Oil pools are valuable underground accumulations of oil, and oil fields are regions underlain by one or more oil pools. When an oil pool or field is discovered, wells are drilled into the ground. When the well reaches a pool, oil usually rises up the well because of its density difference（密度差） with water beneath it or because of the pressure of expanding gas trapped above it. Although this rise of oil is almost always carefully controlled today, strong natural flows of oil were common in the past. Gas pressure gradually dies out, and oil is pumped from the well. Water or steam may be pumped down neighboring wells to help push the oil out.

　　As oil becomes increasingly difficult to find, the search for it is extended into more unfriendly environments. The development of the oil field on the North Slope of Alaska and the construction of the Alaska pipeline are examples of the great expense and difficulty involved in new oil discoveries. Offshore drilling platforms extend the search for oil to the ocean’s continental shelves. More than one-quarter of the world’s oil and almost one-fifth of the world’s natural gas come from offshore, even though offshore drilling is six to seven times more expensive than drilling on land.

　　Of course, there is far more oil underground than can be recovered. Even given the best exploration techniques, only about 30 to 40 percent of the oil in a given pool can be brought to the surface. The rest is far too difficult to reach and has to remain underground.

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