PETROLEUM and coal are called “fossil fuels” because they are believed to have formed from the remains of plants that grew long ages ago. It appears that the organic material of the buried plants, screened off from atmospheric oxygen that would promote ordinary decomposition, was converted into hydrocarbon compounds. The great pressure and elevated temperatures under the earth’s surface, operating over many millenniums, are probably the essential factors in the formation of petroleum and coal.
The hydrocarbons vary greatly in hydrogen content. The highest is in methane, the chief constituent of natural gas. There is less hydrogen in the complex liquid hydrocarbons that make up petroleum, and less yet in asphalt, which is solid. Finally, in coal, all but a few percent of the hydrogen has been squeezed out by more extreme temperature and pressure. These chemical reactions must have been going on in the earth long before man’s creation.
If this understanding of the origin of petroleum and coal is correct, the energy they contain came in the first place from the sun, being fixed in organic compounds by photosynthesis in green plants. However, if the formation of these fuels is continuing now, it is certainly not keeping pace with man’s use of them.