The quality of the environment is adversely affected by the activities of the petroleum industry. During exploration, drilling, transportation, processing, and storage, the petroleum industry discharges gaseous, liquid and solid wastes and products into the environment threatening the health of the population and damaging the quality of the environment by rendering farmlands and water bodies unusable. In particular, oil spills are especially damaging to lands and the Marine environment. Environmental resources at risk are those of direct economic value like fisheries, and those of intrinsic value to human society, like aesthetically pleasing environment.
Once discharged, petroleum spills undergo several processes. When crude oil or refined petroleum products are released into an aquatic environment, immediate changes occur in their physio-chemical properties as a result of several weathering processes. These processes are evaporation, spreading, advection, dispersion, dissolution, emulsification, adsorption onto suspended particulate material, sedimentation (agglomeration and sinking), oxidation and biodegradation. To a certain degree, the relative rates of these processes are affected by the position of oil release and the relative magnitudes and importance of each weathering process are also dependent on the composition. A knowledge of these processes and their interactions affect temporal changes of the physio-chemical properties of the spilled oil. Simulation models of these processes can lead to the prediction of the movement of the oil slick with the postulation of pollution control strategies for clean-up purposes enhancing the control of its movement away from valuable resources. This, of course, is an addition to the use of other strategy for accelerating the natural control processes through the use of dispersants or limiting spreading by containment. The processes of spreading, dispersion, and dissolution are most important during the early stages of a spill, whilst oxidation, sedimentation, and biodegradation are long term processes which determine the ultimate fate of the oil.
It is easy to identify the importance of source identification of an oil spill. The question to be answered is: if we notice a large oil spill in any of our water ways, how do we determine who is responsible? We need to know who is responsible for many reasons, the most obvious being the liability for clean up and compensation. Once the issue of culpability is resolved, the complementary issue of the best strategy for oil pollution control is best tackled by designing predictive mathematical models for the understanding of the different processes the oil slick undergoes. This issue is made more complex by the interaction of two or more of these weathering processes in a general scheme.
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For full understanding of the twin problem of oil spill characterization and identification, it is necessary to discuss issues connected with the chemical composition and the properties and how these inform the process of spill identification and prediction of oil slick movement.
Chemistry of Petroleum Hydrocarbons: Crude oils contain thousands of different chemical compounds owing to process during petroleum formation from the biosynthesis of organic earth matters of sediments over millions of years of geological time. Hydrocarbons are the most abundant compounds found in crude oils, accounting for 50-98% of the total composition, majority of crude oils contain the higher relative amounts of hydrocarbons. While carbon and hydrogen are the main elements in petroleum, sulphur, nitrogen and oxygen are minor constituents and functional groups. Compounds containing N, S, O as constituents are often collectively referred to as NSO compounds. Crude oils contain widely varying concentrations of trace metals such as V, Ni, Fe, Al, Na, Ca, Cu, and U.
The Alkanes: The alkanes, or aliphatic hydrocarbons, consist of the fully saturated normal alkanes also known as paraffins and branched alknanes of the general formula with n ranging from 1-40, although compounds with 60 carbons have been reported. The most important group of branched compounds is the isoprenoid hydrocarbons consisting of isoprene building blocks. Pristine and Phytane are the most abundant isoprenoids, and while the isoprenoids are often the major petroleum constituents.
The Cycloalkanes: These are the compounds with saturated ring structures. They are also called cycloparaffins or naphthenes. They consist of important major constituents that like the isoprenoids, have specific animal or plant precursors such as steranes, diterpanes, triterpanes and that serve as important molecular makers in oil spill and geochemical studies.
The Aromatic Compounds: Aromatic hydrocarbons although usually less abundant than saturated hydrocarbons, contain one or more aromatic (benzene) rings connected as fused rings or lined rings.
Petroleum contains many homologous series of aromatic hydrocarbons consisting of unsaturated or parent aromatic structures such as phenanthrene and like structures with alkyl side chains that replace hydrogen atoms. Alkyl substitution is more prevalent in the 1-,2-, and 3- rings aromatics, although the higher polynuclear aromatic compounds do contain alkylated side groups. The polycyclic aromatics with more than 3 rings consist mainly of Pyrene, chrysene, benzanthracene, benzopyrene, benzofluorene, benzofluo-ranthene and perylene structures. The naphtheno aromatic compounds consist of mixed structures of aromatic and saturated cyclic rings. This series increases in importance in the higher boiling fractions along with the saturated naphthenic series. The naphtheno-aromatics appear related to resins, kerogen and sterols. Petroleum generation usually involves the formation of some naphtheno-aromatic structures.
Non-Hydrogen Constituents: The non -hydrogen petroleum constituent, can be grouped into six classes which are sulphur compounds, nitrogen compounds, porphyrins, oxygen compounds, asphaltenes and trace metals. Sulphur compounds comprise the most important group of non-hydrogen constituents. Most sulphur present is organically bound such as heterocyclies. The organo sulphur compounds of thiols, disulphides, sulphides, cyclic sulphides and thiophenes. The benzothiophenes and dibenzophenes apparently have the highest molecular weight of sulphur heterocyclics.
Nitrogen is presenting in all crude oils in compounds such as pyridines, quinolines,benzoquinolines, acridines, pyroles, insoles, carbazoles and benzocarbazoles. The porphyrins are nitrogen containing compounds derived from chlorophyll consisting of four linked pyrrole rings. Porphyrins occur as organometallic complexes of vanadium and nickel.
Oxygen compounds in crude oils are found primarily in distillation fractions above 400 degrees centigrade and consist of phenols, carboxylic acids, ketones, esters, lactones, and ethers.
Petroleum contains significant fraction of materials of higher molecular weight consisting of both hydrocarbon and NSO compounds called asphaltenes. These compounds, consisting of 10-20 fused rings with aliphatic and naphthenic side chains, contribute significantly to the properties of petroleum in geochemical formations and in spill situations in relation to emulsification behaviour.
Vanadium and nickel are the most abundant metallic constituents of petroleum, sometimes reaching thousands of parts per million. For instance, crudes from Nigeria, Libya, and USA contain variable amounts less than 1 ppm up to 1200 ppm vanadium and 150 ppm nickel in Venezuelan crude. While they are primarily present in Porphyrin complexes and other organic compounds. The stable isotope ratio of Nitrogen, carbon and sulphur in fractions and in total lipid fractions of sediments and organisms are being used in the area of oil identification. In the low sulphur crude oils like those found in Nigeria and Libya, the nickel is frequently more prevalent than vanadium while in higher sulphur crudes like those found in the Middle East, Vanadium is frequently more prevalent than nickel.
Types and Sources of hydrocarbons: Hydrocarbons are ubiquitous to Marine environment. These compounds may originate from:
1) Biogenic sources- Marine and terrestrial. 2) Petrogenic sources. A) anthropogenic petroleum inputs from a variety of sources including municipal discharges, storm water runoff, tanker washings, accidents and offshore activities (produced water, chronic spillages, drilling cuttings discharges, blow outs). B) Natural petroleum sources that is petroleum seeps: 3) pyrogenic (incomplete combustion) sources-from anthropogenic combustion of oil, coal, wood, peat and from natural fire: 4) diagenetic sources-the production or alteration of hydrocarbons in sediments mediated by time, temperature, and microbial activity. These sources vary substantially in their hydrocarbon composition.
The chemical and physical properties of crude oils and refined petroleum hydrocarbon products are significant as they relate to the fate of the spilled oil in the Marine environment. Crude oils and refined products are often characterized by a wide variety of parameters which include specific gravity, viscosity, pour point, boiling point range, paraffin, olefin, naphtha, and PONA content and percent non-distillable residuum.
PROPERTIES OF PETROLEUM
Specific Gravity :
The specific gravity organ oil is it’s density in relation to pure water. Most oils are lighter than water and have specific gravity below 1. The density of crude oils and petroleum products is usually expressed in terms of API gravity.
In addition to determining whether or not the oil will float, its density can also give a general indication of other properties of the oil. For instance, oils with a low specific gravity tend to be rich in volatile components and are highly fluid.
The distillation characteristics of an oil describe its volatility. As the temperature of an oil is raised, different components reach their boiling point in turn and are distilled. The distillation characteristics are expressed as the proportions of the parent oil which distill within given temperature ranges.
Viscosity: The viscosity of an oil is its resistance to flow. High viscosity oils flow with difficulty whilst those with low viscosities are highly fluid. Viscosities decrease at higher temperatures and so, sea water temperature and the extent to which the oil absorb heat from the sun are important considerations. This property of oil is important as it controls the rate of spreading in the gravity-viscous regime.
Pour point: The pour point of an oil is the temperature below which an oil will not flow. If the ambient temperature is below the pour point, the oil will essentially behave as a solid. In terms of the distribution of oil, once it is spilled into the Marine environment, the pour point is an extremely important parameter as the degraded oil becomes more viscous because of loss of the lighter components.
The compositional characteristics of a variety of crude oils and refined products are important especially as they relate to oil spill scenarios. Typical analyses of three different crude oils present physical characteristics and chemical properties that permit a comparison of such factors as their specific gravity, trace element composition and their relative proportions of aliphatic, naphthenic and aromatic compounds.
These compositional characteristics and physical properties are important when oil is released into the environment because the presence of nitrogen, sulphur, and oxygen compounds generally increases the water solubility of the petroleum compounds and along with the viscosity, can be significant in the formation of both oil in water and water in oil emulsions. The presence of NSO compounds is also important in that some of their degradation products are more toxic than the parent materials. The Inputs of Petroleum into the Sea: Petroleum oils enter the Marine environment from various sources. The best estimate of the total input of petroleum into the marine environment from all sources is about 3.2 million metric tonnes per year. Accidental spills from ships with offshore exploration and production activities account for about 0.47 million metric tonnes per year, a relatively small fraction considering the world’s current production of 3 billion metric tonnes per year. Half of this production figure is transported by sea. Sources of petroleum spills include natural sources, offshore oil production, marine transportation (operational discharges, drydocking, marine terminals, bunker operations, bilge and fuel oil transfer and accidental discharges, the atmosphere, coastal, municipal and industrial waste and run off and ocean dumping.
Natural Sources: The direct input of petroleum from natural sources into the marine environment occur naturally by means of two main processes that is submarine seepage and erosion of sedimentary rocks.
Offshore Production: Petroleum entering the Marine environment from offshore petroleum production result from operational (produced water) discharges, minor and major spills.
Marine transportation : Marine transportation activities include operational discharges, of ballast and tank washing water, drydocking, spillages resulting from human errors during Marine terminal, bilge, and fuel oil discharges. Of the 1.47 million metric tonnes of petroleum entering the sea each year as a result of marine transportation, 0.7 million metric tonnes are due to cargo residues remaining on board after discharge.
Accidental Spillages: Accidental spillages could be grouped into two categories to wit: tanker and non tanker accidents. Accidental spills from tankers contribute 400,000 metric tonnes annually. Analysis of tanker spills occurring throughout the world shows that about 75% occur in port during routine ship operations such as loading, discharging, and bunkering. In comparison, accidents such as collisions and groundings give rise to less than 10% of all spills from tankers, but a quarter of these are larger than 700 metric tonnes. Major pollution incidents such as blow outs are rare but contribute roughly 75% of the 50,000 metric tonnes lost annually from offshore platforms.
Pipeline Failures : Petroleum enters the seas through spillages resulting from pipeline or hose failures, submarine pipeline rupture, or storage tank ruptures either at a terminal or from flow lines piping from well heads.
Other sources: Other sources of petroleum input into the marine environment include atmospheric, coastal, municipal and industrial wastes and run off. All these fluxes introduce petroleum into the seas.
Classification of Crude Oils: This classification of crude oils will be made based on the chemical composition and physical properties. Many classifications have been proposed by both geochemists and petroleum refiners. The petroleum refiners are mostly interested in the amount of successive distillation fractions for instance viscosity, pour point, etc. Geologists and geochemists are more interested in identifying and characterizing the crude oils and its relationship to source rocks and their grade of evolution. Therefore, they rely on the chemical and structural information of crude oil constituents, especially on molecules which convey genetic information. In such cases, molecules at relatively low concentrations such as high molecular weight n-alkanes, steroids and terpenes are of great interest.
Hence, this classification of crude oils is based primarily on the content of the various structural types of hydrocarbons: alkanes, cycloalkanes (naphthenea), and aromatics plus NSO compounds (resins and asphaltenes). It also takes into account the sulphur content. The peculiar distribution of crude oils and the geochemical consequences derived from it are the basis for the classification. For instance, the concentration of saturates in crude oil is negatively correlated with aromatics, resins, asphaltenes, and sulphur content.
The first class of this classification is called paraffinic or naphthenic if the total content of saturated hydrocarbons is over 50%. The other major class is the aromatic where the total content of saturated hydrocarbons is less than 50%. This implies that the total content in aromatics, resins and asphaltenes is more than 50%. These two major classes are then delineated into subclass so that there are six classes of crude oils.
1. The paraffinic class comprises of light crude oils, some being fluid, and some high wax, high pour point crude oils. The viscosity of these high pour point oils is high at room temperature due to a high content of n-alkanes. The viscosity becomes normal at slightly higher temperatures. Some of these crude oils can be found from oils in North Africa, United States and South America, West Africa, Libya, and Indonesia. Some high wax crude oils occur in Indonesia, West Africa and Utah in the United States.
Specific gravity is usually below 0.85. The amount of resins plus asphaltenes is below 10%. Viscosity is generally low except when n-alkanes of high molecular weight are abundant. Aromatic content consists of mono and diaromatics, frequently including monoaromatic steroids. Benzothiophenes are very scarce, sulphur content is low to very low.
2. The class of paraffinic-naphthenic oils has a moderate resins-asphaltenes content which is usually 5-15 % and a low sulphur content which is usually 0-1%. Aromatics amount to 25-40% of the hydrocarbons. Benzo and dibenzothiophenes are moderately abundant. Density and viscosity are usually higher than in the paraffinic class but remain moderate.
3. The class of naphthenic oils consist of few crude oils. This class includes mainly degraded oils which usually contain less than 20% n-plus isoalkanes. They originate from biochemical alteration of paraffinic or paraffinic-naphthenic oils and they are usually low in sulphur. Examples are found in Gulf Coast area in the North Sea and in the USSR.
4. The aromatic intermediate class is made up of heavy crude oils with resins and asphaltene content between 10-30% and sulphur content higher than 1%. Aromatics amount to between 40 and 70% and the content of monoaromatic, especially those of the steroid type, is relatively low. Thiophene derivatives that is benzo and dibenzothiophene are abundant with about 25-30% of the aromatics. Specific gravity is usually more than 0.85. This class includes crude oils from the Middle East, from West Texas in the United States, from Alberta in Canada and from Venezuela, California and the Mediterranean.
5. The classes of aromatic-naphthenic and aromatic-asphaltic oils are mostly represented by altered crude oils. During biodegradation, alkanes are the first to be consumed. More severe degradation may involve removal of monocycloalkanes by oxidation. Therefore, most aromatic-naphthenic and aromatic-asphaltic oils are heavy viscous oils resulting from the degradation of paraffinic, paraffinic-naphthenic or aromatic-intermediate oils. The resin and asphaltic content is higher than 25% and may reach 60%. The aromatic-asphaltic oils includes a few true aromatic oils, apparently degraded from Venezuela and West Africa. It includes heavy, viscous or even solid oils, resulting from alteration of aromatic-intermediate (particularly high sulphur) crude oils. The result is usually am asphaltic oil, whose sulphur content is higher than 1% and may reach up to 9% in extreme cases. Most degraded tar sands from Western Cananda fall in this class.
The classification in the above discussion is very helpful in presenting a general classification based on the chemical composition of the crude oil. However, it is not very useful in providing discrimination of crude oils from contiguous fields with similar chemical constituents. Therefore, it is necessary to look for an alternate classification scheme that has as basis the genetic information of the crude oil. Such a classification is provided by geochemical fossils which are biological markers that can convey information about the two types of organisms contributing to the organic matter incorporated in sediments. Because they are generic, they can be used for characterization, correlation, and/or reconstitution of the depositional environment, in the same manner as macro or micro fossils are commonly used by geologists. It is important to emphasize that other molecules may also be used for correlation, provided they are characteristic enough. For example, the molecules found in ancient sediments may range from unchanged biogenic molecules like n-alkanes, and less frequently, acids or alcohols, through compounds very close to the original molecule such as steranes, through compounds very close to the original molecule, such as steranes and triterpanes, to molecules keeping only the cyclic skeleton, like aromatic steroids, Geochemical fossils frequently convey genetic information and they are used for correlations ( oil-oil and oil-source rock) for reconstitution of depositional environments and also as indicators for diagenesis and catagenesis. Identification of the major sources of organic material and of the importance of reworking by microbes, can be achieved by using fossil hydrocarbons. More specific geochemical fossils such as alcohols, acids, etc.may provide a more refined interpretation in terms of plant or animal groups, climate, etc.