17
Second,
there should be a porous rock nearby in which oil can
accumulate (it is often sandstone). If the holes in the
rock are interconnected, then oil can flow easily out of the rock.
This condition is called
permeability
. The porous rock must have
good permeability, ­which is why studying the structure of
rocks is an important step to finding oil.
Third,
there is usually a ‘cap rock’ or seal to trap
the oil in the underground reservoirs and prevent it
from seeping to the surface. Within these reservoirs,
hydrocarbons are typically organized like a
three-layer cake—with a layer of water
below the oil and a layer of
gas above it.
Much of the oil that escapes to the
surface often evaporates into the air.
But it can leave behind deposits of
residual hydrocarbons called
bitumen
.
Underground
Oil Accumulation
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