Petroleum, or crude oil, is a natural blend of liquid hydrocarbons and other fossil substances, often in combination with gaseous hydrocarbons (natural gases) and solid ones (bitumen, asphalt). It derives from the transformation of organic matter (especially algae and aquatic micro-organisms) deposited and accumulated on the seabed along with clayey or calcareous mud. When quickly covered over with other sediment, or when the environment is low in oxygen, the organic matter does not decompose. Instead, it sinks deep, often transforming into oil as a result of high pressure and high temperatures. The sediment, instead, becomes compact and forms the ‘mother rock’ from which petroleum is then extracted. Crude oil can migrate for considerable distances, reaching ‘reservoir rocks’: rigid yet porous and permeable rocks (as sand bodies or reef limestones) where the deposit forms.
