Energy

Fossil fuels or mineral fuels are fossil source fuels, that is, hydrocarbons found within the top layer of the earth’s crust. It is generally accepted that they formed from the fossilized remains of dead plants and animals by exposure to heat and pressure in the Earth's crust over hundreds of millions of years.

It was estimated by the Energy Information Administration that in 2005, 86% of primary energy production in the world came from burning fossil fuels, with the remaining non-fossil sources being hydroelectric 6.3%, nuclear 6.0%, and other (geothermal, solar, wind, and wood and waste) 0.9 percent.

Fossil fuels are non-renewable resources because they take millions of years to form, and reserves are being depleted much faster than new ones are being formed. Concern about fossil fuel supplies is one of the causes of regional and global conflicts. The production and use of fossil fuels raise environmental concerns.

Levels of primary energy sources are the reserves in the ground. Flows are production. The most important part of primary energy sources are the carbon based fossil energy sources.

Levels (reserves) (EIA oil, gas, coal estimates, EIA oil, gas estimates)

• Oil: 1,050,691 to 1,277,702 billion barrels (167 to 203 km³) 2003-2005
• Gas: 6,040,208 - 6,805,830 billion cubic feet (171,040 to 192,720 km³) 6,805.830*0.182= 1,239 BBOE 2003-2005
• Coal: 1,081,279 million short tons (1,081,279*0.907186*4.879= 4,786 BBOE) (2004)

Flows (daily production) during 2002 (7.9 is a ratio to convert tonnes of oil equivalent to barrels of oil equivalent)

• Oil: (10,230*0.349)*7.9/365= 77 MBD
• Gas: (10,230*0.212)*7.9/365= 47 MBOED
• Coal: (10,230*0.235)*7.9/365= 52 MBOED

Years of production left in the ground with the most optimistic reserve estimates (Oil & Gas Journal, World Oil)

• Oil: 1,277,702/77/365= 45 years
• Gas: 1,239,000/47/365= 72 years
• Coal: 4,786,000/52/365= 252 years

Note that this calculation assumes that the product could be produced at a constant level for that number of years and that all of the reserves could be recovered. In reality, consumption of all three resources has been increasing.