Oil - Definitions and chemistry

OIl is a mixture of hydrocarbons. Each deposit has it's own blend of hydrocarbons. In the oil refinery they are separated into fractions by boiling them off in a fractionating column.

• Refinery gas – methane, ethane, propane and butane. 
• Petrol / Gasoline – hydrocarbons with 5 to 8 carbon atoms, boiling range 40– 80°C 
• Kerosene (paraffin oil) -  hydrocarbons containing 11 or 12 carbon atoms, boiling range 160°C – 250°C
• Diesel oil - a mixture of hydrocarbons containing 13 to 25 carbon atoms, boiling range 220°C – 350°C 
• Black tar (asphalt or bitumen) 
• Aromatics – hydrocarbon compounds containing a benzene ring 
• Cycloalkanes – hydrocarbon compounds containing a ring of carbon atoms joined by single bonds.

The proportion of petrol obtained from crude oil can be increased by 'cracking' i.e. breaking down the heavier hydrocarbons into the lighter hydrocarbons that make up petrol. Refining, cracking and other processes,  can consume up to 5% of the energy of the oil entering the refinery.  

Crude oil is a liquid consisting of hydrocarbons buried in sedimentary basins. It is formed as organic-rich rocks are buried and heated over geological time.
Condensate is a mixture of pentane and heavier hydrocarbons found in gas fields. Though it is a gas underground due to the temperature, it cools and condenses when brought to the surface.
For more, see: Refining process
  Boiling  Critical temp  
Methane -160 -83 Heating gas
Ethane -90 32 Polythene
Propane -40 97 LPG
ButaneLPG 0 152 LPG
Pentane 35 196 Petrol
Hexane 70 234 Petrol
Heptane 100 267 Petrol
Octane 130 295 Petrol

Above the critical temperature a substance is a gas, and cannot be liquefied by any pressure. Below the critical temperature it is a vapour and can be liquefied by pressure. This is why methane must be cooled to be liquefied.

Oil, gas and condensate

It is difficult to discuss oil and gas separately as they often occur together.

Underground where the temperature is high, many hydrocarbons exist as a gas/vapour, but when brought to the surface and cooled, they condense as liquids. These liquids are classified as "condensates".