Origin of natural oil and gas
What is the origin of natural oil and gas?
Most scientists agree that hydrocarbons (oil and gas) are biogenic - formed from the remains of plants and animals that lived hundreds of millions of years ago. This theory is supported by the fact that many underground oil deposits contain chemicals that are only known in plants that live on the Earths surface.
Organisms, mostly algae and tiny animals called plankton, died and sank to the bottom of the seas in which they lived. They were covered with mud and sand then, over millions of years, buried deep under other sediments. Temperature in the Earth"s crust increases with depth. This has warmed the plant and animal remains, causing them to produce oil and gas.
Oil forms at temperatures between about 50°C (120°F) and 175°C (350°F). At higher temperatures, gas is formed and any oil that has already been produced starts to turn into lighter oils and eventually into Methane gas, the lightest and simplest hydrocarbon. At temperatures above about 260°C (500°F), plant and animal remains turn completely to carbon and no more oil or gas are produced.
Methane gas can also be formed at low temperatures at shallow depths. This includes swamp gas, formed by plants decomposing in swamps.
Some gas comes from coal, which is made from deep layers of vegetation from ancient forests. This vegetation has been also buried and "cooked".