ABSTRACT

The oily seepage from Ugwueme in Awgu L.G.A of Enugu State was characterized by uv, ir and heavy metal contents analyses. Proportions of saturates, aromatics, resins and asphaltenes (SARA) components in the seepage were determined by solvent fractionation based on solvent polarity and solubility of the seepage in various solvents. FTIR indicated presence of aromatic C=C, aliphatic  C-O, C=O, C-N and N-H stretching vibrations. The seepage displayed the presence of a conjugated system in the UV region. AAS results showed relatively low levels (mg/kg) of trace metals notably Cu (118.2), Ni (102.6), Zn (52.7), V (2.8), Cd (86.6), Pb (25.4), Fe (103.7), Mn (62.9), and Co (38.5).The divergent ratios V/Ni, V/V+Ni and Co/Ni were calculated and found to be 0.03, 0.03 and 0.375 respectively. The dorminant major inorganic components identified are Fe₂O₃, CaO, SrO, K₂O, SiO₂, Al₂O₃ and TiO₂. Their mode of occurrence indicates calcium rich origin and major aluminosilicate detrital input evidence by Ti. XRD results implicated hematite, chrysotile, petalite, quartz and ramsdellite as the major mineral components of the asphaltene pointing to the source rock that generated the seepage oil. The comparative examination of the chemical composition of the weathered seepage asphaltene indicated terrestrial origin, deposited in an oxic environment with high biogenic carbonate and aluminosilicate detrital matter.

 

 

 

TABLE OF CONTENTS

Title page                                                                                                                                i

Certification                                                                                                         ii

Dedication                                                                                                  iii

Acknowledgements                                                                                     iv

Table of contents                                                                                        v

List of figures                                                                                             viii

List of tables                                                                                               ix

Abstract                                                                                                      x

CHAPTER ONE                                                                                       1

Introduction                                                                                                         1

1.0     History of crude oil in Nigeria                                                                     1

1.1     Oil seepages                                                                                                2

1.2     Off shore seepages                                                                                      5

1.3     Characteristics of seepages                                                               8

1.4     Weathering of seepages                                                                     8

1.5     Importance of seepages in human social and economic history                  11

1.5.1  Importance of micro and macro seepages                                         11

1.6     Origin of Ugwueme oil seepage                                                                  13

Location of study area                                                                                18

Description                                                                                                 19

1.7     Anthropogenic activities in Ugwueme                                              20

1.8     Statement of problem                                                                       20

1.9     Aim and Objectives                                                                           21

1.10   Literature gap                                                                                   22

Scope                                                                                                                   22

CHAPTER TWO

2.0 LITERATURE REVIEW                                                                            23

2.1 Origin of petroleum                                                                              25

2.1.1 Diagenesis                                                                                         26

2.1.2 Catagenesis                                                                                                 28

2.1.3. Metagenesis and metamorphism                                                      29

2.2. The oil window                                                                                    30

2.3 Crude oil Constituents                                                                          33

2.3.1. Saturates:                                                                                         33

2.3.2. Aromatics:                                                                                        34

2.3.4. Resin:                                                                                               34

2.3.5. Asphaltene:                                                                                                35

2.4. Separation                                                                                           36

2.5. Standard method for separation of asphaltene                                              37

2.5.1. Molar mass of asphaltene                                                                 39

2.5.2. Self Aggregate of Asphaltene Molecules                                           39

2.5.3. Variables affecting asphaltene precipitation                                              41

2.5.4. Molecular Structure of Asphaltene                                                   42

2.5.5. Deposition of Asphaltenes                                                               45

2.6. Sources of metals in crude oil                                                              47

2.6.1. Nickel (N_i)                                                                                        49

2.6.2. Vanadium                                                                                         50

2.6.3. Manganese (Mn)                                                                               51

2.6.4. Iron (Fe)                                                                                           51

2.6.5. Zinc (Zn)                                                                                           52

2.7. Heavy Metal                                                                                        53

2.7.1. Lead                                                                                                  54

2.7.2. Cadmium                                                                                          55

2.7.3. Chromium                                                                                        57

 

CHAPTER THREE

3.0 Experimental                                                                                        59

3.1 Sample decomposition by acid digestion                                                       61

3.2 UV-Visible Spectrophotometry                                                            61

3.3 X-ray Diffractometry                                                                            61

3.4 X-ray Florescence (XRF)                                                                      62

3.5 Fourier Transform Infrared Spectroscopy (FTIR)                                62

3.6 Seepage solubility tests and melting point determination of the

Asphaltene                                                                                           63

 

CHAPTER FOUR

Results and Discussion                                                                               64

4.0 Solubility test                                                                                       64

4.1 UV-Visible Interpretation                                                                      66

4.2 FT-IR Spectroscopy                                                                                       67

4.3 X-ray diffraction                                                                                   68

4.4 The major oxides                                                                                  71

4.5 Sulphur content                                                                                    72

4.6 Biomarker indicators                                                                                     73

4.7 Minerological analysis                                                                          75

4.8 Recommendation                                                                                  76

4.9 Contributions to knowledge                                                                           77

4.9.1 Conclusion                                                                                        78

REFERENCES                                                                                           80

APPENDIX                                                                                                                            98

 

 

CHAPTER ONE

INTRODUCTION

• History of crude oil in Nigeria

Crude oil is the single and most important commodity in the entire world today and also the largest resources for man’s demand for energy. Crude oil is the oil believed to have originated from plants and animals remains over a long period of time. It is derived from organic molecules formed by living organisms million years ago, and this substance, organic compound formed over millions of small plant and animal and so it is known as hydrocarbon. Petroleum is a naturally occurring hydrocarbon as it contains compounds mainly that are composed of carbon and hydrogen only ^{1, 2,} and do also contain any hetero-atoms (nitrogen, oxygen, and sulphur as well as compounds containing metallic constituents, particularly vanadium, nickel, iron, and copper). Oil production in Nigeria dates back to 1903 when the British mineral Survey Company began mineralogical studies of the country. In 1914, the British government passed the mineral oil ordinance. Shell BP and other developers in the pursuit for commercially available petroleum found oil in Nigeria in 1956. During the pursuit, many Nigerians thought the developers were looking for palm oil. Nigeria joined the Organization of Petroleum Exporting Countries (OPEC) in 1971 and established the Nigerian National Petroleum Company (NNPC) in 1977; a state owned and controlled by 6 companies, which is a major player in both the upstream and downstream sectors ³. But after, nearly 50 years searching for oil in the country, Shell –BP discovered the oil at Oloibiri in Niger delta. The first oil field began production in 1958. Nigeria has a total of 159 oil fields and 1481 wells in operation according to the ministry of petroleum resources ⁴. Oloibiri oil field is an onshore oil field located in Oloibiri in Ogbia L.G.A.   Bayelsa State, Nigeria. This is about 13.75 square kilometers (5.31sqm) and lies in a swamp, Oloibiri, usually named after the host community where it is located or local land mark. In 1958 Shell/D Achy discovered oil, twelve areas in the Niger delta of which Oloibiri, Afam and Bomu were the most promising. This oil discovery was also made in the tertiary Agbada formation subsequently fifteen wells were drilled around this discovery well so making Oloibiri, Nigeria’s first commercial oil field. The oil produced from the field is sour and heavy with an API gravity of 20.6 ⁵. The discovery of these commercial oil fields led to the discovery of other seepages virtually in all the states in the south east.

1.1 Oil seepages

Oil seepage is defined as a visible evidence at the earth’s surface of the present or past leakage of oil, gas or bitumen from the subsurface. Much large seepage represent tertiary migration, that is, migration from on accumulation that has been disturbed by tilling of strata, changes in depth of burial, and erosion, or development of new avenues of escape to the subsurface, such as fractures and faults ⁶. The first oil wells of Canada, Pennsylvania, Oklahoma, California and Texas were drilled near oil seepage; even the giant Masjid-i- Suliman field in Iran was the first big oil and gas seepages. Links ⁷, stated that less than 15% of the hydrocarbons generated by source rock becomes recoverable oil and gas in reservoirs, while the rest in partly disseminated in rock throughout the subsurface and partly lost at the surface. Seepages are common at the out crop of uncomformities and permeable hormoclinal beds. They occur wherever a permeable pathway leads to the surface from mature source strata or from leaking petroleum reservoirs. Link ⁷, recorded the worldwide occurrence of seepages and classifies them into five groups depending on their origin as follows:

• Seepages emerging from homoclinal beds, the ends of which are exposed where these beds reach the surface.
• Seepages associated with beds and formations in which the oil was formed.
• Seepages from large petroleum accumulations that have been barred by erosion or reservoirs that have been ruptured by faulting and folding.
• Seepages at the outcrops of unconformities
• Seepages associated with intrusions such as mud volcanoes, igneous intrusions and pier cement salt domes.

Link ⁷ did his study before the concept of plate tectonics became accepted. Since then it has been recognized that the rupturing of source rock and seepages of oil through small fractures and faults is common in earth quake-prone areas along the edges of crustal plates where the continents collide. Associated fracturing and faulting have permitted the upward escape of oil and gas from cretaceous reservoirs. Near the centre of this earth quake belt, the supergiant Burgan field of Kuwait has large heavy-oil seepage directly over its structural crest.

Worldwide, there is a correlation between seepages and earthquake activities, with majority of the seepages close to plate boundaries.

Selley ⁸ documented 173 seepages and impregnations in Great Britain. The seepages were found primarily around the margins of basins where permeable carrier beds uncomformably overlie “basement” rocks. The main sources of the petroleum seepages were believed to be Devonian oil shales and carboniferous shales and coals. Macgregor ⁹ compared the global occurrence of seepages with tectonics and subsurface petroleum reserves. In the north and south Sumatra basins he found that most of the visible seepages came from the smaller, shallower traps and those most strongly affected by diapirism or faulting. The larger, deeper fields not connected to the surface by faulting rarely showed visible seepages. Macgregor’s ⁹ global studies offer the following guidelines for evaluating seepages:

• Seepage patterns are strongly controlled by regional and local tectonics. Seepages are most common in over pressured diaper-rich basins and in active thrust belts. They are rare in tectonically inactive basins.
• Most large, deep accumulations do not seep directly to the surface.
• Intracratonic and foreland basins show a small number of seepages relative to reserves, and thrust belts show an anomalously large number of seepages.
• The presence of seepages over basins or prospects often considerably reduces the exploration risk. The absence of seepages over a tectonically active basin or shallow faulted prospect increases the risk.

Macgregor ⁹ felt that the prime value of visible seepages in frontier basins is at the regional level, in providing information on the hydrocarbon potential of a basins source system.

1.2 Off shore seepages

Bitumen washing up on beaches in tectonically active areas may come from under water seepages, tanker spills or accidental spills in coastal areas. Much active seepages are found off shore on the continental shelf, coming from formations that produce oil off shore as well as those that produce oil on nearby land area. They are irregularly distributed along an east-west trend of faulted anticlines. The asphalt becomes less dense than sea water through the loss of gas and light hydrocarbons and the accumulation of sediment material. Some of the vented asphalt escapes to the surface, but much of it contributes to the growth of the mounds, which are encrusted by marine organisms. Some of the asphalt mounds, especially those nearest point conception, originate from low API gravity crudes that are among the least mature migrant products of type 11.s kerogen. Their initial API gravity may be <10^o 10.

Recently the different styles of underwater petroleum seepages were described in three areas ¹¹.

• The Gulf of Mexico from the Florida escarpment west across the Sigsbee escarpment and south to the Campeche bank off Yucatan, Mexico.
• California from the Oregon border to los Angeles and
• Off shore Alaska

All these areas contain major oil and gas fields. The U.S National Academy of Sciences concluded from its study of petroleum in the marine environment, that approximately 1.5 million barrel of oil is being introduced into the world’s ocean annually by natural seepages ¹².

Off shore oil seepages usually are first recognized by bitumen floating on the water, but clusters of gas bubbles venting from the sea floor. Bubbles reflect the pulse and show up as a dark vertical line against the reflection-less background response of sea water. Sieck ¹³, correlated these bubble clusters with gas charged sediment cones or chimneys below the seafloor. These cones create volcano shaped mud lumps on the seafloor, along with circular sea-bottom depression (pockmarks) where they vent into the seawater. High resolution data can be obtained by combining several seismic systems such as side scan sonar, bathometer, turned transducer, and a sparker or air gun system. The world wide occurrences of pockmarks and their relationship to gas seepages were recorded by Hovland and Judd ¹¹. The recording shows a pockmark (P.M) and a mound (M), the later having an elevation of about 1m above the seafloor. Pockmarks in this area vary in sizes and depth. The floor and the inside wall of an 80m wide pockmark contain crusts of sandstone cemented with aragonite and magnesium calcite crystals. The ∂¹³C of the cement 15-59%, which indicates that the carbon in the cement is coming from methane escaping from the seafloor, was converted by bacteria to CO₂ deficient in C-13- deficient carbon crystals. Sample of methane collected in the vicinity of these pockmarks had ∂¹³C value of -39 to 45% which indicates a thermogenic origin for the gas ¹⁴.

Conclusively, jet coring, guided by suitable interpreted seismic data is an effective way to trace the origin of gas seepages and to assess the probability of a seepage trapping structure containing hydrocarbons.

 

1.3 Characteristics of seepages

For petroleum seepage to be called a sedimentary basin or a hydrocarbon province it must possess 5 key attributes:

• Contain liquid or gas hydrocarbon.
• Contain mature petroleum source rock (which is a sedimentary rock example coal and organic rich mudstone that contain sufficient organic matter that when heated will drive off liquid or gas hydrocarbon) with a permeable pathway to reservoir rock.
• Adequate reservoir rock with porosity and permeability (the reservoir unit will have sufficient connected void space typically porosity between grains which allows commercial concentrations of oil to be stored beneath a trap).
• Structure or stratigraphic traps (traps could be a sealed container defined by an appropriate geometry such that the buoyant oil/ gas is constrained
• Contain seals to confide the oil and gas which are buoyant because they are less dense than water ⁶.

1.4 Weathering of seepages

When oil leaks to the surface, it undergoes series of changes that considerably alter its physical and chemical composition. They are as follows:

• Evaporation of the more volatile hydrocarbons; in the first two weeks after an oil reaches the surface, it loses its hydrocarbon content up, through about C₁₅, then in sub-sequent months, additional hydrocarbons are lost, up through about C₂₄.
• Leaching of water soluble constituents; the most soluble nitrogen, sulphur and oxygen compounds along with some lighter aromatic hydrocarbons may be leached out by groundwater.
• Microbial degradation; hydrocarbons leaking to the surface are subjected to microbial attack.
• Polymerization; polymerization is the combining of the intermediate-to-larger molecules to form very large complex structure after the elimination of water, carbon dioxide and hydrogen.
• Auto-oxidation; many constituents of petroleum absorb sunlight and oxygen, which convert the oil to asphalt high in oxygen, with exposure to air and sunlight, seepages can take up more than 6 wt % oxygen.
• Gelation; is the formation of a rigid structure. This may begin to develop over time depending on the type of petroleum.

All these reactions lead to solidification of the original oil. The crude will generally be converted from liquid oil to an asphaltite and eventually to a substance physically close to a pyrobitumen. Consequently, unless seepage is supplied by a continuous flow of fresh oil, it ultimately hardens into bitumen (used to indicate a thermal maturity). Several studies on surface bitumens reveal that severe weathering can mimic late stage maturation without any change in maturation indicators such as biomarkers ¹⁵.

1.5 Importance of seepages in human social and economic history.

Seepages played important role in human social and economic history. The use of asphalt as a building material from seepages in the Middle East dates back to 3,000 B.C. Burning gas seepages have existed in the Baku area since several centuries before Christ. California Indians living near the seepage hand –formed cakes of asphalt to make trade goods. As a symbol of mourning, crude oil was sometime smeared on the faces and hair of widows and grieving female relatives. Some widows form more lumps of asphalt stung into necklaces. Shamans painted their faces with heavy oil before dancing, believing the oil had supernatural powers. Egyptian mummies dating from 1,295 B.C to A.D 300 have been found to contain bitumen in their embalming material. Early settlers used oil from seepages for water proofing, lubrication and lamp oil. Seepages, sometimes provide water to plants and animals, for example an insect known as the petroleum fly lives exclusively, in and around the seepage ¹⁶.

1.5.1 Importance of micro and macro seepages

The importance of observable surface seepage reveals little about the sources, type or thermal maturity within a basin, hydrocarbons of large range of components, from the simplest, lightest methane molecules to very large and complex molecular structures. The light hydrocarbons, methane, propane and butane (C₁-C₄), migrate to the subsurface in gaseous form, heavier hydrocarbons (C₅₊) migrate in the liquid phase. This difference is important with respect to seepage detection. Consequently observable macro-seepages are rare and have often followed tortuous pathways to the surface whereas the light gaseous hydrocarbons, however, are more mobile in the subsurface and require a much less open pathway to be focused at the surface. They are also more likely to be located closer to their subsurface sources. These micro seepages are much more widespread and, although generally invisible to the naked eye, but are present in small concentrations that can now be sampled in soils, measured and mapped. The relative concentrations of these light hydrocarbons (C₁-C₄) are directly related to production type and thermal maturity of hydrocarbons in the basin ¹⁷. In other words, oil productive basins will contain greater proportion of ethane, propane and butane relative to methane. While gas prone area will contain greater proportions of methane. Thus, light hydrocarbon surveys can be considered as a sources rock tool applied at the surface. Although the presence of oil seepage provides the existence of oil generating sources, it does not provide adequate information on the level of thermal maturity and the potential gas available to charge the subsurface reservoirs. This type of information is not generally present in liquid seepages because of weathering effect. The thermal maturity of the subsurface hydrocarbons that source the surface seepages can be determined by sampling the light hydrocarbons in the vicinity of observable seepages.

Seepages reduce exploration risk; they are particularly important to exploring new basins or areas. Only about one explored basin in three in the world is petroliferous enough to contain producible oil and gas, and only one in six contains even one very large oil field ¹⁸. A look at the exploration history of the important oil areas of the world proves conclusively that oil and gas seepages gave the first clues to most oil producing regions of the world. Many great oil fields are the direct result of seepage drilling ¹⁹. Nevertheless nearly all the important oil producing regions of the world were first discovered by surface seepages.

The importance of seepage tends to be minimized in this era of increased use of highly sophisticated instrumentation and decreased use of ground survey.

1.6 Origin of Ugwueme oil seepage

The hydrocarbon play concept started, when Ekweozor and Unomah discovered and reported an oil shale deposit at Lokpanta and its surrounding villages, located 80 km north of Port Harcourt, on the Port Harcourt –Enugu express road ²⁰. They further stressed that this shale was composed of highly bituminous sediments and corresponds to Ezeaku shale which consist of interbedded calcareous shale and marl. With the full support of the comprehensive programme of geological and geochemical mapping of Abakaliki fold Belt, the different locations were cored as follows:

• Lokpanta (core Hole #1)
• Acha near Uturu/Okigwe (core Hole #2)
• Onoli Awgu (core Hole #3)

According to Ekweozor, some areas appeared to be more elevated in terms of total organic carbon (TOC 3-10 wt %) with corresponding hydrogen index values (200-600mgHe/g TOC). These areas were proposed to occur at same time with the nose of Abakaliki, anticlinorum and were differentiated as two separate regions namely.

• Greater Acha Area (consisting of Uturu, Acha, Ndiobasi and Ndiokoroukwu).
• Greater Lokpanta Area (including Lokpanta, Lokpaukwu, Lekwesi and Onoli Awgu)

Ekweozor proposed that the sediments be recognized as a separate member of Ezeaku group, because they were quite distinct from the main stream Ezeaku shale²¹.

Another impressive view was seen about 300 metres upstream from the bridge over the river at Onoli Awgu on Awgu/Ishiagu road. It comprises of over 30 units across 1000 centimeter of section of rhythmically interbedded marl and calcareous mud stone, containing numerous nodules of various sizes especially within the shale strata.

 

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