Petroleum, also called crude oil, is a thick, flammable, yellow-to-black colored liquid. Petroleum was first found oozing out of rocks on Earth's surface. Hence, its name comes from the Latin words petra, meaning rock, and oleum, meaning oil. Petroleum is a hydrocarbon, an organic compound containing only carbon and hydrogen. It is a mixture of other hydrocarbon compounds such as natural gas, gasoline, kerosene, asphalt, and, probably most important, fuel oil.
Today, most scientists agree that oil was formed from the remains of plants and tiny animals that settled to the bottom of ancient oceans. These remains or sediments were buried by layers of mud and sand. Gradually, over millions of years, the weight of these accumulating layers built up great pressure and heat. The sediments packed together and became rock. The organic (once living) remains were changed into kerogen, a waxy substance that forms oil and natural gas. Most of the world's petroleum is more than 100 million years old, and is thus called a fossil fuel.
Unlike coal, which stays in one spot unless moved by Earth's shifting crust, oil slowly migrates upward through cracks and pores, or tiny holes, in nearby rocks. Eventually the oil reaches a solid layer of rock and becomes trapped underneath in a reservoir (pool). Natural gas often occurs in association with oil. Most of the world's petroleum reservoirs lie deep underground in structures called anticlines—gently folded layers of rock that form an arch above the deposit. Petroleum can also be trapped by fractured layers (or faults), salt formations, and stratigraphic (rock) traps. Some oil is also contained in shales (clay) and sands.
Although petroleum is found throughout the world, the Middle East possesses nearly two-thirds of all recoverable oil. Latin America contains about 13 percent, while the continents of Europe, North America, Asia, and Africa have only 4 to 8 percent each. Most North American oil is extracted in Alaska, Texas, California, Louisiana, and Oklahoma. The former Soviet republics, Saudi Arabia, and China are among the world's other leading oil producers. Their petroleum is sent to the United States for refining. While the United States possesses little of the world's petroleum supply (it must import more than 50 percent of its oil), it is one of the world's leading refiners. It is also the world's heaviest consumer of oil.
Currently, petroleum is among our most important natural resources. We use gasoline, jet fuel, and diesel fuel to run cars, trucks, aircraft, ships, and other vehicles. Home heat sources include oil, natural gas, and electricity, which in many areas is generated by burning natural gas. Petroleum and petroleum-based chemicals are important in manufacturing plastic, wax, fertilizers, lubricants, and many other goods.
Different types of petroleum can be used in different ways. Refineries separate different petroleum products by heating petroleum to the point where heavy hydrocarbon molecules separate from lighter hydrocarbons. As a result, each product can be isolated and used for a specific purpose without waste. Thus, tar or asphalt, the dense, nearly solid hydrocarbons, can be used for road surfaces and roofing materials. Waxy substances called paraffins can be used to make candles and other similar products. And less dense, liquid hydrocarbons can be used for engine fuels.
The oil industry faces strong challenges. Environmental concerns are forcing companies to reevaluate all of their operations. Political unrest in the Middle East causes concern about access to oil supplies. And it is only a matter of time before oil supplies finally run out. According to some experts, that could be as soon as the mid-twenty-first century.