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Oil has
been a defining force in the 20th
century. Oil has shaped political
boundaries, defined the outcome of
battles and wars, created and disrupted
economies, and remade the urban
environment. It has also taken a heavy
toll on the environment, through air
pollution and oil spills. We are burning
through our supplies of oil so quickly
that oil may become a uniquely 20th
century phenomenon. American oil
production is fading fast, even as
demand rises. Worldwide reserves of oil
could supply 40 to 60 years of
consumption at current rates. By the
middle of the 21st century, world oil
supplies could be dwindling.
Environmental pressures may lead to
restrictions on its use before then.
How Oil Is Formed
Oil is formed from ancient plants and
animals deposited since the Cambrian
period 500 million years ago. This
organic matter settled on the sea floors
of that era, mixing with and being
covered by sedimentary rocks, mostly
sandstone and limestone. Over many
years, deposits of more sediments,
tectonic plate movements or volcanic
activity buried these deposits under an
impermeable layer of stone or
mud.Trapped under high pressure, with no
oxygen present, bacteria broke down the
dead plants and animals into
hydrocarbons, such as coal, oil and
natural gas. Once formed in this
sedimentary layer, oil can be squeezed
out or washed out by underground water
flows. Since natural gas is less dense
than oil, and oil is less dense than
water, the gas tends to float on top,
with oil under it, supported by
water.Oil is not actually in pools
underground; it is held in permeable
rock, such as sandstone or ancient coral
reefs, like a sponge. Gas, oil and water
would rise to the surface over time, but
they are held underground by a “cap
rock” such as a layer of shale. Oil and
gas become concentrated in areas where
there are high spots, or domes, under
the layer of cap rock.The most common
type of oil trap occurs in an
“anticline.” Anticlines are formed when
the cap rock is buckled by geological
movement, as when mountains are formed.
The most recent major anticline
formation was 20 million years ago, when
the Alps, the Himalayas and the Rocky
Mountains
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