Oil is thought to have formed over millions of years from the break down of tiny dead creatures. Natural gas is formed alongside oil.
The dead organisms sank to the bottom of lakes or seas and became trapped in muddy sediments. As the sediments built up, the lower layers were under pressure. They eventually turned to rock. If there was no oxygen in the sediments, heat and pressure turned the remains of the organisms into oil and natural gas.
Some rocks are porous - they have a network of tiny holes in them.Sandstone and limestone are examples. Oil is a liquid so it seeps into porous rocks. Gas also diffuses into these rocks.
Porous rocks may also contain water. Gas and oil do not mix with water. They are less dense than water. This means they form layers above the water.
Sometimes the rock layers form so that the oil and gas are trapped under the rock such as shale that is not porous. Large amounts of oil and gas may collect in a porous rock. The pressure on the oil may build up so much that when a hole is drilled through the rock cap, oil gushes out.
Crude oil is a mixture of many thousands of different compounds with different properties. They are called hydrocarbons because they only contain the elements hydrogen and carbon.
To make crude oil useful, batches of similar compounds with similar properties need to be sorted. These batches are called fractions and they are separated by fractional distillation.
The theory behind this technique is that some of the compounds in crude oil are easily vaporised, for example, they are volatile due to their low boiling points. Others are less volatile and have higher boiling points.
In fractional distillation, the crude oil is heated to make it vaporise. The vapour is then cooled. Different fractions of the oil are collected at different temperatures.
|Fraction:||No. of carbon atoms:||Colour:||Boiling point range oC:||Uses:|
|Refinery gas||1 - 4||Colourless||Below room temp.||Gaseous fuel, making chemicals.|
|Gasoline (petrol)||4 - 12||Colourless to pale yellow||32-160oC||Motor car fuel, making chemicals.|
|Kerosine (paraffin)||11 - 15||Colourless to yellow||160-250oC||Heating fuel, jet fuel.|
|Diesel oil||15 - 19||Brown||220-350oC||Diesel fuel for lorries, trains, etc. and heating fuel.|
||C||Dark brown||Above 350oC||Fuels for power stations, ships etc. Some is distilled further to give lubricating oils, waxes, etc.|
|20 - 30|
|30 - 40|
|50 and above|
As the hydrocarbon molecule chain increases its boiling point increases, it becomes more viscous, becomes more difficult to light, the flame becomes sootier and it develops a stronger smell.