Carbon Cycle

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Since carbon is the major ingredient in fossil fuels, it is relevant to discuss the carbon balance in the atmosphere. Carbon exists primarily in the form of carbon dioxide, which is present in the air, dissolved in ocean water, or trapped in rocks and in plants through photosynthetic processes. Animals (including humans) consume plants as a source of nutrients and are part of the carbon cycle as well.

Depending on the ultimate fate of dead animals and plants, carbon may return to the atmosphere as carbon dioxide or turn into fossil fuel. If the environment dries up, dead plants and animals will become exposed to air and react with it to produce carbon dioxide, thus completing the carbon cycle (Figure 1). However, if the environment is fl ooded with mud, stagnant water, silt, or sand, oxygen is cut off and the proteins and carbohydrates decompose from bacteria into a wax-like, organic, insoluble material called kerogen (a). The time it takes for kerogen to transform into fossil fuel ranges from tens to hundreds of millions of years, and depends on its depth in the ground, temperature, and pressure.

Figure 1 Carbon Cycle in the Atmosphere.
Figure 1 Carbon Cycle in the Atmosphere.

Whether a fossil ultimately ends up as coal, oil, or gas depends on the original constituents of the kerogen and the conditions under which it is kept. If the origin of kerogen is from giant plants and stronger parts of plants such as lignin (material that strengthens the trunks and branches of trees), then carbon forms into very complicated structures and rings. These rings eventually connect together into a graphitic structure called coal. If the kerogen is formed from algae or plankton, they break into shorter chains that depending on their lengths, become a mixture of liquids (crude oil) and gases (natural gas). The deeper in the earth and more aged the kerogen, the greater the probability that the chains will break into lighter materials and a higher fraction of the kerogen will be in the form of natural gas. Th e carbon cycle is completed when fossil fuels are extracted and burned to form carbon dioxide and water.



(1) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005

Additional Comments

(a) Keros is the Greek word for wax, and gen means more.

Further Reading

Berkowitz, N., Fossil Hydrocarbons: Chemistry and Technology, Elsevier Academic Press, 1997.

Deff eyes, K. S., Hubbert’s Peak: Th e Impending World Oil Shortage, Princeton University Press, Princeton, N. J., 2001.

Campbell, C. J., Th e Coming Oil Crisis, Multi-Science Publishing Company, 2004.

Tariq Ali, Th e Clash of Fundamentalisms: Crusades, Jihads and Modernity, Verso, 2002.

Pelletiere, S., Iraq and the International Oil System: Why America Went to War in the Gulf, Praeger Publishing, 2001.

Oil and Gas Journal, Technology, news, statistics, special reports, and analysis (

Journal of Petroleum Technology, The official journal of Society of Petroleum Engineers, Dallas.

The Petroleum Engineer, Petroleum Engineer Pub. Co.

Journal of Petroleum Science and Engineering, Elsevier, covers the fields of petroleum (and natural gas) exploration, production and flow.

External Links

National Energy Technology Laboratory: Th e Strategic Center for Coal (

National Petroleum Technology Office (

US Geological Survey (

Organization of Petroleum Exporting Countries (OPEC) (

Society of Petroleum Engineers (