section_path: Composition
section_name: Composition
paragraph_text: Composition
  
  Petroleum consists of a variety of liquid, gaseous, and solid components. Lighter hydrocarbons are the gases methane, ethane, propane and butane. Otherwise, the bulk of the liquid and solids are largely heavier organic compounds, often hydrocarbons (C and H only). The proportion of light hydrocarbons in the petroleum mixture varies among oil fields.
  
  An oil well produces predominantly crude oil. Because the pressure is lower at the surface than underground, some of the gas will come out of solution and be recovered (or burned) as associated gas or solution gas. A gas well produces predominantly natural gas. However, because the underground temperature is higher than at the surface, the gas may contain heavier hydrocarbons such as pentane, hexane, and heptane (" natural-gas condensate ", often shortened to condensate.) Condensate resembles gasoline in appearance and is similar in composition to some volatile light crude oils.
  
  The hydrocarbons in crude oil are mostly alkanes, cycloalkanes and various aromatic hydrocarbons, while the other organic compounds contain nitrogen, oxygen, and sulfur, and traces of metals such as iron, nickel, copper and vanadium. Many oil reservoirs contain live bacteria. The exact molecular composition of crude oil varies widely from formation to formation but the proportion of chemical elements varies over fairly narrow limits as follows:
  
  Four different types of hydrocarbon appear in crude oil. The relative percentage of each varies from oil to oil, determining the properties of each oil.
  
  The alkanes from pentane (C H) to octane (C H) are refined into gasoline, the ones from nonane (C H) to hexadecane (C H) into diesel fuel, kerosene and jet fuel. Alkanes with more than 16 carbon atoms can be refined into fuel oil and lubricating oil. At the heavier end of the range, paraffin wax is an alkane with approximately 25 carbon atoms, while asphalt has 35 and up, although these are usually cracked in modern refineries into more valuable products. The lightest fraction, the so-called petroleum gases are subjected to diverse processing depending on cost. These gases are either flared off, sold as liquefied petroleum gas, or used to power the refinery's own burners. During the winter, butane (C H), is blended into the gasoline pool at high rates, because its high vapour pressure assists with cold starts.
  
  The aromatic hydrocarbons are unsaturated hydrocarbons that have one or more benzene rings. They tend to burn with a sooty flame, and many have a sweet aroma. Some are carcinogenic.
  
  These different components are separated by fractional distillation at an oil refinery to produce gasoline, jet fuel, kerosene, and other hydrocarbon fractions.
  
  The components in an oil sample can be determined by gas chromatography and mass spectrometry. Due to the large number of co-eluted hydrocarbons within oil, many cannot be resolved by traditional gas chromatography. This unresolved complex mixture (UCM) of hydrocarbons is particularly apparent when analysing weathered oils and extracts from tissues of organisms exposed to oil.
  
  Crude oil varies greatly in appearance depending on its composition. It is usually black or dark brown (although it may be yellowish, reddish, or even greenish). In the reservoir it is usually found in association with natural gas, which being lighter forms a "gas cap" over the petroleum, and saline water which, being heavier than most forms of crude oil, generally sinks beneath it. Crude oil may also be found in a semi-solid form mixed with sand and water, as in the Athabasca oil sands in Canada, where it is usually referred to as crude bitumen. In Canada, bitumen is considered a sticky, black, tar-like form of crude oil which is so thick and heavy that it must be heated or diluted before it will flow. Venezuela also has large amounts of oil in the Orinoco oil sands, although the hydrocarbons trapped in them are more fluid than in Canada and are usually called extra heavy oil. These oil sands resources are called unconventional oil to distinguish them from oil which can be extracted using traditional oil well methods. Between them, Canada and Venezuela contain an estimated 3.6 trillion barrels (570 × 10 m) of bitumen and extra-heavy oil, about twice the volume of the world's reserves of conventional oil.