Like their name suggests, fossil fuels were originally formed when the remains of ancient organisms decomposed in anaerobic conditions under layers of rock and soil. The formation of fossil fuels has occurred over a long period of time. Because of this, fossil fuels are considered practically non-renewable. 

Oil, natural gas, and shale gas have formed from the remains of ancient marine organisms. Some concentrations of these resources are still located in the bottom of the oceans, while some have remained inside bedrock as a result of plate tectonics. Oil and natural gas are extracted by drilling into oil or gas pockets that are buried deep under the ground. Shale gas is extracted from rocks that contain organic substances with the help of pressurized water. 

Oil is the most important energy source in the world. Approximately one third (33 %) of the total energy consumed on the planet is produced with oil. Crude oil contains a number of different hydrocarbons, and it can be refined into various kinds of fuels, lubricants, plastics and bitumen. In addition to its versatility, it is also easy to transport and store. Oil is especially important fuel in traffic and transportation. In contrast, natural gas covers approximately one quarter (24 %) of Earth's total energy consumption. 

Approximately one fourth of Finland's total energy comes from crude oil. It is used as fuel for transport, as well as in heating and electricity production. Almost all of the crude oil used in Finland comes from Russia. 

Natural gas is formed through similar processes as crude oil. It, too, is mostly imported into Finland from Russia, which is home to the largest natural gas deposits on the planet. It is mainly used to produce heating and energy for population centers and industrial production. It is also used in some cars. Natural gas is mainly comprised of methane, and it is by far the least harmful of all fossil fuels. 

The planet's coal supplies were formed when the thick peat layers of ancient fern forests decomposed in anaerobic conditions. It is a solid fuel that is mined from bedrock. Coal is a lasting, cheap and easy energy source. It is used especially in heavy industry and electricity production. In Finland, coal is usually used as a reserve energy source for heating during the winter. It is one of the most environmentally harmful fossil fuels.


A coal power plant.

Like other fossil fuels, peat is also formed from the decomposed remains of ancient organisms. Peat is produced in bogs and is technically renewable, but this renewal occurs at such a low rate that it cannot be considered a renewable energy source. Peat collection is an important source of employment in Finnish regions where other jobs are in low supply. It is also easy to process and use. However, burning peat produces a lot of greenhouse gases, and its collection destroys bog ecosystems.