When oil was struck in the Forties Field under the North Sea in 1969, it led to the discovery of at least 350 million tons of oil. However, by the year 2020, the world's known oil reserves are due to run out. By then, new oil fields will need to be found, probably in more and more inaccessible places. Prospectors looking for oil look for sedimentary basins which could be oil-bearing, magnetic surveys and gravity surveys are often used. All rocks are magnetic, but the magnetism varies slightly from one rock to another, giving geologists clues to the structure and type of rocks that lie underground. Other clues include the density of the rock. When the production wells have been drilled and lined with casing, a perforating gun is lowered down them to drive explosive charges through the casing and cement and into the rock beyond to allow the oil to get into the wells. As oil is extracted, pressure may be maintained by injecting water or gas into the reservoir rock to displace the oil towards the production wells. Even with the help of modern techniques, however, such as electrical and mechanical pumps, it is seldom possible to extract more than 30%% to 50%% of the oil in a field. Perhaps a means will be found to get all the oil out of a "dig." If so, millions more gallons would be available out of the wells which have previously been drained "dry."
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