Petroleum or 
crude oil, is a naturally occurring 
flammable liquid consisting of a complex mixture of 
hydrocarbons of various molecular weights and other liquid 
organic compounds, that are found in 
geologic formations beneath the 
Earth's surface. A 
fossil fuel, it is formed when large quantities of dead organisms, usually 
zooplankton and 
algae, are buried underneath 
sedimentary rock and undergo intense heat and pressure.
Petroleum is recovered mostly through 
oil drilling. This comes after the studies of structural geology (at the reservoir scale), sedimentary basin analysis, reservoir characterization (mainly in terms of porosity and permeable structures). It is refined and separated, most easily by 
boiling point, into a large number of consumer products, from 
petrol (or 
gasoline) and 
kerosene to 
asphalt and chemical 
reagents used to make 
plastics and 
pharmaceuticals. Petroleum is used in manufacturing a wide variety of materials, and it is estimated that the world consumes about 88 million barrels each day.