Fossil Fuel

What is a fossil fuel?
Fossil fuels are hydrocarbons such as coal, oil and natural gas, sourced from the organic remains of prehistoric organisms.

From left: a lump of coal, liquid petroleum anda gas flame.
When these fuels are burnt, the energy released can be harnessed to produce electricity, power vehicles, heat homes, cook food and much more. They are also used in the production ofimportant materials such as plastics.

How do fossil fuels form?
In order to answer this question, we must first distinguish oil and gas from coal. Generally speaking, oil and gas are formed from theorganic remains of marine organisms which become entrained within sea-floor sediments. Coal, by contrast, is typically formed in non-marine settings from the remains of land vegetation.
Oil and gasformation begins with the accumulation of organics on the sea-floor; these are the dead remains of organisms living in the water column, such as microscopic plankton, which rain down on the sea floorbelow. This will only occur in rather unusual settings, where the sea floor is stagnant such that there is no oxygen present to break the organic remains down and no sea-floor dwelling organisms presentthat might feed on the organics. A high sediment accumulation rate of may also help to bury the organics before the action of decay can break them down. As the sediment pile becomes deeper theorganics within it are subjected to heat and pressure which leads to formation of oil and then gas. For oil and gas extraction, it is important that the source rock is not 'over-cooked' or the hydrocarbonswill be destroyed. There must be suitable reservoir-rock, such as a porous sandstone, into which the hydrocarbons can migrate and accumulate. This must be overlain by an impervious cap-rock, such as aclay, which prevents the hydrocarbons from escaping to the surface. Finally, the geometry of the reservoir and cap-rock bodies must be such that the hydrocarbons become trapped; usually folding...