Petrol, diesel, jet fuel, bottled gas and the black surface of many roads all start as one thick, dark liquid pumped up from deep underground: crude oil.
Crude oil is a mixture of thousands of different hydrocarbons, the compounds of only carbon and hydrogen that you have met many times. Some are chains, the alkanes. Many are rings of carbon atoms, such as cyclohexane, and some carry the flat benzene ring. Their sizes run from methane, with one carbon, to big molecules with dozens.
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Made from tiny sea life
Crude oil formed over millions of years from plankton: tiny algae and animals that drifted in ancient seas. When they died, they sank and were buried in mud on the sea floor, where there was too little oxygen for them to rot away. More mud and sand piled on top and slowly turned to rock. Deep underground, heat and pressure turned their remains into crude oil and natural gas.
Oil and gas are lighter than water, so they creep upwards through porous rock, rock such as sandstone with tiny spaces between its grains. Where they meet a layer of cap rock, rock with no spaces between its grains such as shale, they collect beneath it. Oil companies drill down through the cap rock to reach them.
Straight from the ground, crude oil has few uses. A refinery sorts it into useful parts, and the next section shows how.