Fossil fuel is a general term for any hydrocarbon or carbonaceous rock that may be used for fuel: chiefly petroleum, natural gas, and coal. These energy sources are considered to be the lifeblood of the world economy. Nearly all fossil fuels are derived from organic matter, commonly buried plant or animal fossil remains, although a small amount of natural gas is inorganic in origin. Organic matter that has long been deeply buried is converted by increasing heat and pressure from peat into coal or from kerogen to petroleum (oil) or natural gas or liquids associated with natural gas (called natural gas liquids). Considerable time, commonly millions of years, is required to generate fossil fuels, and although there continues to be generation of coal, oil and natural gas today, they are being consumed at much greater rates than they are being generated. Fossil fuels are thus considered...