Current 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. This report describes the characteristics of fossil fuels that make it necessary to use precise terminology, summarizes the major terms and their meanings, and provides a brief summary of the United States' endowment of fossil fuels and the relationship between the U.S. fossil fuel energy endowment and those of other nations. Fossil fuels are categorized, classified, and named using a number of variables. It is important to keep in mind that 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). These characteristics are graphically portrayed in Figure 1 as a resource pyramid. At the top of the pyramid are the deposits that are high quality and easy to access. These deposits have been generally discovered and produced first. Examples of the deposits at the top of the resource pyramid are the large oil deposits of Saudi Arabia and the enormous natural gas deposits of Qatar. Moving down the pyramid, the quality and/or accessibility declines, and production becomes more difficult and expensive. A large oil deposit in the deep waters of the Gulf of Mexico would be further down the pyramid than a comparable deposit on land because of the added expense and technology required to produce it. It is important to note that the deposits at the bottom of the pyramid may be quite extensive. Deposits may be of poor quality or diffuse, but may occur in vast quantities. Examples of fossil fuel deposits that would be found at the bottom of the pyramid are oil shale and methane hydrates (both discussed further under " Sub-Economic Oil and Natural Gas Resources "). Oil shale and methane hydrate deposits contain massive amounts of oil and natural gas, but their mode of occurrence, poor accessibility, and difficult recovery make them difficult to produce and/or sub-economic currently. The economic threshold for producing deposits further down the pyramid is partly a function of commodity price. That threshold is also moved by the development of new extraction technologies that make production feasible at lower cost. For U.S. oil deposits, the resource pyramid ( Figure 2 ) indicates that many of the high-quality, easy-to-find deposits have already been produced. Current proved reserves (see " Terminology ") include many deposits that are of lower quality or with poorer access than some historical production, but which are still economic under current market conditions. As long as demand for oil continues, the exploration and production process will move down the pyramid under the influences of price (including environmental costs in some cases) and technology. Whether the vast deposits of oil shale that are lower on the pyramid will be produced depends on the price of oil, the cost of production (including environmental cost), and the availability of technology to produce it. Although this example is for oil, similar relationships exist for natural gas and coal. A search for energy statistics in the literature quickly reveals a large number of terms used to describe amounts of fossil fuels. Most of these terms have precise and legitimate definitions, and even a careful comparison of statistics for diverse forms of fossil fuels can become quite difficult to reconcile or understand. Not only do oil, natural gas, and coal occur in many diverse geologic environments, but each commodity may occur in different modes or in different geologic settings that impose vastly different economics on their recovery and delivery to market. A vocabulary of terms has developed over the decades to capture the nature of deposits in terms of their likelihood of being developed and their stage of development. Examples of terms used for fossil fuel deposits (not an exhaustive list) include Proved reserves Probable reserves Possible reserves Unproved reserves Demonstrated reserve base Undiscovered resources Probable resources Possible resources Speculative resources Potential resources Technically recoverable resources Economically recoverable resources Definitions for several of these terms are included in the Appendix . It is important to note that no systematic statistical data exist for most of these categories. Although data for more categories may be available in the future, the critical categories for which data are currently collected and compiled are proved reserves and undiscovered resources. It is critical to understand some key distinctions among these categories in order to understand statistical reporting of oil and natural gas. The first key distinction is between proved reserves and undiscovered resources; the second key distinction is between conventional and unconventional deposits of fossil fuels. For oil and natural gas, a major distinction in measuring quantities of energy commodities is made between proved reserves and undiscovered resources. Understanding these terms will help avoid confusion about statistical energy data. Proved reserves are those amounts of oil, natural gas, or coal that have been discovered and defined at a significant level of certainty, 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 Energy Information Administration (EIA), and to the Securities and Exchange Commission because those reserves are considered capital assets. Because proved reserves are defined by strict rules, they do not include all of the oil or gas in a region, but only those amounts that have been carefully confirmed by drilling or some other geophysical technique. Because proved reserves are, by definition, economically recoverable, the proportion of the oil in the ground that qualifies as proved reserves increases when prices are high, and decreases when prices are low. That is, even without new discoveries, oil that may be sub-economic at $70 per barrel might become economic at $100 per barrel and so the total proved reserves increase simply because price increases. In addition to the volumes of proved reserves, there are other deposits of oil and gas that have not yet been discovered, which are called undiscovered resources. The term "resource" has often been used in a generic sense to refer to quantities of energy (or other) commodities in general. Observers may refer to resource-rich nations, or speak about a large resource base, for example. But the phrase "undiscovered resources" has a specific meaning. Undiscovered resources are amounts of oil and gas estimated to exist by examining geologic characteristics in unexplored areas. Certain geologic processes and characteristics are required for the formation and accumulation of hydrocarbons, and careful geologic study can reveal whether those characteristics exist in a particular area, and whether the potential for hydrocarbon resources is large or small. The proportion of undiscovered resources that might be recovered is determined by comparing the area with production wells in a known area and volumes are constrained by current technology and production practices. Thus the phrase "undiscovered technically recoverable resources" (UTRR) refers to estimates of oil and gas that might be produced in the future using current proven technology, but without regard to cost. The cost factor is an additional applied to UTRR to determine the "economically recoverable" resources for an area, which are reported as separate figures. Estimates of undiscovered resources for the United States are made by the U.S. Geological Survey for resources on land, and by the U.S. Bureau of Ocean Energy Management, Regulation and Enforcement (formerly the Minerals Management Service) for resources offshore. These assessments are based on observation of geological characteristics similar to producing areas and many other factors. Reported statistics for undiscovered resources may vary greatly in precision and accuracy (determined retrospectively), which are directly dependent upon data availability, and their quality may differ for different fuels and different regions. Because estimates of undiscovered resources are based partly on current production practices, they are generally reported as undiscovered technically recoverable resources. Another term sometimes used in the fossil fuels literature is "in-place" resources. 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. Because only a small proportion of the total amount of the fossil fuel in a deposit is ever recovered, there are often large discrepancies between volumes of in-place resources and the proportion of those resources that are technically recoverable. In-place resource estimates are sometimes very large numbers, which may be misleading if the reader does not appreciate that usually only a small proportion of the in-place volume of a resource can ever be produced or recovered. Research by a USGS geologist during the 1980s and 1990s revealed that a rock formation in the Williston Basin of North Dakota, South Dakota, Montana, and southern Canada contained an abundance of hydrocarbons dispersed throughout layers of shale and sandstone. The numbers reported were quite impressive; estimates of 271 to 503 billion barrels of oil attracted the attention of the oil industry. However, those estimates, while huge, represented "in-place" oil. That is, the total volume of oil was huge, but it was disseminated throughout thousands of square miles of shale and only a small portion of that total would be technically recoverable using technologies available at that time. With the development of new production technologies, some of the oil in the Bakken Formation has become producible and, subsequently, the USGS has conducted a more detailed and thorough estimate of the technically recoverable resources. USGS now estimates that the Bakken Formation contains 3.65 billion barrels of undiscovered technically recoverable oil and 1.85 trillion cubic feet (tcf) of undiscovered technically recoverable natural gas. These estimates are still substantial in volume, and production in the Bakken Formation is increasing. But the gap between estimates of in-place oil and technically recoverable oil demonstrates vividly the importance of knowing what the numbers represent. The first oil and gas deposits discovered consisted of porous reservoirs in geologic formations, capped by an impervious rock "trap" within which migrating fluids such as oil, natural gas, and water would accumulate. Within the reservoir, natural gas would be the least dense fluid and would have accumulated at the top of the reservoir. Oil is more dense than gas, but less dense than water and would pool in a layer below the gas cap. Below the oil and gas, water would fill the confined reservoir. This layered arrangement of natural gas, oil, and water within a reservoir contained by a trap is called a conventional deposit and has historically provided most of the oil and natural gas that has been produced. In recent decades, geologists began to realize that considerable volumes of oil and natural gas exist outside conventional reservoirs in sedimentary rocks situated in geologic basins. The distribution of oil or natural gas throughout a geologic formation over a wide area, but not in a discrete reservoir, is called an unconventional deposit (sometimes called a continuous deposit). The amounts of oil and gas contained in unconventional deposits may be very large, but recovering those deposits is sometimes difficult and expensive. Oil sands provide an example of an unconventional oil deposit in which the oil is distributed widely through the sandstone formation. Recovering the oil from oil sands requires special technologies and treatments such as heating, steam flooding, or even excavation. An example of an unconventional natural gas deposit is coalbed methane. The natural gas (methane) does not exist in a discrete reservoir but is distributed throughout the pore spaces of coal and held in place by water. When the water is removed from the coal, the gas is released and can be produced. Another type of unconventional natural gas deposit is shale gas, which is discussed below (see " Shale Gas "). There is no direct correlation between the economic recoverability of a deposit and whether it is conventional or unconventional. Some conventional deposits are not economically recoverable because they are too small, too deep, or lack surface access. On the other hand, certain unconventional deposits such as oil sands and coalbed methane are economically recoverable in some locations. For example, coalbed methane production was 1.9 tcf in 2009 out of a total U.S. natural gas production of 22.5 tcf (approximately 8%), and is an important component of the U.S. natural gas supply. Many individuals and institutions have attempted to compile and publish estimates of resources. However, the statutory responsibility for collecting and publishing authoritative statistical information on the various types of energy sources in the United States has been given to specific federal agencies. The Energy Information Administration (EIA) was originally created as the Federal Energy Administration (FEA) and is charged with the responsibility of monitoring and reporting U.S. energy reserves and production. The Energy Information Administration (EIA) was created in response to the need for additional Federal initiatives to collect and disseminate energy-related information, and to evaluate and analyze this information. These needs were revealed as the United States sought to respond to the energy crises of the 1970s. The first law to address these needs was the Federal Energy Administration Act of 1974 and, over the years, many subsequent laws have contributed to EIA's evolution and growth. [The law c]reated the FEA and mandated it to collect, assemble, evaluate, and analyze energy information; provide energy information and projections to the Federal Government, State Governments, and the public; and provide Congress with an annual report summarizing these activities. It also provided FEA with data collection enforcement authority for gathering data from energy producing and consuming firms. Likewise, the responsibility for assessing onshore undiscovered technically recoverable oil and gas resources in the United States resides with the U.S. Geological Survey (USGS), in the Department of the Interior. The USGS has conducted a number of national assessments of undiscovered technically recoverable oil and natural gas resources over several decades. The most recent complete comprehensive national assessment for onshore oil and gas was completed in 1995, but USGS updates that assessment on an ongoing basis as new data become available for individual basins. Responsibility for assessment of offshore undiscovered technically recoverable oil and natural gas resources belongs to the Bureau of Ocean Energy Management, Regulation and Enforcement, also within the Department of the Interior. EIA and USGS have responsibilities for evaluating the nation's endowment of coal that are similar to their responsibilities for oil and natural gas. In addition to purely governmental assessments, several expert groups provide perspectives on individual fuels, geographic areas, or industry sector. Some of these groups are composed of government, industry, and academic experts (e.g., the Potential Gas Committee ), expert advisory committees for federal agencies (e.g., National Petroleum Council ), independent study groups (e.g., the National Research Council, Committee on Earth Resources ), or professional societies (e.g., American Association of Petroleum Geologists or the Society of Petroleum Engineers ). Each of these groups applies considerable expertise to the assessment and evaluation of oil and gas reserves and resources, and their reports are considered to be authoritative and serious, but none have the responsibility to provide a consistent, timely statistical review of U.S. oil and natural gas resources. When using estimates generated by these expert groups, it is important to look for clear and transparent explanation of assessment methodology; in the absence of explanation, it will not be clear what is being estimated, and the value and usefulness of the statistics is diminished. In addition to these estimates of volumes and trends in specific hydrocarbon resources, the BP Statistical Review of World Energy provides an excellent compilation of data from authoritative sources worldwide for oil, natural gas, coal, and other energy sources. U.S. proved reserves of crude oil (plus condensate) as reported by EIA total 22.3 billion barrels. The data are from the latest full compilation, at the end of calendar year 2009. There is generally a delay of over a year between the end of a reporting year and the compilation of the data because the process is time-consuming and quality control is essential. The EIA value for proved reserves includes both onshore and offshore reserves. Compiling oil reserves is not a simple arithmetic exercise. Each year, volumes of individual components change significantly. Below is a list of how the reserves might change as a function of the normal exploration, production, and business processes conducted by oil companies. A more detailed description of these terms is in the Appendix . Adjustments (+,-) Revision Increases (+) Revision Decreases (-) Sales (-) Acquisitions (+) Extensions (+) New Field Discoveries (+) New Reservoir Discoveries in Old Fields (+) Estimated Production (-) U.S. proved reserves of dry natural gas, also reported by EIA for 2009, total 272.5 tcf. It should be noted here that, though this is the latest available number for natural gas reserves, it is for 2009 and therefore does not include the substantial increases in gas reserves attributable to the rapidly developing shale gas during 2010 and 2011. Like oil, the compilation of proved reserves of natural gas involved keeping track of several kinds of production and adjustments. Total proved reserves of gas are reported as dry natural gas and include both onshore and offshore reserves. The following list shows a breakdown of how natural gas production is reported prior to separation into its gas and liquid components. Natural Gas, Wet After Lease Separation Natural Gas Nonassociated, Wet After Lease Separation Natural Gas Associated-Dissolved, Wet After Lease Separation Natural Gas Liquids (included with crude oil reserves in this report) As mentioned previously, the responsibility for assessing the undiscovered technically recoverable resources (UTRR) of oil and natural gas is split between the USGS for onshore resources, and the Bureau of Ocean Energy Management, Regulation and Enforcement (BOEMRE) (formerly the Minerals Management Service (MMS)) for offshore resources. USGS and BOEMRE use somewhat different assessment methodologies. The combined results of the onshore and offshore assessments of UTRR rely on the availability of geologic data, which may be quite limited in some regions, especially for certain offshore areas. Nevertheless, the estimates are complementary and are tabulated in Table 1 and Table 2 . The USGS distinguishes between conventional and unconventional undiscovered resources, and they are reported separately for oil, natural gas, and natural gas liquids. BOEMRE reports its estimates for each offshore planning region but does not distinguish between conventional and unconventional deposits. The total endowment of technically recoverable oil and natural gas for the United States is obtained by summing proved reserves from EIA with the onshore and offshore undiscovered technically recoverable resources from USGS and MMS, as shown in Table 3 . Although values for proved reserves reported by EIA include proved reserves of shale gas, the value for total technically recoverable natural gas (1,176.21 tcf) does not include much of the emerging prospective shale gas being explored in the United States. According to a 2009 report by the Potential Gas Committee, a consortium of industry, academic, and government experts, the total "future supply" of natural gas is 2,074 tcf, which includes substantial volumes of shale gas (see " Shale Gas "). After coal, oil shale represents the most abundant fossil fuel in the United States. However, despite government programs in the 1970s and early 1980s to stimulate development of the resource, production of oil shale is not yet commercially viable. The need for massive capital investment and the cost of production itself have been the major barriers. A further economic factor lies in the fact that liquids produced from oil shale have a unique chemical composition and, unlike conventional crude oil, cannot be distilled to produce gasoline, but would be primarily a source of other liquid middle distillate fuels such as jet fuel or diesel oil, fuels for which there is significant national demand. In addition, production of liquids from oil shale requires large amounts of water, an important factor since most of the resource is located in water-scarce regions of western Colorado, Utah, and Wyoming. Other environmental problems include the difficulty in disposing of tailings if excavation is used as the extraction process, and the production of greenhouse gases. In light of these difficulties, efforts to aid in the development of oil shale are focused on pilot projects to test alternative technologies of production. Estimates of the amount of hydrocarbon fuel in U.S. oil shale resources are highly speculative, given the small amount of development that has taken place. The Department of Energy (DOE) Office of Naval Petroleum and Oil Shale estimates that approximately 1.38 trillion barrels of shale oil are potentially recoverable from the roughly 7.8 million acres of federal oil shale. A more conservative estimate by the RAND Corporation is that 800 billion barrels may be recoverable. Shale gas is an emerging type of natural gas deposit, and exploration for and production of shale gas is increasing. Some shale gas is currently economic, and improved production methods are leading to increasing production. Shale gas is a classic unconventional type of deposit; the gas is distributed throughout the low-permeability shale formations rather than in a localized accumulation in a more permeable reservoir. The occurrence of gas in this manner requires special production techniques that often involve horizontal drilling into the gas-bearing formation, followed by hydrofracturing of the rock (exerting pressure in the gas well so high that it causes brittle rock to fracture) to release the gas from the rock. The use of hydrofracturing has caused some environmental concerns arising from the injection of large amounts of water into the well, concerns about the chemical composition of the injected fluids, disposal of production fluids after hydrofracturing is completed, fears that the fractured rock will expose local water wells to non-potable waters, and the observation that some hydrofracturing jobs have apparently created small earthquakes. However, industry officials insist that any environmental concerns could be mitigated through careful production practices. Shale gas is being developed in several basins within the U.S. and the proportion of total natural gas reserves comprised of shale gas is rising, though precise reserves figures are difficult to obtain in this rapidly changing sector. As the Energy Information Administration stated in its 2011 Annual Energy Outlook: Estimates of technically recoverable shale gas are certain to change over time as new information is gained through drilling and production, and through development of shale gas recovery technology. Over the past decade, as more shale formations have been explored and used for commercial production, estimates of technically and economically recoverable shale gas resources have skyrocketed. However, the estimates embody many assumptions that might prove to be untrue in the long term. No systematic assessment of undiscovered technically recoverable shale gas resources has been completed for the entire United States, though government, industry and academic experts estimate that the technically recoverable volumes of natural gas from these shale deposits are very large. The Potential Gas Committee has estimated that the United States has 616 tcf of "potential natural gas resources" occurring as shale gas. The proportion of that resource that will actually be produced will depend on further development of exploration and production technology, the price of natural gas, and the ways in which states deal with potential environmental issues. However, EIA has significantly increased the projections for shale gas production in its 2011 Annual Energy Outlook. Another form of fossil fuel with potentially vast resources is natural gas in the form of methane hydrate. Methane hydrate (sometimes called natural gas hydrate, or just gas hydrates) is being investigated as an energy source by both DOE and USGS. Methane hydrate is a crystalline solid composed of methane and water that forms in porous rocks under very specific conditions of temperature and pressure. Deposits occur most commonly offshore in the sediments or rocks of the continental shelf and slope, or in cold climates such as northern Alaska and Canada. Although considered a scientific oddity until the 1990s, methane hydrates are now known to exist in hundreds of locations around the world, often in small, isolated deposits, but sometimes in massive quantities. Total worldwide in-place resources of methane hydrates are probably huge, perhaps thousands of trillion cubic feet, but hydrates have never been produced commercially. Current efforts by the United States, Canada, Japan, India, and several other nations are aimed at developing technologies to exploit this large and widespread form of natural gas. The mean in-place gas hydrate resource for the entire United States is estimated to be 320,000 tcf of gas, with approximately half of this resource occurring offshore of Alaska and most of the remainder occurring beneath the continental margins of the lower 48 states. The USGS estimates that there are about 85 tcf of undiscovered technically recoverable gas resources within gas hydrates in northern Alaska, and recent studies have shown that methane hydrates are more abundant in the sediments of the Gulf of Mexico than previously believed. Improved understanding of the occurrence and behavior of these important natural gas deposits, and improved technology for producing them, may make methane hydrates a viable source of natural gas in the future. Heavy oil, so-named because it is more dense and more viscous than light crude oil, constitutes substantial deposits of oil in Canada, Venezuela, and other parts of the world. Canada's Athabasca oil sands and Venezuela's Orinoco oil sands are the largest deposits of this type. Canada's oil sands contain an estimated 173 billion barrels of technically recoverable oil and account for more than half of Canada's oil production. The Orinoco oil sands are estimated to contain 1.36 trillion barrels of extra heavy oil in-place, of which approximately 270 billion barrels are technically recoverable. Oil sands generally require special production techniques such as excavation or steam flooding, and the oil produced is often limited to certain refineries equipped to handle the heavy oil. U.S. heavy oil is found in California, Alaska, and Wyoming, and is estimated to constitute in-place resources of up to 100 billion barrels of oil, though production of heavy oil in the United States is declining because of the depth of the resource and the cost of production. EIA is the authoritative source for coal reserves and resource estimates for the United States. EIA compiles data on coal reserves and resources from state sources and federal sources, including from geologic assessment work done by the USGS. The terminology used for coal is slightly different than for oil and natural gas. The primary statistic reported by EIA is the demonstrated reserve base (DRB), which is comprised of coal resources that have been identified to specified levels of accuracy and may support economic mining under current technologies. For the latest reporting period, calendar year 2009, the U.S. demonstrated reserve base was 486 billion short tons. Because the United States produces and consumes about 1 billion short tons of coal per year, the demonstrated reserve base would appear to provide hundreds of years' supply of coal, if U.S. users continue to consume it at the same rate. However, because coal production often requires ground disturbance, especially for open-pit mining, the amount that is technically recoverable is not always available. EIA has applied an availability factor that reduces the technically recoverable amount to 261 billion short tons that would actually be available for mining. Detailed availability studies by the USGS have indicated that, at least in some cases, the available and economically recoverable coal might be substantially less than the technically recoverable amount for a variety of reasons: a significant portion of the coal resources less than 4,000 ft (1,219.2 m) in depth are also typically subeconomic due to a number of restrictions that further limit their availability and recoverability. Some of these restrictions are technical constraints (using existing technology) such as coal beds too thin to recover or dipping too steeply. Many societal or environmental restrictions such as the presence of towns, wetlands, or other environmentally sensitive areas may also preclude coal recovery. Both regional mine planning and economic studies are necessary to derive estimates of the coal reserves for any given area. For example, in one specific case in Wyoming, 47% of the in-place coal is technically recoverable, but the available, economically recoverable coal is only about 6% of the in-place coal. While these proportions may vary between 5% and 20%, depending upon the specific conditions for each coal-mining area, very large coal numbers are viewed with some caution because in-place numbers, or even recoverable numbers, may not provide a realistic assessment of the coal that could actually be produced. It is sometimes useful to equate the different types of fossil fuels in order to compare the energy content or to gauge the magnitude of one type of fossil fuel in terms of another. Fossil fuels may be liquid, gas, and solid; oil is a liquid measured in barrels, natural gas is a gas measured in cubic feet, coal is a solid measured in short tons, and all three types of fossil fuels vary in composition, quality, and heat content. Therefore, converting one type of fossil fuel to an equivalent amount of another is a slightly problematic calculation. For example, the energy content of coal varies by at least a factor of three depending on grade. However, government and industry sources commonly use rule-of-thumb measures to make these conversions. For example, EIA provides a conversion tool on its website that assumes the following heat contents: Using these rule-of-thumb heat values, we can express natural gas and coal units in terms of barrels of oil equivalent (BOE): Using these conversion factors, we present a crude comparison of U.S. energy reserves plus resources in Table 4 . These values are for combined reserves and UTRR for each fossil fuel. For all the reasons mentioned above, these calculations should be viewed as approximations rather than precise calculations. Reliable values for proved reserves and undiscovered resources outside the United States are less available than for the United States. The only source of data for some countries is one of the nation's ministries (energy, resource, interior, commerce, etc.), and those data may not be completely accurate because of the lack of good geologic data and assessment methodology, or because the information is purposely withheld. In fact, even if all nations wished to report their resource estimates reliably, it would not be possible to collect uniform data because different methods, accounting rules, and terminology are used in each country. Therefore, some reserve statistics reported outside the United States are not consistent with the U.S. data. Furthermore, only reserves and production statistics are reported for most nations. There has been no reliable source for estimates of undiscovered oil and natural gas resources internationally since the U.S. Geological Survey completed its World Petroleum Assessment in 2000. Data for proved reserves and production in all countries are most reliable. Production statistics can be obtained for the Organization of the Petroleum Exporting Countries (OPEC) and for the Organization for Economic Cooperation and Development (OECD) countries from the International Energy Agency, an arm of OECD. For international statistics, the EIA relies on the Oil & Gas Journal (a publication of PennWell Corporation) or World Oil (a publication of Gulf Publishing Company) for foreign oil and natural gas reserves. These energy industry trade publications monitor individual national sources for information, as described by the Oil & Gas Journal: OGJ does not make its own estimates of a country's reserves but rather compiles the estimates of proved reserves from an annual survey of official sources, including government agencies and ministries. Since most countries do not assess their reserves annually, many of the figures in this report are unchanged from a year ago. World Oil summarizes their data sources this way: World Oil's tables are produced with data from a variety of sources, including governmental agencies. Operating companies with drilling programs also contributed to this year's survey. Our survey is not scientifically randomized, and new environmental and political challenges may emerge at any time. In some cases, a country may not have responded to our surveys, in which case we might use proxies such as rig counts and third-party sources, both public and private. A source of global oil and gas information commonly used by a number of analysts is the BP Statistical Review of World Energy. Some of BP's data also come from Oil & Gas Journal and World Oil, but are supplemented with additional data: The reserve numbers published in the BP Statistical Review of World Energy are an estimate of proved reserves, drawn from a variety of official primary sources and data provided by the OPEC Secretariat, Cedigaz, World Oil and the Oil & Gas Journal and an independent estimate of Russian oil reserves based on information in the public domain. Oil reserves include field condensate and natural gas liquids as well as crude oil. They also include an estimate of Canadian oil sands 'under active development' as a proxy for proved reserves. This inclusive approach helps to develop consistency with the oil production numbers published in the Review, which also include these categories of oil. The BP Statistical Review of World Energy uses data from the World Energy Council (WEC) for coal reserves. The WEC is a global consortium of national committees that compile energy statistics for their own countries. WEC estimates for oil and natural gas reserves differ somewhat from the Oil & Gas Journal and World Oil values, but not dramatically. For the United States, the U.S. Energy Association (USEA) is the WEC national committee, and USEA cites EIA sources for its estimates for the United States. When using any international fossil fuel statistics, users should be cognizant of ultimate sources of data among these energy data organizations. Using the best-available data, it is possible to draw a comparison of the total endowment of fossil fuels for nations. Table 5 includes the basic data for oil, natural gas, and coal for selected nations, with calculations of the total fossil fuels in each nation, expressed in billions of barrels of oil equivalent (billion BOE). Using only proved reserve numbers for the United States and other nations shows that the United States remains among the top nations in proved reserves of all fossil fuels taken together. Importantly, however, more than 92% of the fossil fuel reserves of the United States is coal. Values for technically recoverable oil and natural gas resources estimated by the USGS contain greater uncertainty than the statistics for proved reserves. Nevertheless, adding the estimates for undiscovered technically recoverable oil and natural gas provides a more inclusive estimate of total endowment of technically recoverable fossil fuels. Table 6 adds technically recoverable oil and natural gas resources to the proved reserve figures of Table 5 to provide a more complete tabulation of technically recoverable fossil fuels. Values for total fossil fuels in Table 6 include the estimates for coal reserves in the first column but do not include any estimates for undiscovered coal resources; those data simply do not exist in any consistent form for various nations. As an example of how such undiscovered coal resources might affect the ultimate total endowment of fossil fuels, the U.S. coal resource estimates do not include some potentially massive deposits of coal that exist in northwestern Alaska. These currently inaccessible coal deposits have been estimated to be more than 3,200 billion short tons of coal. Only a portion of that coal would likely be technically recoverable even if development were pursued but, nevertheless, it suggests other fossil fuel deposits in many parts of the world that have not been estimated or are not available for extraction. Again, it is noteworthy that approximately two-thirds of the total fossil energy endowment (reserves plus UTRR) of the United States is coal. That proportion may shift somewhat as new deposits of tight oil and shale gas are identified. In addition, a comparison of the ultimate endowments of fossil fuels among nations would include the important deposits of oil, natural gas, and coal that are lower on the resource pyramid in Figure 1 , and that might be exploited in the future given the appropriate technology, economic viability, and environmental acceptability. However, the uncertainty associated with estimates of those deposits is too great to produce meaningful comparisons. For example, the values in Table 6 could be amended further by including estimates of oil shale or methane hydrate resources, but the final tally would have very little meaning considering the difficulties in estimating those resources. The United States has considerable amounts of fossil fuels, both conventional and unconventional, both discovered and undiscovered, that are not currently economically viable. However, it is likely that other nations contain similar deposits but lack any comprehensive assessment of those resources. Finally, comparisons of global fossil fuel resource estimates should consider that apples-to-apples comparison of resources among countries is very difficult for a variety of reasons. Differences in definitions used in each nation, the extent to which reliable data are collected, and the transparency of data reporting all contribute to the estimates of a nation's fossil fuel endowment. The data represented in this report should be viewed only as gross approximations based on data available today, and may not represent the ultimate rankings of national endowments. An absence of data—either because the data don't exist or because they are not made available—produces imprecise and perhaps inaccurate estimates and, in fact, estimates made in the absence of specific data are often assumed to be zero. As data become available through exploration and development of resources in a particular region, the estimates become more accurate and more precise and sometimes grow larger. For example, the United States is an extremely well-studied and well-developed area, so estimates may be larger than an equivalent area elsewhere in the world. A country with vast areas of unexplored territory might have estimated resources that look small by comparison because there are no data to support a larger estimate. Therefore, rankings may change over time as different regions are explored and developed. To provide some scale for the reserves and undiscovered resource values reported above, Table 7 lists production and consumption of oil, natural gas, and coal by the United States. For a more complete summary of U.S. energy supply and demand, see CRS Report R40187, U.S. Energy: Overview and Key Statistics , by [author name scrubbed] and [author name scrubbed]. Reserves and Resources Terms Definitions of terms taken from U.S. Department of the Interior, Survey of Available Data on OCS Resources and Identification of Data Gaps, Report to the Secretary, OCS Report MMS 2009-015, Appendix A, List of Terms Used, http://www.doi.gov/ocs/report.pdf . Proved reserves. The quantities of hydrocarbons estimated with reasonable certainty to be commercially recoverable from known accumulations under current economic conditions, operating methods, and government regulations. Current economic conditions include prices and costs prevailing at the time of the estimate. Estimates of proved reserves do not include reserves appreciation. Reserves. The quantities of hydrocarbon resources anticipated to be recovered from known accumulations from a given date forward. All reserve estimates involve some degree of uncertainty. Reserves appreciation. The observed incremental increase through time in the estimates of reserves (proved and unproved) of an oil and/or natural gas field as a consequence of extension, revision, improved recovery, and the additions of new reservoirs. Resources. Concentrations in the earth's crust of naturally occurring liquid or gaseous hydrocarbons that can conceivably be discovered and recovered. Undiscovered resources. Resources postulated, on the basis of the geologic knowledge and theory, to exist outside of known fields or accumulations. Undiscovered technically recoverable resources (UTRR). Oil and gas that may be produced as a consequence of natural pressure, artificial lift, pressure maintenance, or other secondary recovery methods, but without any consideration of economic viability. They are primarily located outside of known fields. Undiscovered economically recoverable resources (UERR). The portion of the undiscovered technically recoverable resources that is economically recoverable under imposed economic and technologic conditions. Unproved reserves. Quantities of hydrocarbon resources that are assessed based on geologic and engineering information similar to that used in developing estimates of proved reserves, but technical, contractual, economic, or regulatory uncertainty precludes such reserves from being classified as proved. Key Terms Used in Oil Statistics Key Terms Used in Natural Gas Statistics

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 Energy Information Administration, and also 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, which are called undiscovered resources. The term has a specific meaning: undiscovered resources are amounts of oil and gas estimated to exist by examining geologic characteristics 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 22.3 billion barrels, and reserves of natural gas total 272.5 trillion cubic feet. Undiscovered technically recoverable oil in the United States is 139.6 billion barrels, and undiscovered technically recoverable natural gas is 1445.3 trillion cubic feet. The demonstrated reserve base for coal is 486 billion short tons, of which 261 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.