The fossil fuels are non-renewable sources of energy. These are actually hydrocarbons that remain inside the earth's crust. There are different types of fossil fuels like petroleum, methane, coal and so on. Some of these are made up of pure carbon and others are of low ratio of carbon and hydrogen. According to the most common theory of fossil fuels, it is believed that these fuels were formed from the corpse of animals as well as plants that got buried under the earth's crust due to certain reasons. There are two different theories of fossil fuels that are known as Biogenic theory and Abiogenic theory.
According to the Biogenic theory that is put forward by a number of geologists, the formation of fossil fuels is caused by the heating and compression of the organic products for a long time. As per this theory, oil has been formed from the remains of the algae and zooplankton in the sea bed or lakes. These materials are believed to be of prehistoric age and since then these were under anoxic conditions. On the other hand, coal has been formed from the terrestrial plants. These plants remained buried under the crust of the earth and due to heat and pressure, chemical changes took place.
These changes turned these products into coal. Again, these organic materials, due to heavy pressure and high temperature, changed into kerogen. This process is known as diagenesis. Further rise in the pressure and temperature levels changed kerogen into gaseous or liquid hydrocarbon. This process is termed as catagenesis.
There is another group of geologists who believe in the abiogenic theory. According to this theory, hydrocarbons (of inorganic nature) were present on this planet since the very beginning. Now, the densities of the petroleum hydrocarbons are lower than the aqueous pore fluids and because of this, these hydrocarbons move upward and get trapped in the oil reservoirs. This movement was made possible by the fracture networks. This theory further demonstrates that the biomarkers that remain in the petroleum are actually due to the microbial life-forms that remain in the rocks. Although this is an important theory of fossil fuels but there are not enough evidences to support this theory.
Both these theories hold that millions of years are needed for the creation of fossil fuels. The reservoirs of these fuels are becoming exhausted very rapidly and the rate of new fossil fuel formation is very low. Thus, these fuels are marked as non-renewable fuels.