Geologists describe petroleum, as it comes out of the ground, using the term crude oil. This oil varies dramatically from place to place. Some is relatively thin and pours about as easily as the oil used to lubricate auto engines. These types of petroleum are known as conventional crudes. In contrast, there is a broad range of oils considered unconventional. These are thick and sometimes even tarlike. In the most extreme cases, they will pour about as easily as peanut butter (which means that at room temperature they really won’t pour at all).
All petroleum consists of a complex mix of molecules known as hydrocarbons. These molecules (and there are many dozens) get their name from the fact that they are combinations of hydrogen and carbon atoms that nature has chemically bound together.
Petroleum hydrocarbons developed out of the decaying remains of plants and animals that lived throughout the world’s oceans before the time of dinosaurs. Over millions of years, they became deeply buried under layers of sand and sediment. Compressed by the weight (pressure) of those upper layers, the decaying remains heated up and then underwent chemical reactions. Heavy crudes developed where microbes were able to feast on and remove the smaller hydrocarbons, leaving only big (and heavy) molecules behind.
Chemical companies prefer the thinner crude oils because they can more easily convert them — through processes known as refining — into products that consumers prize. These include gasoline, diesel fuel, jet fuel, kerosene and petrochemicals (which include organic solvents and plastics). Conventional oils tend to leave the ground with fewer natural impurities that must be removed prior to refining, such as sulfur and metals.
“Because conventional light oil can typically be produced at a high rate and a low cost, it has been used before other types of oil,” note  Richard F. Meyer and Emil D. Attanasi of the U.S. Geological Survey in Reston, Va. But a heavy demand for products made from crude oil over the past 80 years has led oil companies to pump enormous quantities from the ground. Many huge natural reservoirs of this oil have now been virtually exhausted.
This has prompted oil companies to take a new interest in the heavier, hard-to-work-with types. Unconventional oils contain fewer of the small hydrocarbon molecules and more large hydrocarbons.
Among these are the so-called heavy crudes and bitumen (or tar sands). Venezuela’s Orinoco Basin in South America holds an estimated 90 percent of the world’s supply of recoverable heavy crude, Meyer and Attanasi report. Canada’s western province of Alberta is home to more than 80 percent of the world’s known (extractable) supplies of bitumen — a super-heavy crude oil with the consistency of tar. Together, world supplies of these heavy and super-heavy crudes about equal remaining extractable quantities of conventional oils, according to the USGS researchers.
This means substantial quantities of unconventional oils remain. However, the costs of refining them into the products that consumers want is far higher than for light crudes, requires the use of far more energy and produces far more pollution.
bitumen (also known as tar sands or oil sands) A type of extra-heavy crude oil that is tarlike at room temperature.
conventional oil A crude oil that leaves the ground as an easy-to-pour natural mix of hydrocarbons.
diesel fuel A fuel obtained during petroleum refining or from blends of refining distillates mixed with residual oil used in motor vehicles. Diesel fuels are heavier than gasoline and have a higher boiling point.
gasoline A complex mixture of relatively volatile hydrocarbons with or without small quantities of additives. They are blended to form a fuel for use in spark-ignition engines.
hydrocarbons A group of many compounds made by linking together different numbers of hydrogen and carbon atoms. Examples include methane, propane and ethylene.
kerosene A thick oil obtained from petroleum and used both as a fuel and solvent.
organic A term that refers to a range of carbon-based molecules (other than carbon dioxide and carbon monoxide).
petroleum A term that usually refers to crude oil or products obtained by refining, or processing, crude oil.
production (of crude oil and gas) Moving oil and natural gas to the surface from underground reservoirs and then treating these to obtain liquid hydrocarbons for use as fuels or the ingredients to make commercial chemicals.
refining An energy-intensive industrial process that transforms crude oil into fuels such as gasoline and heating oil or into chemicals used to make plastics, synthetic rubber, solvents and a range of other products.
unconventional oil A crude oil that leaves the ground as an especially thick (viscous) natural mix of hydrocarbons.
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U.S. Energy Information Administration. “Energy kids: Oil (petroleum) .”