petroleum: A naturally occurringoil that consists chiefly of hydrocarbonswith some other elements, suchas sulphur, oxygen, and nitrogen. Inits unrefined form petroleum isknown as crude oil (sometimes rockoil). Petroleum is believed to havebeen formed from the remains of livingorganisms that were deposited,together with rock particles and biochemical and chemical precipitates,in shallow depressions, chiefly in marineconditions. Under burial andcompaction the organic matter wentthrough a series of processes beforebeing transformed into petroleum,which migrated from the source rockto become trapped in large undergroundreservoirs beneath a layer ofimpermeable rock. The petroleumoften floats above a layer of waterand is held under pressure beneath alayer of natural gas.
Petroleum reservoirs are discoveredthrough geological exploration:commercially important oil reservesare detected by exploratory narrowboredrilling. The major known reservesof petroleum are in SaudiArabia, Russia, China, Kuwait, Iran,Iraq, Mexico, USA, United Arab Emirates,Libya, and Venezuela. The oil isactually obtained by the sinking ofan oil well. Before it can be used it isseparated by fractional distillation inoil refineries. The main fractions obtainedare:
(1) Refinery gas A mixture ofmethane, ethane, butane, andpropane used as a fuel and for makingother organic chemicals.
(2) Gasoline A mixture of hydrocarbonscontaining 5 to 8 carbon atoms,boiling in the range 40–180°C. It isused for motor fuels and for makingother chemicals.
(3) Kerosine (or paraffin oil) A mixtureof hydrocarbons having 11 or 12carbon atoms, boiling in the range160–250°C. Kerosine is a fuel for jetaircraft and for oil-fired domesticheating. It is also cracked to producesmaller hydrocarbons for use inmotor fuels.
(4) Diesel oil (or gas oil) A mixtureof hydrocarbons having 13 to 25carbon atoms, boiling in the range220–350°C. It is a fuel for diesel engines.The residue is a mixture of higher hydrocarbons. The liquid componentsare obtained by vacuum distillationand used in lubricating oils.The solid components (paraffin wax)are obtained by solvent extraction.The final residue is a black tar containingfree carbon (asphalt or bitumen).
A mixture of hydrocarbons
formed originally from marine animals and
plants, found beneath the ground trapped
between layers of rock. It is obtained by
drilling (also called crude oil). Different
oilfields produce petroleum with differing
compositions. The mixture is separated
into fractions by fractional distillation in a
vertical column. The main fractions are:
Diesel oil (gas oil) in the range 220–350°C,
consisting mainly of C13–C25 hydrocarbons.
It is used in diesel engines.
