Our energy resources include petroleum and petroleum products, coal, uranium (nuclear reactions), and geothermal resources. At present, about 90 percent of the energy needs of the United States are supplied by coal, oil, and natural gas. Petroleum is important also in the production of plastics, asphalt, and thousands of related products.
Fossil fuels. Fossil fuels are oil, natural gas, and coal. The general term petroleum includes both natural gas and crude oil. Crude oil is a liquid containing hydrocarbons (molecules made from only hydrogen and carbon) that forms in organic‐ or fossil‐rich sediments and rocks. The hydrogen and carbon in the oil comes from the breakdown of the organic material over time. Natural gas is a gas that contains hydrocarbons and that usually occurs with crude oil.
Petroleum forms in marine sedimentary rocks that contain abundant organic remains from microscopic organisms such as algae. Continental shelves contain basins that capture thick accumulations of organic debris. This material lithifies into a source rock that is buried by overlying sediments, and the resulting increased pressure and temperature conditions convert the organic material into hydrocarbons.
In response to the confining pressure, petroleum moves outward and upward along zones of increased permeability into a reservoir rock. Reservoir rock, such as sandstone or limestone, has the high porosity and permeability necessary to hold large accumulations of petroleum. The petroleum migrates into a trap (either structural or stratigraphic) in the reservoir rock. Structural traps include faults between permeable and impermeable rocks, thrust faults, and folds such as anticlines. For example, petroleum will collect in a porous limestone reef below the contact with an overlying, impermeable unit such as shale, forming a pocket. A salt dome results when a bed of rock salt is under pressure; the salt extends upward plastically through a sedimentary sequence, disrupting the sediments and creating open spaces that trap petroleum. Stratigraphic traps are naturally occurring changes in a sedimentary sequence that trap migrating oil and gas, such as a porous reef structure in a limestone unit. A sandstone unit surrounded by shale is another stratigraphic trap. The occurrence of oil pools in a number of traps in one area is called an oil field.
Crude oil and gas are recovered from oil fields through a series of drilled wells. The petroleum may rise to the surface through the well as a result of its high confining pressure, or it may need to be pumped. Also, water or steam can be pumped into the oil pool from the surface to increase the pressure on the oil and its viscosity. The oil is shipped to a refinery and separated into natural gas, gasoline, kerosene, other oils, and asphalt. A huge variety of petrochemicals produced from petroleum are used in nearly every manufactured product we depend on today, including plastics and synthetic rubber. As the more easily discovered oil fields are pumped dry, oil companies have moved offshore to undertake risky and high‐cost exploration drilling along continental shelves across the world.
With only about 5 percent of the world's population, the United States annually consumes over a quarter of the world's total oil production. At present, the United States has only a twenty‐five‐year supply of oil remaining and imports nearly half of the oil it uses. Similarly, natural gas reserves in the United States are expected to be depleted within thirty‐five years. Future sources for natural gas will include gas trapped in coal beds.
Oil sands (tar sands) are sandstone deposits that have been cemented with tar or asphalt (blackish, solidified residues from petroleum). Famous deposits include those of the La Brea Tar Pits in Los Angeles, California, and the Athabasca Oil Sand in Alberta, Canada. Oil sands are strip‐mined and processed. Venezuela also has large reserves of oil sand. Heavy crude is a dense, viscous petroleum that flows so slowly it is usually left behind in an oil field. Efforts are continuing to extract this material, including pumping in steam or other solvents to make the crude less viscous.
Oil shales are organic‐rich shale formations from which oil can be extracted. The shale formed from muds on the bottom of large shallow lakes. Oil shales tend to be low‐grade and difficult and costly to mine. New technologies are being used to explore ways to extract the oil from the rock in place, including heating the rock with microwaves to separate the oil. The United States has large oil shale resources in Montana, Utah, Colorado, and Wyoming, but at present they are not feasible to mine. Oil shales will ultimately be exploited when the cost of finding new oil fields gets too high.
Coal is a dark‐colored sedimentary rock that contains a high percentage of organic plant material. Coal is representative of warm, lush, swampy environments and usually contains abundant plant fossils. Different kinds of coal result from different degrees of compaction and depth of burial. Peat is unlithified organic material that is solid enough to be cut into blocks and burned for fuel. Burial and increasing pressure and temperature convert peat into a soft, brown coal called lignite. Continued pressure results in subbituminous coal and bituminous coal, which are soft, black, banded, and sooty. Metamorphism converts these varieties of coal into anthracite, a hard, black, shiny form of coal that is dust free. Coal beds, or seams, range in thickness from a few centimeters to nearly 30 meters. Coal is mined using underground, open‐pit, and strip‐mine methods. The United States has an impressive coal resource of nearly four trillion tons and consumes or exports about a billion tons a year. Most of the coal in the United States is produced in Kentucky, West Virginia, and Pennsylvania.
Coal once provided nearly all of the United States' energy needs. That figure has dropped to less than 25 percent because of the abundance of petroleum, oil, and natural gas and the negative environmental effects of burning coal. Coal is an important ingredient in manufacturing steel. Oil and gas can also be produced from coal. New, cleaner ways of using coal are being researched as the country's petroleum reserves are being depleted.
Uranium and geothermal sources. Uranium is used to generate nuclear power. It is found in the minerals pitchblende and carnotite, which are mined from sandstone deposits in the western United States and Canada. Lower‐grade uranium also occurs in organic black shales and phosphate deposits. Nuclear generators are used to produce electricity. Nuclear power supplies about 8 percent of the United States' energy needs.
Geothermal sources can also generate electricity but represent less than 1 percent of the energy consumption in the United States. More geothermal power may be used in the future, especially if deeper heat sources across the nation can be located and exploited.
Renewable resources. Research is continuing on developing energy from renewable resources—power from sources such as wind, sun, running water, waves, and ocean‐currents and from burning hydrogen from the breakdown of water. The obvious attraction is that these resources are renewable, and they will become more important as our supplies of nonrenewable resources continue to dwindle.