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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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