Fossil fuel

Fossil fuel is the name given to natural fuel, recovered from underground or surface deposits. Both coal and liquid or gas hydrocarbons deposits are believed to be derived from ancient plants, buried by sediment under anaerobic conditions, and then subjected to geologic heat and pressure over geologic time-spans.

Coal

Coal is a solid fuel.

Natural gas

Natural gas consists of buried deposits of lighter hydrocarbons that are gaseous at room temperature, like methane, propane. Natural gas is recovered from under domes where impermeable rock caps porous sandstone, or other rocks with pores. Recovery consists of drilling through the impermeable rock, and piping the gas to where it is to be used, or to where it might be liquified for transport in special tankers.

Natural gas is considered most desirable fossil fuel from an environmental point of view, having fewer impurities. Transportation is more difficult. The special tankers for shipping liquid natural gas require cooling the gas to very low temperatures. The tankers are inherently more vulnerable than oil tankers.

Oil companies in some jurisdictions are allowed to simply burn off natural gas, when they recover crude oil from remote oil fields.

Crude oil

Crude oil consists of a mixture of hydrocarbons that are liquid at room temperature. Like natural gas it is also often found under caps of impermeable rock, where it is often capped by a layer of natural gas. Rarely crude oil seeps to the surface.

The most desireable crude oil, "light sweet crude" consists of lighter liquid hydrocarbons, and has a relatively low level of impurities. Sulphur is the most common impurity in oil and gas, and, if it is not removed before the fuel is burned, it forms sulphur dioxides when burned, which fall as sulphuric acid when precipitated -- one of the major components of acid rain.

Crude oil is easier to transport than other forms of fossil fuel. It is transported through pipelines and in oil tankers.

Crude oil is typically fractionally distilled into its component hydrocarbons in large refineries. Once the fractions are distilled they are recombined into the synthetic fuels, like gasoline and kerosene.

Tar and pitch

Trinidad's pitch lake, one of the largest pitch seeps in the world.

There are relatively uncommon surface seeps of very heavy, viscous hydrocarbons, call tar or pitch. Because there is less use for heavy hydrocarbons the long heavy hydrocarbon molecules tar and pitch consists of are heated until they "crack" into lighter, more useful hydrocarbon molecules. This makes refining tar and pitch consume more energy.

Tar sands

Tar sands (also known as oil sands) consists of heavier hydrocarbons that are too viscous to be recovered through drilling, which are embedded in some other material. Refining is more costly, and consumes more energy. Turning tar sands into crude oil that can be transported by pipeline has typically required two to three times units of fresh water for every unit of crude produced.

The separation of the oil from the sand also leaves large volumes of toxic tailings, in the form of a slurry.

Known deposits of tar sands are relatively uncommon. But the two largest known deposits, the Orinoco Tar Sands, in Venuzuala, and the Athabasca Tar Sands, in Alberta, Canada, are each estimated to contain as much oil as was found in Saudi Arabia's oil fields, the largest known deposit of crude oil.

Oil shale

Like tar sands, oil shale consists of porous rock that incorporates very heavy hydrocarbons in its pores. Like tar sands the oil cannot be pumped out.