Petroleum
Introduction
The word petroleum is made of two Latin words, petra (rock) and oleum (oil). Its first use is lost in the darkness of antiquity, but we know that the builders of Babylon used asphaltum (a natural tar-like substance that washes ashore from oil seepages beneath the water surface) to bind together the rocks from which the city was built. The Greek historian Descorides Pedaners recorded that citizens of Agrigentum in Sicily burned petroleum in lamps long before the birth of Christ. Oil was used for various purposes by Persians, Hindus, Incas, Aztecs, and Indians.
Three miles above Oil City, Pennsylvania, there was an oil spring in the middle of Oil Creek. In 1790, Nathaniel Carey dug and walled shallow holes in the creek bed and skimmed the oil as it rose on the still water. He sold it as "Seneca Oil" for 25 cents a gill (~118 ml). This was, probably, the first sale of crude petroleum in the United States.
The theories that try to explain the origin of petroleum in nature may be divided into those that ascribe their origin to inorganic substances and those that recognize an organic source of petroleum. The theories of abiogenic origin of oil are diverse, but the best known are based on the assumption of existence of carbides deep within the Earth, from which hydrocarbons were formed. The organic (biogenic) theories are those which ascribe the origin of petroleum to animal or/and plant sources. Extensive research traced oil origin to the lipids contained in planktonic plants and animals which live in brackish water such as blue-green algae and foraminifera (a large group of amoeboid protists). At least three lines of evidence support biogenic origin of petroleum: (1) the oil is found only in association with sedimentary rocks; (2) polarized light passing through any petroleum undergoes a rotation that is the same as for the organic oils; (3) oil contain compounds which organic origin is certain such as chlorophyll and hemin.
Oil composition
Crude oil is probably the most complex mixture of organic compounds that occurs on Earth. Recent advances in mass spectrometry have allowed the identification of more than 17,000 distinct chemical components. Different crude oils have a range of chemical and physical properties that affect their impact on ecosystem and environmental fate. Crude oils contain four main groups of chemicals: saturated hydrocarbons and aromatic hydrocarbons, and the more polar, non-hydrocarbon components - the resins and asphaltenes.
Light oils are typically high in saturated and aromatic hydrocarbons, with a smaller proportion of resins and asphaltenes. Heavy oils, which result from the biodegradation of crude oil under anoxic conditions in situ in petroleum reservoirs, have a much lower content of saturated and aromatic hydrocarbons and a higher proportion of the more polar chemicals, the resins and asphalenes.
Oil properties
The main physical properties which affect the behavior of petroleum spilled at sea are:
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Specific gravity or relative density
This is density of the oil in relation to water. Most oils have specific gravity below 1 and are lighter than sea water which has a specific gravity of about 1.025. The American Petroleum Institute gravity scale is commonly used to describe the specific gravity of crude oil, and is calculated as follows:°API = 141.5/specific gravity - 131.5
Low specific gravity (high °API) indicates whether oil will float as well as that it may contain a high proportion of volatile components and to be of low viscosity. -
Distillation characteristics
As the temperature of the oil is gradually raised, different components reach their boiling point one after another and evaporate (become distilled). The distillation characteristics are expressed as the proportions of the oil which distil within given temperature ranges. Some bituminous, waxy or asphaltenic compounds do not distil even at high temperatures. -
Viscosity
Viscosity describe internal friction existing within a liquid which determines its resistance to flow. Heavy oils are more viscous than light oils. Viscosity of oil increases when temperature falls. In cool water oil can become semi-solid and very difficult to clean up by skimming or pumping. -
Pour point
Pour point is the temperature below which an oil will not flow. It depends on wax and asphaltene content of the oil. As an oil cools, it so-called "cloud point", the temperature, at which the wax components begin to form crystalline structures. Pour point of oil can be as high as 67°C (Shenyang oilfield, China).
Oil can affect living organisms in many ways. Components containing nitrogen, oxygen, or sulfur are usually the most toxic. Toxicities of oils vary according to their composition. In general, crude oils are less toxic than refined oil products.
References
- Oil history timeline
- Haney M. Petroleum. The Scientific Monthly, Vol. 17, No. 6 (Dec., 1923), pp. 548-561.
