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Types of Petroleum Oils

Characterization of crude oils and refined petroleum products in a release situation is one of the earliest response tasks that must be undertaken. Proper classification and an understanding of the chemical and physical properties of these substances helps determine the hazard to personnel and wildlife, the effects that may be observed on adjacent shorelines or estuaries (for spills into water), and the form a response should take. Non-petroleum-based oils also pose a potential threat to human health and the environment. This discussion focuses only on crude oils and refined products; non-petroleum oils are discussed elsewhere.

Chemical Composition

Crude oils and refined petroleum products consist largely of hydrocarbons, which are chemicals composed solely of hydrogen and carbon in various molecular arrangements. Crude oils contain hundreds of different hydrocarbons and other organic and inorganic substances including atoms of sulfur, nitrogen, and oxygen, as well as metals such as iron, vanadium, nickel, and chromium. Collectively, these other atoms are called heteroatoms. Certain heavy crude oils from younger geologic formations (e.g., Venezuelan crudes) contain less than 50 percent hydrocarbons and a higher proportion of organic and inorganic substances containing heteroatoms. The refining process removes many of the chemicals containing these heteroatoms. All crudes contain lighter fractions similar to gasoline as well as heavier tar or wax constituents, and may vary in consistency from a light volatile fluid to a semi-solid.

Petroleum products used for motor fuels are essentially a complex mixture of hydrocarbons. Gasolines are mixtures of hydrocarbons that contain 4 to 12 carbon atoms and have boiling points between 30 and 210 degrees Celsius. Kerosenes used for jet fuel contain hydrocarbons with 10 to 16 carbon atoms and have boiling points between 150 and 240. Diesel fuels and bunkering fuels contain hydrocarbons with higher numbers of carbon atoms and higher boiling points. In addition, diesel fuels and bunkering fuels have greater proportions of compounds containing heteroatoms.

Upon release, the hydrocarbons that are composed of fewer carbon and hydrogen atoms vaporize, leaving behind a heavier, less volatile fraction. Gasolines contain relatively high proportions of toxic and volatile hydrocarbons, such as benzene, which is known to cause cancer in humans, and hexane, which can affect the nervous system. Gasoline and kerosene releases are exceptionally hazardous due to their high flammability. Crude oils and semi-refined products, such as diesel and bunkering oils, may contain cancer-causing polycyclic aromatic hydrocarbons and other toxic substances.

The petroleum industry often characterizes crude oils according to their geographical source, e.g., Alaska North Slope Crude. Oils from different geographical areas have unique properties; they can vary in consistency from a light volatile fluid to a semi-solid. Classification of crude oil types by geographical source is generally not a useful classification scheme for response personnel because they offer little information about general toxicity, physical state, and changes that occur with time and weathering. These characteristics are primary considerations in oil spill response. The classification scheme provided below is more useful in a response scenario.

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