Barrons AP Environmental Science

beneath a dome-shaped or folded layer of nonporous rock (e.g., limestone). In
other formations, the oil is trapped at a fault or a break in the layers of the crust.
Natural gas is usually present just below the nonporous layer and immediately
above the oil. The oil is released from this formation by drilling a well and
puncturing the limestone layer. The oil is usually under such great pressure that
it flows naturally, and sometimes with great force, from the well. However, in
some cases, this pressure later diminishes so that the oil must be pumped. Once
the oil has been collected, it is sent to a refinery. Refining involves separating the
components of oil by their boiling points (e.g., gasoline, heating oil, diesel oil,
asphalt).
Up to 70% of Earth’s global crude oil reserve has already been depleted, and
it is estimated that there is about a 50-year supply of oil left on Earth, with
countries in the Middle East owning about half of what’s left. The United States
owns 3% of the world’s oil reserves but uses 30% of the oil extracted in the
world each year. Increased competition for foreign oil by China and India
increases the world’s cost of oil.
Oil imports in the United States have decreased in the last few years because
of energy-efficiency improvements, higher fuel economy standards, and new
techniques for extraction (e.g., fracking). Of all known oil reserves, 65% is
found in 1% of all fields—primarily in the Middle East.