A mixture of hydrocarbons that stay liquid or semi-solid in subsurface reservoir.

The usual composition ranges from C₁ to C₃₂ and API gravity is between 4° and 42° (Specific Gravity is 0.816 ÷ 1.0).).

It may also include minor organic and inorganic chemical substances (< 7% in weight).

The total range of oil density is often classified in narrow ranges:

Light Oil	Medium Oil	Heavy Oil	Extra Heavy Oil (Bitumen)

In case oil does not contain dissolved gas it is called Dead Oil otherwise it is called Live Oil.

See also