'''Petroleum''', named for "rock oil
", is a [[fossil fuel]] usually found deep within the [[earth]], although some deposits are very close to the surface.
It is a heavy dark [[liquid]], consisting of many different [[hydrocarbon]] [[chemical compound|compound]]s.
Crude oil is extracted from the ground and [[distillation|fractionated]] into its components by volatility in an oil refinery. Some of the main products distilled from crude oil are:
The following table<ref>Stainforth, J., Nourse, R. & Nosiara, M., 2007, ''Basin Oriented Geology - Petroleum Systems Module Course Manual''. </ref> compares the qualities of various types of petroleum:
| align=left bgcolor=lightgrey | '''''[[Fraction]]'''''
| align=center bgcolor=lightgrey | '''''
[[API ]] gravity'''''
| align=center bgcolor=lightgrey | '''''[[Viscosity]]<br>(mp)'''''
| align=center | >10<sup>5</sup>
Oil deposits are often found in concurrence with [[natural gas]] in a variety of geological structural or [[stratigraphy|stratigraphic]] traps.
Geologists]] have deduced that petroleum is the remains of huge amounts of algal material that was laid down millions of years ago, and altered in form under high temperatures and pressures. Layers of buried plant material tend to produce [[coal]] or lignite.
A minority belief is that petroleum is synthesized naturally by unspecified underground processes, slowly rising towards the surface for our use.