(From Grolier Multimedia Encyclopedia)


Petroleum is so critical to industry, agriculture, transportation, and communication systems throughout the world that its cost and availability have become vital factors in the economic health of entire nations. It is little wonder that this valuable nonrenewable resource is sometimes called black gold.

Early Uses

Although the first commercial oil well was not drilled until 1859, petroleum has been collected for thousands of years wherever it seeped to the surface of Earth. Archaeologists have discovered that it was used by the Persians nearly 6,000 years ago as mortar in buildings and as glue in other applications. The Greek traveler and historian Herodotus, who lived in the 5th century B.C., detailed the uses of petroleum in the Persian Empire. He described the process of drawing oil from shallow surface wells, commenting that it was "dark and evil-smelling."

Throughout the region that is now called the Middle East, there were a great many seepages of petroleum. These were sometimes ignited by chance and became the "eternal fires" of the Persian fire worshipers. The Persians also used petroleum in warfare, shooting arrows tipped with burning oil into the ranks of their enemies. In the Bible, the substance called "pitch," which Noah used in building the Ark, was probably crude petroleum.

The Venetian traveler Marco Polo visited the oil fields of Baku on the Caspian Sea toward the end of the 13th century on his way to the court of Kublai Khan, the great Mongol emperor of China. Polo told of "a fountain from which oil springs in great abundance, inasmuch as 100 shiploads might be taken from it at one time," and added that "this oil is not good to use with food, but it is good to burn." He also mentioned that the oil was applied as an ointment to cure camels of mange, a skin disease. By the middle of the 20th century, Baku, which is now the capital of Azerbaijan, became one of the world's great oil-producing fields.

The early explorers of North America found oil oozing from the ground or floating on the surface of water in many places. Native Americans rubbed their bodies with oil, believing that it toned their muscles and made them active and quick. European colonists who settled in the eastern part of the continent also used the oil. Sometimes they gathered it by soaking blankets in the seepages and then squeezing the oil out and catching it in vessels. They also skimmed floating patches of oil from the surface of water. The amounts obtained by such means were small, and much of it was sold at a high price for medicinal purposes. Peddlers offered it under such names as Seneca oil or Indian oil, and claimed that it would cure rheumatism when rubbed on externally, and have therapeutic effects for an array of other ailments when taken internally.

First Wells

In 1846, Canadian geologist Abraham Gesner discovered that a flammable oil could be distilled from coal. Gesner named his invention kerosene, and formed a company to manufacture it. The company was so successful that other coal-oil manufacturers soon entered the business. In a series of landmark experiments in 1855 in Pennsylvania, Professor Benjamin Silliman of Yale College showed that kerosene could also be made from crude oil. Suddenly people had a great deal of interest in petroleum.

As demand for petroleum increased, owners of salt wells that had been ruined by petroleum contamination began to market their oil. Many such brine wells were located around Titusville, Pennsylvania. A group of businessmen decided to drill specifically for oil there, making use of the derricks and other equipment already employed in the salt industry. The group engaged a retired railroad conductor, Edwin L. Drake, to take charge of the enterprise. When Drake's well, powered by an old steam engine, struck oil in the summer of 1859, the petroleum industry was born. Within a few years, so much oil was being pumped from the ground around Titusville that the price fell, temporarily, to as little as 10 cents per barrel (one barrel contains 42 gallons, or 159 liters).

Composition and Origin

Petroleum is a thick, dark, combustible liquid made up mostly of hydrocarbons—compounds containing only hydrogen and carbon. The hydrocarbon content of petroleum ranges from 50 percent to 98 percent. The remainder of it is composed chiefly of organic compounds containing oxygen, nitrogen, or sulfur.

How was petroleum formed? According to one widely held theory, the remains of countless small marine animals and plants dropped to the ocean bottommillions of years ago and were gradually covered by mud. Over the course of time, many layers of plant and animal remains and mud accumulated. These sediments were subjected to great pressure and heat, and were often squeezed and distorted as Earth's crust moved. Gradually, they were converted into layers of sedimentary rock.

Under heat and pressure, the plant and animal remains within the sedimentary rock were eventually transformed into petroleum and natural gas. The dynamics of this transformation are not well understood. Some scientists theorize that a portion of the hydrocarbon content of the plant and animal cells was preserved after the plants and animals decayed, and that, in time, petroleum was formed from those hydrocarbons. Others believe that bacteria removed oxygen, sulfur, and nitrogen from the matter and converted it into a petroleum-like substance containing hydrocarbons. Perhaps both processes were at work.

Today the oceans have receded from many of the areas in which these great transformations originally took place millennia ago. In other areas, rock formations containing petroleum products still lie far beneath the waters of the seas.

Where Most Oil Is Found

With the exception of a few seepages or springs, most petroleum deposits are located far underground. They are contained, together with brine and gas, in porous, spongelike layers of rock, such as limestone and sandstone, that can be tapped only by drilling. Most deposits are contained in a trap that encloses a petroleum reservoir, preventing the petroleum from escaping. Gas, oil, and water within the trap form three distinct layers, with the gas at the top and the water at the bottom. The upper boundary of a reservoir trap is known as caprock. It is always impermeable. The lower boundary is called the oil-water contact.

Most of the world's oil deposits occur in a type of trap known as an anticline—an arching fold of stratified rock. A deposit may also be trapped by a fault—a fracture in Earth's crust—when a porous layer has become hemmed in by nonporous layers. The formations called salt domes are often associated with petroleum deposits. They consist of intrusive bodies of rock salt that have forced their way through the overlying sedimentary rock, forming a dome.

Natural gas and water are often trapped with the oil. Sometimes the pressure of escaping gas forces the oil to the surface of Earth when a drill reaches a trap. Wells of this sort are called gas-driven. In water-driven wells, the water in the trap forces the oil upward. Until oil engineers devised a means of controlling the upward pressure, a great deal of oil was wasted when a successful drilling operation created a geyser of petroleum called a gusher.

Other Deposits

Petroleum can also be extracted from oil shale, a compact sedimentary rock. In the process called destructive distillation, the shale is first crushed, and then heated in a furnace kept free of air. The temperature is kept high enough to stimulate the chemical decomposition of the shale. The principal products of this distillation are oil, gases, and water solutions of organic acids and other substances. Scotland and Australia produce some oil in this way.

Major shale deposits have been discovered in Brazil and in the United States in Colorado, Wyoming, and Utah. Experts estimate that these deposits could yield more than 2 billion barrels of oil. Unfortunately, obtaining oil in this way requires that large amounts of shale be mined and processed—a costly endeavor that can potentially harm the environment. Economical mining methods are being investigated, but large-scale commercial facilities are not yet operational in the United States.

Another form of oil deposit that cannot be tapped by ordinary methods is tar sand. Tar sands are mined and then washed in hot water to remove the tar from the sand. The tar is then heated and broken down, or cracked, into simpler molecules that are upgraded and blended to produce synthetic oil.

The two largest known tar-sand deposits in the world are the tar belt of eastern Venezuela and the Athabasca tar sands in the northern part of Canada's Alberta province. The Athabascan region contains one of the largest known deposits of petroleum in the world. Geologists estimate that perhaps 700 billion barrels of oil could potentially be recovered from the region. Suncor, a Canadian energy company, currently operates a commercial oil plant near Fort McMurray, Alberta.

Locating the Resource

The amount of oil obtained from oil shale or tar sands by the distillation process represents as yet but a small fraction of the world supply. Most oil is recovered by drilling through rock into various traps. It is an expensive procedure, causing the price of oil to vary according to the depth at which it is found and the hardness and thickness of the rock that must be drilled to reach it. A single well can cost several million dollars. Drillers therefore need to be reasonably sure that oil is under the spot where they erect their rigging. A geologic prospector helps obtain this information.

Geologists study Earth to determine which areas are likely to be petroliferous, or petroleum-bearing. Surveys of various kinds are made, and regions are mapped using aerial photography. Devices such as gravity meters, magnetometers, and seismographs are also employed to locate oil-bearing rock formations deep underground.

Gravity meters measure differences in the pull of gravity at the surface of Earth. Porous rocks tend to decrease gravitational pull, so a low reading on a gravity meter can indicate the presence of oil-bearing porous rocks. Different kinds of underground rocks also affect Earth's magnetic field, which can be measured on a magnetometer. Seismographs, used primarily for measuring and locating earthquakes, can also be helpful in locating oil domes or pockets under Earth. Geophysicists create miniature earthquakes by setting off charges of dynamite, and then use portable seismographs to determine the speed at which echoes return through each kind of rock. Scientists can then chart underground rock formations to determine which areas are worth drilling.

To check even more closely on the nature of the rocks and the presence of oil, gas, and water, scientists run further tests while the well is being drilled. This research is done by means of an electric device that is lowered into the well. It sends an electric current through the surrounding strata, recording differences in resistance to the current that indicate the presence of oil. In spite of these and other sophisticated scientific tools, many drilling operations are unsuccessful.

Drilling Methods

Early petroleum wells were drilled by punching into the ground with a heavy, sharp bit, or cutting tool, attached to the end of a cable. Using this method, called cable-tool, or percussion, drilling, the bit was repeatedly raised and dropped until the necessary depth was reached. Cable-tool drilling was widely used until about 1920, but it could not drill deep wells, and was ultimately replaced by a method called rotary drilling.

When engineers sink a modern oil well using the rotary method, they first build a platform to hold machinery and pipe connections. A steel framework tower called a derrick is then erected. The derrick, which is used for raising and lowering drilling equipment out of and into the well, may be as tall as a 17-story building.

In rotary drilling, a hole is bored by a bit that is attached to a hollow drill pipe. The bit and pipe are connected to a large, flat wheel, or turntable, resting on the floor of the derrick. The turntable is power-driven to rotate, thus turning the pipe and bit. A variety of bits are used; all are large, heavy, and hollow through the center, and all are made of extremely hard steel. As the bit turns and cuts, penetrating deeper into the ground, new sections of pipe are fitted to the top. When oil has been reached, the drill pipe and bit are removed.

To keep the rotating steel drill bit from overheating, a stream of mud is constantly forced from the surface down through the drill pipe and the hollow center of the bit. The mud is a mixture of clays, chemicals, and water.

After the mud passes through the hollow center of the bit, it returns outside the bit and pipe to the surface, carrying rock shavings with it. This mixture plasters the walls of the hole, helping to prevent cave-ins. When the gas, oil, and water are finally reached, the mud holds back the pressure of these liquids so that their flow can be more readily controlled.

Oil Flow

As the borehole goes deeper into Earth, its sides are lined with steel pipe called casing. Each length of pipe fits into the one above it as the well extends farther into the ground. Then, when the borehole reaches the depth where oil lies, a special kind of tubing, about 2 inches (5 centimeters) in diameter, is lowered through the casing until it runs the entire length of the well. The space between the tubing and the casing is sealed, forcing the oil to go through the tubing to reach the surface. Valves and meters attached to the top of the tubing control and measure the flow of the oil. The natural pressure of the oil and gas usually makes a newly drilled well flow adequately. If, as sometimes happens, there is inadequate natural pressure, pumps help raise these substances to the surface.

As the oil and gas rise toward the surface, the pressure decreases, causing the two substances, which have been in solution, to separate. At the top of the well, the mixture passes into a separator that completes the process by piping the gas-free oil into gathering tanks. If little gas is in the solution, it is often burned off at the well. Larger quantities are gathered and piped from the well to a natural-gas plant, where the natural gas is processed and broken down into various products. Some of these gases are occasionally sent back into the oil wells to keep up the pressure, while some are used to make carbon black, or are fed to main transmission pipes for use as fuels for cooking, heating, and other purposes.

Most oil fields include a number of large storage tanks clustered together in what is called a tank farm. The tanks vary in capacity from a few hundred barrels to as much as 80,000 barrels. Other groups of storage tanks are located at key points along pipelines, at ports where oil is loaded onto tankers, and at the refineries where crude oil is processed into various products for the market.

Transporting Oil

Transportation is a vital factor in the petroleum industry. In earlier days, when refineries were always located near oil fields, various forms of oil were transported in barrels by wagons, barges, and railways. In recent years, however, great oil fields have been developed in regions far from the centers of population and industry. As a result, the crude oil taken from these fields must be delivered to refineries by pipeline if it must be transported overland, or by tanker or barge if by water.

The only way to transport petroleum across large bodies of water is by oil tanker. Petroleum from the oil-producing U.S. states along the Gulf Coast is often carried by tankers to East Coast ports. Lake tankers transport oil from Canadian pipeline terminals through the Great Lakes. Great strings of barges float down the inland waterways of the United States, bound for southern and western refineries.

Most transport of petroleum across the oceans is performed by huge tankers carrying as much as 1 million barrels of petroleum each. While such supertankers are less expensive to operate than smaller tankers, they often require special deepwater ports offshore. The oil is then piped from these ports to tank farms onshore.

Tankers are built with many safety and fire-prevention devices, and use sophisticated navigational equipment. But accidents do occur, and those involving very large tankers often cause widespread environmental damage. In March 1989, the tanker Exxon Valdez ran aground and spilled some 11 million gallons (42 million liters) of crude oil into the waters off Alaska. In late 2004, the collision of two cargo vessels at the mouth of the Pearl River led to the largest oil spill in China's history. Though damage to each ship was minimal, 450 tons of oil oozed into the South China Sea, creating a slick some 11 miles (17 kilometers) in length and up to 650 feet (200 meters) wide.


Pipes of one sort or another are the most widely used and economical way to transport petroleum overland. The major oil-producing nations have built pipelines totaling hundreds of thousands of miles. In the United States, a famous example is the Little Big Inch. Built during World War II to carry oil from Texas to New York, it now transports natural gas. The 800-mile (1,300-kilometer)-long Trans-Alaska Pipeline System (TAPS) brings oil from Prudhoe Bay in northern Alaska to Valdez in the south.

In Canada, the Edmonton–Great Lakes line extends about 1,900 miles (3,060 kilometers) from Alberta to Superior, Wisconsin. In mid-2005, a Canadian company and two U.S. energy firms announced plans to construct a new oil pipeline from Wyoming to Utah.

Pipelines often raise controversy because of the potential environmental risks. In the building of the TAPS, thousands of square miles of wilderness were made accessible. In 2005, the U.S. government approved oil drilling in Alaska's Arctic National Wildlife Refuge. Once the measure is implemented, it may take years for the oil to become available for public use. Opponents insist that petroleum deposits in the refuge are small, and that the inevitable network of pipelines and roads will disturb a fragile wildlife habitat.

Sabotage is also a frequent concern. Attacks on Iraqi pipelines in 2004 reduced the country's output and caused a sharp increase in oil prices.


Crude petroleum must be refined before it can be used for fuel and other purposes. The basic steps in the refinery process—distillation and cracking—require elaborate complexes of towers, tanks, pipelines, and chemical-engineering facilities.

Distillation separates crude oil into a variety of hydrocarbon groups, or fractions, by boiling them until they vaporize. Some petroleum liquids boil at temperatures below 68° F (20° C); others require temperatures of more than 600° F (316° C).

In the widely used fraction-distillation process, crude oil is pumped through a pipe that winds around a heated chamber. By the time the oil has reached a temperature of about 645° F (340° C), much of it has turned to vapor. The vapor-liquid mixture then passes into a cylindrical steel tower called a fractionating tower, where the vaporized oil is separated into different fractions, or groups. Among these fractions are gasoline, kerosene, heating oil, and lubricating oil.

Fractionating towers are as high as 100 feet (30 meters), and are equipped from bottom to top with a series of perforated horizontal trays spaced from 10 to 24 inches (25 to 60 centimeters) apart. Steam is introduced into the bottom of the tower to make the separation of the different fractions easier.

The hot oil vapor cools as it rises through the perforated trays on its way to the top of the tower. The hydrocarbons with the highest boiling point condense on the bottom trays, while those with lower boiling points condense in trays higher up the tower. The heaviest fraction—the so-called residue—is drawn off as a sluggish liquid from the bottom of the tower for use as heavy fuel oil or as asphalt. Higher up on the column, lubricating oil condenses on the trays and is led off in liquid form. Farther up the fractionating tower, liquids with lower boiling points—liquids such as fuel oils, light heating oil, light diesel fuel, and kerosene—condense; high-octane gasoline collects at the very top. Uncondensed gases are piped from the top of the fractionating tower for further processing.

Cracking increases the amount of gasoline that can be obtained from crude oil. This process actually breaks the molecules in the heavier part of crude oil into the lighter molecules required for gasoline. After the cracking process is complete, the oil is redistilled to isolate the additional gasoline.

Several methods of cracking have been developed. In thermal cracking, the bonds between carbon atoms in molecules are broken by the action of heat alone. This method was developed in about 1912 and is still employed. The major process used today, however, is catalytic cracking, in which a finely granulated catalyst is used. (A catalyst is a substance that can accelerate a chemical reactionto a rate not possible otherwise.) Petroleum vapors are mixed with the catalyst and piped to a unit called a reactor. There, under high temperature, a reaction occurs that breaks down heavier hydrocarbons into lighter ones. The spent catalyst then passes to another unit—the regenerator—where it is cleansed of carbon deposits. The catalyst can then be reused.

Researchers have discovered numerous ways to take hydrocarbon molecules apart and rearrange them to fit particular purposes, such as forming special kinds of gasoline. In one process, called isomerization, carbon and hydrogen atoms are rearranged to make gasoline engines run more efficiently. In another process, called hydrogenation, hydrogen atoms are added to molecules deficient in this element. Alkylation is a method used to combine molecules of natural and cracked refinery gases. In polymerization, smaller gaseous molecules are united to form larger ones—the reverse of cracking.

Petroleum Derivatives

Once crude petroleum has been refined into its component parts, it forms the raw material of a surprising variety of substances. Tens of thousands of products of virtually every description are derived, directly or indirectly, from crude oil.

Common Types


 Almost 50 percent of the total yield of crude oil goes into the production of gasoline, making it the petroleum product in greatest demand. Automobiles consume about 90 percent of all gasoline produced; the remaining 10 percent is used by airplanes, tractors, and various other types of motorized equipment. Fuel for jet planes is primarily a mixture of gasoline and kerosene.

American drivers burn about 382 million gallons (1.4 billion liters) of gasoline daily. Environmental concerns as well as a fear of oil embargoes imposed by foreign producers have led many in the United States to seek the development of alternative fuels and energy sources—such as ethanol, biofuels (liquid fuels created from organic material such as wood or crops), natural gas, and electricity—to help reduce the transportation sector's reliance on petroleum. In California and elsewhere, gasoline is increasingly being replaced or supplemented with methanol (an alcohol-based fuel made from coal or wood), butane, and propane. Butane and propane are derived both from crude oil and from natural gas, a fuel that is plentiful in the United States.


 In 1909, this light and volatile liquid fuel represented 33 percent of the total volume of petroleum production. Today, production has plummeted to only a tiny percentage. Originally, kerosene was used chiefly as a lamp oil. It now more often serves as fuel for cooking, space-heating installations, and farm equipment. In the United States, Canada, and many other nations, kerosene is primarily used in jet fuel. Many developing countries rely on kerosene for lighting and heating.

Diesel fuels.

 The American Petroleum Institute reports that, in the past few decades, demand for diesel fuel has increased annually by about 11 percent. Diesel fuel powers ships, locomotives, motor vehicles, and power plants.

Light fuel oils.

 Light oils are used as fuel in automatic household-heating burners, in small commercial-heating units, and in various industrial applications, including smelters.

Residual fuel oils.

 The heavy fractions of crude petroleum are viscous, or slow-flowing, fuels. They are used in specially designed burners for commercial heating, for marine and railroad steam engines, and other purposes. Residual-oil fuels—the least costly of the petroleum fuels—account for about 3 percent of the total volume of petroleum production.


 Lubricants make up a rather small percentage of the total of petroleum products. They are extremely important, however, since the moving parts of most machinery require constant lubrication. Specialized lubricants have been developed for general-purpose machinery, steam turbines, textile-mill spindles and looms, steam-engine cylinders, and for every other place where a wheel, gear, or shaft rotates. Perhaps the most familiar application of lubricants is as motor oil in automobile internal-combustion engines.


 These heavy lubricants are important in servicing hard-to-reach bearings in high-temperature operations and in bearing housings that cannot be made oiltight. They are also frequently used where the dripping or splashing of fluid lubricants might contaminate products.


 Extracted by chilling, filtering, and washing lubricating-oil fractions, wax is used primarily in packaging. It serves to waterproof and vaporproof milk containers and wrappers for breads, cereals, and frozen foods. Wax is also used in the preparation of molds for dentures, for the casting of intricate parts of machinery, and for numerous other applications.


 Asphalt is obtained as a residue from the refining of crude oil. It is used primarily for roofing and road surfacing. In the United States, 94 percent of all roads and highways are surfaced with asphalt.

Petroleum coke.

 A by-product of cracking and destructive-distillation processes, petroleum coke is most often used as a fuel, but it has other applications as well. It serves in the refining of various metals, the manufacture of calcium carbide (from which acetylene gas is made), and the production of abrasives and materials with high heat resistance.

Carbon black.

 This by-product of the cracking process is used primarily in the manufacture of automobile tires and other rubber products. It is also a pigment ingredient in printing ink, paints, phonograph records, and other products.

Liquefied petroleum gases.

 These by-products of the refinery process are stored and handled under pressure to keep them in a liquid state. They are used widely as cooking fuels in areas not served by utility systems, and to operate refrigerators, water heaters, space heaters, and furnaces. They are often added to manufactured gas in order to enrich it. Liquefied petroleum gases are frequently used on farms to heat incubators and brooders, to sterilize milking utensils and other equipment, to dry fruits and vegetables, and to prevent frost damage.


 A great number of organic chemicals and some inorganic chemicals are derived wholly or in part from crude petroleum and natural gas. Among the most important organic petrochemicals are ethylene, propylene, butylene, isobutylene, cyclohexane, and phenol. The inorganic petrochemicals include ammonia and hydrogen peroxide.

Secondary petroleum products.

 These include numerous products derived from petroleum, and their number is increasing yearly. Most synthetic detergents are made from petroleum products. Two of the more important ingredients in the manufacture of synthetic rubber are the petroleum-derived chemicals butadiene and styrene. Among the synthetic fibers made from petroleum or its derivatives are nylon, Orlon, Dacron, Dynel, and Acrilan. These synthetic fabrics are all considered plastics. Other plastic products, including polyethylene "squeeze bottles," adhesives, plastic-based paints, garden hose, draperies, upholstery, luggage, and piping, are also derived from petroleum. Petroleum also yields such secondary products as floor waxes, furniture polishes, disinfectants, antiseptics, shampoos, vanishing and cold creams, hand lotions, lipstick, rouge, nail lacquers, polish removers, ointments, and various drugs.

Distribution of Petroleum Products

Some refineries in the United States distribute their products directly to their customers. In many other cases, wholesalers buy from refineries in bulk quantities and market petroleum products to resellers and consumers. Such "bulk-plant operators" receive shipments by truck, pipeline, tanker, or barge. They keep supplies on hand in storage tanks and operate extensive transportation equipment. Bulk-plant operators supply service stations, commercial consumers, public utilities, transportation companies, and factories. In rural areas, they deliver oil products to farms by tank truck from the bulk plant or terminal. Retail distribution of oil products such as gasoline, diesel fuel, and motor oil is carried out on a truly vast scale by service stations throughout the country.

World Production and Reserves

The United States was the world's biggest oil producer for almost 90 years. The nation was self-sufficient in petroleum, producing enough for its own needs and exporting to other countries as well. In 1948, the United States became a net petroleum importer—buying more petroleum from other nations than it exported. The 1973 Arab oil embargo and subsequent increases in oil prices led the United States to step up production of domestic oil. The Alaskan pipeline, opened in 1977, carried almost one-fifth of the total amount of petroleum produced domestically over the course of the past 25 years.

According to the U.S. Department of Energy (DOE), the nation's dependence on foreign oil is growing. Imported crude now represents nearly 50 percent of U.S. petroleum consumption, and that figure is expected to increase in the coming years. By 2025, petroleum from outside sources is projected to meet 70 percent of the total U.S. demand. As of 2007, U.S. oil imports originated mainly from Canada, Mexico, Saudi Arabia, and Venezuela. Several other nations, including Nigeria, Algeria, and Iraq, also supply the United States with oil.

The petroleum in the United States and abroad is being consumed far faster than it is being formed. The natural petroleum-making process takes thousands of years. In practical terms, petroleum must be considered a nonrenewable resource.

The DOE predicts that world oil consumption will increase by about 1 percent annually between 2006 and 2030. Although no one is certain just when the planet’s proven oil reserves will be exhausted, many experts predict that at the present rate of consumption, the pace of oil depletion may soon overtake the discovery of new supplies. Global petroleum demand has pushed production capacities to their limits.

At present, the total world oil reserves are estimated at about 1.3 trillion barrels. Of this amount, Saudi Arabia is the world leader in petroleum supply, with about 262 billion barrels, followed by Canada with 179.2 billion. The United States ranks 11th on the list, with about 21.8 billion barrels.

Many observers have noted that conservation would help stretch Earth's steadily dwindling oil supply. Certain petroleum products can be derived from coal and natural gas, but clean-burning, renewable alternate sources may be the answer to the world's long-term energy needs.