The Petroleum Industry
  Petroleum is one of the most valuable natural resources in the world. Some people call petroleum black gold, but it may be better described as the lifeblood of industrialized countries. Fuels made from petroleum provide power for automobiles, airplanes, factories, farm equipment, trucks, trains, and ships. Petroleum fuels also generate heat and electricity for many houses and business places.
  The petroleum industry is one of the world's largest industries. It has four major branches. The production branch explores for oil and brings it to the surface. The transportation branch sends crude oil to refineries and delivers the refined products to consumers. The manufacturing branch processes crude oil into useful products. The marketing branch sells and distributes the products to consumers. Gasoline service stations handle the largest share of these sales. Oil companies sell their petroleum products directly to factories, power plants, and transportation-related industries.
  The petroleum industry plays a large role in the economy of many nations. In such developed countries as the United States and Canada, it provides jobs for a great many people. It also is a major buyer of iron, steel, motor vehicles, and many other products. In certain developing but oil-rich countries, petroleum exports furnish most of the national income. Petroleum is also a source of political power for such countries because many other nations depend on them for fuel.
  In addition to fuels, thousands of other products are made from petroleum. These products range from paving materials to drip-dry fabrics and from engine grease to cosmetics.
  Although we use a huge variety of products made from petroleum, few people ever see the substance itself. Most of it comes from deep within the earth as a liquid called crude oil. Different types of crude oil vary in thickness and color, ranging from a thin, clear oil to a thick, tarlike substance. Petroleum is also found in solid form in certain rocks and sands.
  The word petroleum comes from two Latin words meaning rock and oil. People gave it this name because they first found it seeping up from the earth through cracks in surface rocks. Today, petroleum is often referred to simply as oil, and most of it is found in rocks beneath the earth's surface.
  People have used petroleum for thousands of years. But few people recognized its full value until the 1800's when the kerosene lamp and the automobile were invented. These inventions created an enormous demand for two petroleum fuels, kerosene and gasoline. Since about 1900, scientists have steadily increased the variety and improved the quality of petroleum products.

How Petroleum Was Formed
  Most geologists believe petroleum was formed from remains of organisms that died millions of years ago. This organic theory of petroleum formation is based on the presence of certain carbon-containing substances in oil. Such substances could have come only from once-living organisms. The process that produced petroleum also produced natural gas. Thus, natural gas is often found in association with crude oil or dissolved in it.
  According to the organic theory, as the organisms died, their remains settled to the bottom of the ocean. As the sediments became buried deeper and deeper they were subjected to increasingly high temperature and pressures, and so were compressed to form sedimentary rock.
  Geologists believe this movement may have been caused by the presence of water in the rock. Water, which is more dense than oil, could have pushed the oil upward. Another possible cause was the weight of the overlying layers of rock, which would tend to squeeze the oil into holes and cracks in the rock.
  The most common types of petroleum traps are anticlines, faults, stratigraphic traps, and salt domes. An anticline is an archlike formation of rock under which petroleum may collect. A fault is a fracture in the earth's crust, which can shift an impermeable layer of rock next to a permeable one that contains oil. Most stratigraphic traps consist of layers of impermeable rock that surround oilbearing rocks. In a salt dome, a cylinder or cone-shaped formations of salt pushes up through sedimentary rocks, causing the rocks to arch and fracture in its path. Petroleum may accumulate above or along the sides of such a formation.

Where Petroleum is Found
Most crude oil lies in underground formations called traps. In a trap, petroleum collects in the pores of certain kinds of rock. Gas and water are also present in most traps. The most common types of traps are anticlines, faults, stratigraphic traps, and salt domes.