Petroleum or crude oil is a naturally occurring, flammable liquid
Flammable liquid
Generally, a flammable liquid is a liquid that can catch fire.In the USA, there is a precise definition of flammable liquid as one with a flash point below 100 degrees Fahrenheit. Less-flammable liquids are defined as combustible liquids...

 consisting of a complex mixture of hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....

s of various molecular weights and other liquid organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

s, that are found in geologic formations beneath the Earth's
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 surface. Petroleum is recovered mostly through oil drilling. This latter stage comes after the studies of structural geology (at the reservoir scale), sedimentary basin analysis, reservoir characterization (mainly in terms of porosity and permeable structures).

1859    Petroleum is discovered in Titusville, Pennsylvania leading to the world's first commercially successful oil well.

1901    The first great Texas oil gusher is discovered at Spindletop in Beaumont, Texas.

1922    Teapot Dome scandal: United States Secretary of the Interior leases Teapot Dome petroleum reserves in Wyoming.

1938    Oil is discovered in Saudi Arabia.

1938    Mexico nationalizes all foreign-owned oil properties within its borders.

1973    OPEC starts an oil embargo against a number of western countries, considered to have helped Israel in its war against Syria.

1979    Iran hostage crisis: In response to the hostage situation in Tehran, US President Jimmy Carter orders a halt to all petroleum imports into the United States from Iran.

1984    San Juanico Disaster: A series of explosions at the PEMEX petroleum storage facility at San Juan Ixhuatepec in Mexico City starts a major fire and kills about 500 people.

1989    Exxon Valdez oil spill: In Prince William Sound in Alaska, the ''Exxon Valdez'' spills 240,000 barrels (42,000 m³) of petroleum after running aground.