Summary
Discussions of U.S. and global energy supply refer to oil, natural gas, and coal using several terms that may be unfamiliar to some. The terms used to describe different types of fossil fuels have technically precise definitions, and misunderstanding or misuse of these terms may lead to errors and confusion in estimating energy available or making comparisons among fuels, regions, or nations.
Fossil fuels are categorized, classified, and named using a number of variables. Naturally occurring deposits of any material, whether it is fossil fuels, gold, or timber, comprise a broad spectrum of concentration, quality, and accessibility (geologic, technical, and cultural). Terminology is adopted to reflect those characteristics.
For oil and natural gas, a major distinction in measuring quantities of energy commodities is made between proved reserves and undiscovered resources. Proved reserves are those amounts of oil, natural gas, or coal that have been discovered and defined, typically by drilling wells or other exploratory measures, and which can be economically recovered. In the United States, proved reserves are typically measured by private companies, who report their findings to the Securities and Exchange Commission because they are considered capital assets. In addition to the volumes of proved reserves are deposits of oil and gas that have not yet been discovered, and those are
called undiscovered resources. The term has a specific meaning: undiscovered resources are amounts of oil and gas estimated to exist in unexplored areas. If they are considered to be recoverable using existing production technologies, they are referred to as undiscovered
technically recoverable resources (UTRR). In-place resources are intended to represent all of the oil, natural gas, or coal contained in a formation or basin without regard to technical or economic recoverability.
In the United States, certain institutions are designated to determine and report quantities of oil, natural gas, and coal reserves and undiscovered resources. Other institutions also estimate these values, but differences in estimating methodology can produce significantly different values.
U.S. proved reserves of oil total 21.3 billion barrels and reserves of natural gas are 237.7 trillion cubic feet. Undiscovered technically recoverable oil in the United States is 145.5 billion barrels, and undiscovered technically recoverable natural gas is 1,162.7 trillion cubic feet. The demonstrated reserve base for coal is 489 billion short tons, of which 262 billion short tons are considered technically recoverable.
Comparisons of different fuel types can be made by converting all of them to a common unit, such as barrels of oil equivalent, based on their heat content. The amounts of fossil fuels found in other nations as reserves and undiscovered resources are much more difficult to determine reliably because data are sometimes lacking or unreliable, but gross comparisons of national
endowments can be made using available data.
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