and the Cross River which runs north-to-south direction on the eastern border and is the main drainage system in the area, south by Unwana and west by Edda and Amasiri villages. The locations studied are Ndibe, Edobi village, Mac Gregor, Mgbom, Ugwuagu, Ozizza, Ngodo, Kpoghirikpo and Akpughuru (Fig. 2).
Access to the study area is through a network of roads which are main, secondary and minor roads. Some of the access routes are untared which during the rainy season are essentially not motorable thereby making geological fieldworks difficult. Others are footpaths which connect most parts of the study area. Exposures in the southern part of the study area were accessed through the Afikpo-Unwana Road, those in the northeastern part were accessed through Ndibe Beach Road, those in the north through Ngodo Road and those in the northwestern part through Afikpo-Amasiri-Abakaliki Road.
1.3 AIMS AND OBJECTIVES
This research work is aimed at assessing the reservoir quality and characterizing the sandstone units in Afikpo area, Southeastern Nigeria. To achieve these aims, the following are the objectives:
To assess the reservoir quality of the sandstone bodies within the research area by studying the porosity, permeability, quantity of clay present and type of clay.
To interpret the depositional environment within the study area.
To integrate all available data such as Grain Size Analysis, Sand Equivalent Test, Methylene Blue Test, Facies analysis, porosity and permeability, quantity and type of clay present to model reservoir architecture, connectivity and flow properties.
1.4 SCOPE AND METHODOLOGY
Systematic study of the area was carried out in three phases: preliminary studies of literature and materials to get acquainted with the study area and visits to outcrops, deskwork and field studies, and laboratory analyses.
1.4.1 PRELIMINARY STUDY
This is the first step for a good geological mapping and involved collection and evaluation of all existing data on the study area. Such data include geological data from archival records, maps, photographs, reports and publications. An understanding of the history and culture of the people also helped in facilitating the work. This preparatory task involved geological interpretation of air photos to help in delineating rock units, tectonic structures and morphological features which facilitated the planning and execution of the actual field activities and also access into the mapping area. With these data at hand, a preliminary study of the area was made from the 2nd to 4th October, 2012. Since the locations (outcrops) to be studied are sites on private land, a visit was first made to H.R.H Ezeogo John O. Ekuma Izuegu 1 of Amizu Autonomous Community, Afikpo and Chief Gabriel A. Agwo the Onikara of Amizu Autonomous Community, Afikpo to seek permission to enter the lands and study outcrops. With the consent of the council, the cultural code for the area was obeyed which included no permission to access certain sites and certainly not allowed the removal of samples without special permission. Also, there were no removals of samples in some sites in other not to destroy the features exposed for other geologists who may wish to visit the site.
Outcrops were located with the help of field guide/base map of the area, information from tipper drivers and other logistic arrangements were made for detailed mapping phase.
1.4.2 DETAILED MAPPING
This phase involved deskwork and field study. Detailed mapping and logging of the sections were carried out in four phases: from 10th to 14th October, 2012, 22nd to 29th October, 2012, 13th to 17th March, 2013 and lastly 31th July to 4th August, 2013. This was done to get the characteristics of the various lithologic units, physical and sedimentary structures, and other details necessary in this research. During these periods, samples were collected from locations and detailed examination of the lithostratigraphic profiles of the exposures. Series of equipment were used which include:
Relevant topographical maps.
Handheld Global Positioning System (GPS) model GPSmap 76CS x, which uses ultra high-frequency radio wave signals from satellite to trigonometrically derive positions to within a few metres laterally.
The compass-clinometer was used to measure:
o The orientation of geological planes and lineations with respect to north.
o The angle of dip of geological features with respect to the horizontal.
o It was also used together with topographic map to accurately determine outcrops.
Field notebook.
Field tape for measuring vertical and horizontal extents of outcrops.
Geological hammer
Masking tape.
Sample bags.
Digital camera for photographing of outcrops and important sedimentary structures.
1.4.3 LABORATORY ANALYSES
This involved processing the samples collected in other to extract necessary information from
them. Sand Equivalent test was used in determining the cleanliness of the sandstones from clay, Methylene Blue test was used to determine the swelling of the clay while granulometric analysis was useful in the analysis and interpretation of textural data. Results from these analyses were presented in form of tables, graphs, photographs, logs and figures.
1.5 LITERATURE REVIEW
The area of study is within some part of the Southern Benue Trough, Southeastern Nigeria. The Benue Trough of Nigeria, located in the West African Continental Margin is about 80-150km wide long, and extends in a NE-SW direction from the Niger Delta in the Gulf of Guinea to the Chad Basin in the interior of the West African Precambrian Shield. The rocks of the Southern Nigeria sedimentary basin are mostly Palaeogene rocks. The rocks comprise the Niger Delta, Benin Embayment (ex-Dahomey), Anambra Basin, Abakaliki Fold-Belt, Afikpo Syncline and the Calabar Flank.
Reyment (1965) undertook the first detailed study of the stratigraphy of the southern Nigerian sedimentary basin and he proposed many of the lithostratigraphic units in the region. He recorded some paleontological evidence that showed that the Turonian was well developed and enriched by lots of ammonite and other fauna.
Murat (1972) and Burke (1996) observed that the Southern Nigeria sedimentary basin followed the breakup of the South American and African continents in the Early Cretaceous. Various lines of geomorphologic, structural, stratigraphic and palaeontological evidence have been presented to support a rift model (King, 1950; Bullard et al., 1965; Reyment, 1969; Burke et al., 1971; Fairhead and Green, 1989; Benkhelil, 1989; Guiraud and Bellion, 1995).
Hoque (1977) described the Eze-Aku Formation as being dominantly texturally and compositionally immature and attributed this to the proximity of provenance to the deposition of the basin. He also described the second cycle of sandstone as texturally immature and compositionally mature and thus the Afikpo Sandstone as quartz arenite and Eze-Aku Formation as feldspathic arkose.
Nwajide (1979) and Arua (1986) suggested environments that ranged from nearshore (barrier ridge-lagoonal complex) to intertidal and subtidal zones of the shelf environments.
Petters (1980) believe that the Eze-Aku Formation and Awgu Formation are equivalent because they are indistinguishable in the field both in lithology and faunal assemblages.
Zaborski (1983) gave detailed description of the ammonite fauna from southern Nigeria. He assigned Campano-Maastricthian age to the Nkporo Shale and Eze-Aku as being Early Turonian in age.
Fayose and Ola (1990) suggested that the sediments were deposited in marine waters between the depths of 10m and 1000m.
CHAPTER TWO
GEOLOGICAL SETTING
2.1 REGIONAL GEOGRAPHIC SETTINGS
2.1.1 TOPOGRAPHY
Topography is the layout of natural and artificial features or cultural features on the surface of the earth, and the science of their detailed, graphic representations on maps and charts. From the topographic section of the study area and elevation map (Fig. 3 and 4), it is observed that the landscape is essentially irregular in nature and divided into three broad sections of plateau, undulating plains and valleys. This landform steadily rises from about 15.24m in the valleys to as high as above 152.4m in the Ozizza-Ngodo areas. The highest elevation was recorded in the Ozizza sandstone ridges with height of 177m. The sandstone ridges are rugged, occur in groups and extend for several kilometres and terminate at the banks of the Cross River. Behind the ridges are depressions of swampy almost flat surfaces which extend for several kilometres and used for rice plantation and other cash crops. Many of the depressions become flooded during the rains and as such destroy crops, but consist of heaps of sand in the dry season. The inaccessibility of some parts of the study area is attributed to the natural/physical features.
2.1.2 DRAINAGE
Structures and lithological differences have had significant influence on pattern and orientation of the drainage network within the study area. The drainage map (Fig. 5) exhibits the dendritic drainage pattern. This pattern resembles the shape of a tree, with the smallest tributaries being the outermost twigs and the main river channel forming the trunk. The area is dominated by the well navigable Cross River drainage system. Other rivers within the study area are Iyioka and Ubei Rivers. The Iyioka river is located in the southern part of the study area and marks a boundary between Afikpo town and Kpoghirikpo villages. The river has its head in the Afikpo
Sandstone Ridge. Two lakes, Ehoma and Iyieke lakes were located in the south eastern region along the Cross River. Streams in the study area are Orra, Ndibe, Okwukwo, Uji and Amoncha all flowing in the south-east direction The Cross River which enters into the Atlantic Ocean forms a loop in the south-eastern edge of the study area.
The study area is made up of dry valleys, poor ground water resources, impervious rocks and numerous dry channels which contain water during the rains and dry up in the dry season. Owing to the impervious nature of the ground, streams are numerous, incised and flow in a general south-easterly direction into the Cross River. While the undulating plains boast of intermittent and ephemeral streams, there are perennial streams in the plateau region. In the plateau region, the valleys of the streams drain southwards into the Cross River. The upper courses of the streams originate from the head of the plateau gullies which flow over impervious shales and sandstones. The steep slopes in the region promote flash flooding and rapid run-offs into the streams. The area experiences scarcity of water during the dry season.
2.1.3 CLIMATE
The study area falls within the fourth region in fig. 6. Climatic condition in this area is tropical and characterized by three air masses. The period from April to September is controlled by the warm wet tropical moisture air mass or south-westerly wind and forms the rainy season. The rains come mainly during this period and are marked as the planting season. This rain is usually of high intensity at the beginning of this season and comes with lightening, thunderstorms and hailstorms. These bring sudden and torrential down pours and run-offs accompanied by sheet erosion on the slopes. This south-westerly wind causes a lot of damage mostly in the plateau region, but ease off in the undulating plains. The wettest months are July and September. There is a break in August with double maxima of rainfall regime. Warm tropical air mass or north-east trade winds attract the harmattan with low relative humidity from December to February.
During this period, the climate is cool and dusty; the mornings are always cool and misty while the afternoons are hazy. The period from February to March is hot (Ofomata, 1975).
The study area has four months in which precipitation is less than 60mm with the driest months having less than 28.75mm. The annual total, however, ranges from 1600mm to more than 2000mm with variations in rainfall regime (Fig. 7).
Relative humidity is high during the rainy season when humid maritime airmass is predominant and low during the dry season when dry continental airmass is predominant. Generally, the tropical continental airmass has a very low relative humidity. It affects the northern parts of the Eastern States in the dry season. From December to February when the study area is under the influence of this airmass, it has an average relative humidity of between 60% to 70% at 10.00hours before it begins to rise again (Monanu, 1975) (Fig. 8).
Temperature is an important aspect of climate and is similar throughout the study area. Due to its latitudinal location, the study area receives abundant and constant isolation. Because of this, atmospheric temperatures are continually high and only change slightly within the year. The mean daily maximum temperature is usually above 27°C all through the year. It is highest from February to April but does not usually exceed 35°C. In the mornings it is usually cold especially in the months of January and December. It is hottest, one or two hours in the afternoon. It is warm late in the afternoon and cold again at night. The nights are so cold that inversions of temperatures within some localities are common. Two periods of high temperature closely associated with the passage of the overhead sun are recorded within the year. These two periods are separated by two periods of low temperature. The hottest months of the year are February, March and April and they coincide roughly with the passage of the overhead sun. The second period of high temperature, also caused by the passage of the overhead sun, is unduly delayed by the heavy rainfall of September and October and so becomes prominent only in November. This period does not last long and as soon as the cold harmattan wind becomes dominant in December and January, it causes temperature to drop again. The coldest month is usually August. It is the middle of the rainy season and so the south west monsoon winds and the heavy rains of the preceding months cause lower atmospheric temperature. Also, in spite of the “little dry season” in August, there is still a high degree of cloudiness which deflects incoming solar radiation. The days are usually cloudy at this period although the nights may be clear (Ofomata, 1975). (Fig. 9).
2.1.4 VEGETATION
This section indicates the type of soil cover and the nature of the underlying geology, especially where there is natural alignment of the vegetation. The vegetation of the study area falls within the Lowland Rain Forest (Fig. 10) which extends almost fully across the Eastern States of Nigeria in a broad band 130 to 200km wide. Although most of this region has been reduced by human activity to a secondary plant cover-so much so that large parts of the rain forest zone may be termed an “oil palm bush”, from the iniquitousness of oil palms Elaeis quineensis-in protected reserves and in some uncultivated patches between crop farms. The forest is characterized by an abundance of plant species, sometimes exceeding 150 different species per hectare. With regard to the vertical arrangement of plant structures, a storeyed sequence of canopies was observed in most sections of the forest. On the forest floor are such herbaceous genera as Geophila and Costus. Plants of intermediate height (10-25m high) are represented by trees like Musanga Smithii and Albizzia Zygia, while at the highest levels (50-65m) occur the crowns of veritable forest giants: Khaya ivorensis, Chloropgora, Ceiba pentandra, and so forth. Complementing the forest physiognomy is a luxuriant growth of viniferous climbers, e.g. Ficus spp., and epiphytic accretions, e.g. Platycerium and Nephrolepsis spp. Along the stream borders are galaxy of forest, mainly bamboo trees while thick forests occur sparsely in the settled areas. The palm bush is prevalent in the virgin forests, but they are not uncommon around the ancestral shines. The dominant trees are those of the mahogany, iroko, cam wood, silk cotton, oil palm, coconut, native pea, wild mango, paw-paw and sweet orange.
In the undulating plains, the soils include acid sands and lateritic alluvial soil. The plateau region is made up of acid and lateritic soils with poorly developed and structurally unstable profiles, alluvial soils along such major rivers as the Iyioka and Ubei. About 60% of the areas of the undulating plains are suitable for the cultivation of tubers such as yams and cassava which are dominant. The low lands are suitable for the cultivation of the dominant cereal which is rice. Also, the region provides ideal conditions for such economic plants as the oil palm, cocoa, citrus, sugar cane, plantain, banana, coconut, cocoyam, pineapple and maize. Vegetables of all sorts grow well. There are extensive stretches of valleys with marshy dense growth of raffia palm trees. The sandstone ridges are marked by sparse vegetation (Ofomata, 1975).
