Oil or oils? According to the basic definition, an oil is an organic compound that is insoluble or not readily soluble in water. It could be a petroleum-based product as well as a non-petroleum product — both categories comprise a number of different kinds of oils.

"Oil" or "oils" are concepts that do not necessarily refer to petroleum. Oil products used for energy or transportation or as raw material for plastics are mineral oils (petroleum-based oils) produced from crude oil. There are, however, also a great variety of other naturally occurring types of non-petroleum-based oil — lipids, essential (ethereal) oils, and wood-derived oils.


Petroleum means "rock oil", from the Greek petros/Latin petra (rock), and the Greek elaion/Latin oleum (oil). The term petroleum is nowadays used as a common denotation for crude oil (mineral oil) and natural gas, i.e., the hydrocarbons from which various oil and gas products are made. Petroleum, then, is a collective term for hydrocarbons, whether solid, liquid or gaseous.

Reserves of natural gas and crude oil have formed over millions of years as plants and animals have been broken down and undergone chemical change at high temperature and pressure. One finds petroleum in porous rock-forming large sedimentary basins, where the oil and gas has been trapped by some kind of barrier thereby forming a reservoir.

When pumped out of a well on land or in the seabed, crude oil is a complex mixture of thousands of different chemical components, mainly organic compounds — hydrocarbons — which usually make up about 95 per cent of the crude oil (however, hydrocarbon contents as low as around 50 per cent also occur). These hydrocarbons vary in toxicity and degradability, and range from very volatile, light materials like propane and benzene, to heavy compounds such as bitumens, asphaltenes, resins and waxes. The remaining about five per cent of the crude oil are made up of small amounts of oxygen, nitrogen and sulphur, and traces of some fifty other elements, mainly metals. Low-sulphur oil is in particularly high demand, since it does not need to be desulphurized prior to use for heating or as fuel.

The composition of the crude oil depends on the "raw material" from which the crude was originally formed, and on the conditions that prevailed during its formation and thereafter. Physical properties and chemical composition vary from one reserve to another and even between different depths in the same well. Thus, every crude oil is unique. Also, crude oils are often characterized by the petroleum industry according to their geographical source.

Before being used as fuel (for energy generation, machinery and vehicles), or as a raw material in the petrochemical industry, crude oil is refined into different fractions. At the refinery, crude oil is separated into light and heavy fractions, which are then converted into various products, such as petrol, diesel oil, jet fuel, etc.


Lipids, which contain fatty acids, may be of animal origin — such as, e.g., whale, seal and fish liver oil, lard and milk fat — or of vegetable original, for example palm oil, rapeseed oil, linseed oil, sunflower oil, olive oil and coconut oil. Essential (ethereal) and wood-derived oils are usually natural, including e.g. wood-derivated oils like resin/rosin oils, as well as oils from flowers or fruits, such as essence of roses, oil of lavender, jasmine, violet, orange, etc. They can also be manufactured synthetically for use in paints (e.g., silicone fluids, and tung oils), or in foods and perfumes.


The crude mineral oil pumped from wells is a mixture of natural gas, water (formation water or production water) and hydrocarbons. First, the gas is separated from the oil and water and further treated. Water and solid particles are removed from the oil component of the crude, which is transported to a refinery for distillation and other separation and refinement processes . The resulting products from these processes are a number of fractions with different characteristics and ranges of use:

  • natural gas;
  • raw gasoline (benzine and naphta), the end product of which is petrol (gasoline);
  • intermediate distillates, the end products of which include light gas (fuel) oil, diesel oil, aviation fuels, kerosene, etc.;
  • heavy distillates, giving end products like heavy gas (fuel) oil for cracking processes, as well as lubricants, waxes, etc.;
  • residues, the end products of which are heavy fuel oils, asphalt (bithumen), tar and coke.

Important physical and chemical properties of oils that will affect the behaviour and effects of oil in water and aquatic environments are its surface tension, specific gravity, and viscosity. The composition and characteristics of an oil, together with a number of circumstances relating to the time and place of the spill, the amounts of oil, weather conditions etc. will determine how persistent the oil will be, how it will spread, whether it will evaporate or sink, etc. From an oil spill response point of view, crude oils can be classified on a scale from light, volatile oils, over non-sticky oils to heavy, sticky oils and non-fluid oils (see EPA). Similarly, refined products can be classified from lightweight gasoline and kerosene over lightweight and medium-weight fuel oils to medium-weight and heavyweight fuel oils/bunker oils (see EPA).