Themes > Science > Earth Sciences > Geology > Oil and Gas > Formation of Hydrocarbon Generation > How Oil and Gas were Formed
Oil has formed throughout much of the Earth's history, in fact, oil is being formed in some parts of the Earth today. Almost all oil and gas comes from tiny decayed plants, algae, and bacteria. At certain times in the Earth's history conditions for oil formation have been particularly favourable. Oil from the North Sea is mainly found in rocks that formed during the Jurassic period - about 150 million years ago, long before people appeared on Earth.
During this time the seas and swampy areas were rich in microscopic plants and animals.
When these died they slowly sank to the bottom forming thick layers of organic material. This in turn became covered in layers of mud that trapped the organic material.
Oil and gas were formed by the anaerobic decay of organic material in conditions of increased temperature and pressure.
The layers of mud prevented air from reaching the organic material. Without air, the organic material couldn't rot in the same way as organic material rots away in a compost heap. As the layers of mud grew in thickness, they pushed down on the organic material with increasing pressure. The temperature of the organic material was also increased as it was heated by other processes going on inside the Earth.
Very slowly, increasing temperature, pressure and anaerobic bacteria - micro-organisms that can live without oxygen - started acting on the organic material. As this happened the material was slowly cooked and altered, like food in a pressure cooker. In this was the energy first given to the plants by the sun is transferred and the organic matter is changed into crude oil and gas.
Oil forms first, then as the temperature and pressure increase at greater depth gas begins to form.
Temperatures within the Earth's crust increase with depth so that the sediments, and any plant materials they contain, warm up as they become buried under more sediment. Increasing heat and pressure first cause the buried algae, bacteria, spores and cuticles (leaf skin) to join their wax, fat and oil to form dark specks called kerogen.
The cellulose and woody part of plants are converted to coal and woody kerogen. Rocks containing sufficient organic substances to generate oil and gas in this way are known as source rocks. When the source rock starts to generate oil or gas it is said to be mature.
As the source rock gets hotter, chains of hydrocarbon chemicals use this heat energy to break away from the kerogen to form waxy and viscous heavy oil. At greater depth, the temperature rises. At higher temperatures the chains of hydrocarbons become shorter and break away to give light oil and gas. Most North Sea Oil is the valuable light oil. Gas from the Southern North Sea is methane.
Oil and gas are called 'hydrocarbons' because they mostly contain molecules of the elements hydrogen and carbon.
Crude oil is a complex mixture of hydrocarbons with small amounts of other chemical compounds that contain sulphur, nitrogen and oxygen.
Traces of other elements, such as sulphur and nitrogen, were also present in the decaying organic material, giving rise to small quantities of other compounds in crude oil.
Hydrocarbon molecules come in a variety of shapes and sizes, (straight-chain, branched chain or cyclic), this is one of the things that makes them so valuable because it allows them to be used in so many different ways.
Oil and gas form as the result of a precise sequence of environmental conditions: