Natural gas is a fossil energy well known in French homes. They are a third to use it, for heating, hot water or cooking. Who says fossil energy, says deposit and extraction. Where are the main natural gas reserves in the world? How are the deposits exploited? What is the future of natural gas at a time when fossil fuels are increasingly criticized?

What is natural gas?

Natural gas is a fossil energy also called primary energy because it does not come from a transformation of energy (unlike electricity). We mainly think of natural gas for domestic use (feeding the boiler or hobs), but it is also very useful in other areas such as industry or transport. Its consumption has been growing for more than 30 years, in particular because of its environmental qualities: it emits practically no fine particles. In addition, it is also the hydrocarbon that returns the least greenhouse gas in the atmosphere once consumed.

The formation of natural gas deposits

Before being marketed, the natural gas must be extracted from the deposits, then treated and finally transported to the place of consumption. The natural gas that humans extract from the depths is in fact of organic origin and has been formed from the decomposition of plankton and algae over millions of years. Most organic waste is destroyed and digested by bacteria. A small part of the residual material (0.1%) is however deposited at the bottom of the oceans (where it is protected from the action of bacteria because the medium is too poor in oxygen) and mixes with sediments (sand, clay, salt). This mixture forms a layer of mud that hardens over the years to become "mother rock". Under the weight of the new sediments deposited there, the mother rock sinks very slowly in the earth's crust: it can travel a few meters to a few hundred meters per million years. Pressure and temperature increase as the mother layer sinks deeper. The organic matter composing it transforms into Kerosene and, from 2000 meters below sea level, into hydrocarbon (carbon and hydrogen):
  • between 2000 and 3800 meters, kerosene is transformed into oil;
  • between 3800 and 5000 meters sediments form hydrocarbons no longer liquid but gaseous: methane gas (the lightest hydrocarbon);
  • Below 8000 meters, there are no more hydrocarbons because the very high temperatures destroy them.
The hydrocarbons rise to the surface of the earth because they are lighter than water. If nothing stops them, they escape and ooze to the surface of the earth or solidify in bitumen losing their volatile constituent. If they encounter an impervious layer called a blanket, they remain locked in a pocket called reservoir rock where the gaseous part rises above the oil pushing down the water table. Sedimentation evolves on average by 50 meters per million years, so it takes about 60 million years for dead plankton to turn into liquid hydrocarbon (oil) and 85 million for a gaseous hydrocarbon to form. It is therefore understandable why oil or natural gas are not considered as renewable energies. Initially, the natural gas operations appeared as a derivative of the oil operations. The initial prospecting of the deposit being carried out for a petroleum exploitation, the natural gas was initially burned with flares in order to limit the risks of incidents on the extraction sites. Today, the value chain of natural gas is better structured and gas is now also sought for direct use.