Fossil fuels are hydrocarbon fuels or hydrocarbon containing fuels such as petroleum (including natural gas) and coal. The utilization of fossil fuels has fueled industrial development and largely supplanted water driven mills and wood or peat burning for heat.

With nuclear power, it makes up the category of nuclear-fossil energy.

When generating electricity, energy from the combustion of fossil fuels is often used to power a turbine. Older generators used steam generated by the burning of the fuel to turn the turbine, but in newer power plants the gases produced by burning of the fuel turn a gas turbine directly. The burning of fossil fuels is the major source of emissions of carbon dioxide which is one of the greenhouse gases.


There are two theories on the origin of fossil fuels: the biogenic theory and the abiogenic theory. The two theories have been intensely debated since the 1860s, shortly after the discovery of widespread petroleum. According to the biogenic theory, fossil fuels are the altered remnants of ancient plant and animal life deposited in sedimentary rocks. The organic molecules associated with these organisms forms a group of chemicals known as kerogens which are then transformed into hydrocarbons by the process of catagenesis. According to the abiogenic theory, fossil fuels are primordial, being part of the Earth as it formed.

The abiogenic theory was favored early because in the late 19th century it was believed that the Earth was extremely hot (possibly molten rock) during its formation. This would have precluded the accretion of hydrocarbons, which would have been oxidized into water and carbon dioxide. When it was later discovered that all fossil fuels contain traces of biological debris, the biogenic theory gained strength because no one believed that life (even microbial life) could exist at the depths at which petroleum had been found.

Subsequently, it has become clear that the Earth formed by accretion of cold matter. Microbial life has also been discovered 4.2 kilometers deep in Alaska and 5.2 kilometers deep in Sweden. Further, at least ten bodies in our solar system are thought to contain traces of hydrocarbons. These discoveries led to a revival of the abiogenic theory, popularized by Thomas Gold.

In the middle of the 20th century it was possible to divide supporters of the two theories along geographic lines. Petroleum geologists in the United States and many in Europe favored the biogenic theory whilst those in Russia and also many microbiologists favored the abiogenic theory. The dispute is of more than academic significance, as abiogenic theory suggests that oil is an extremely abundant resource — in fact, that oil reservoirs may often refill from below as they are depleted!

Proponents of the biogenic theory have fought back, arguing from significant advances in the understanding of chemical processes and organic reactions and improved knowledge about the effects of heating and pressure during burial and diagenesis of organic sediments. Biogenesis remains the minority theory. But the abiogenic theorists may have the last laugh, because reports from the field in the U.S. and Middle East suggest that some oil patches are in fact refilling from the bottom.

A limited resource

Fossil fuels are a finite resource, but the alarmist reports from the early 1970s (the 1973 energy crisis) that oil supplies would run out in the 1990s have proven wrong. Significant usage of hydroelectricity and nuclear power and scientific advances have reduced the dependency on fossil fuels, of which household usage has increased nonetheless.

Sooner or later we will have to find alternatives (in the form of some kind of renewable energy source), however many people share a viewpoint that the time at which we would run out of fossil fuels is far in the future. As hydrocarbon supplies diminish, prices will rise (the principle of supply and demand). It has therefore been pointed out that higher prices will lead to increased supplies as previously uneconomic sources, such as tar sands or artificial gasolines (which require more expensive production and processing technologies than conventional petroleum reserves) become economically viable.

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