Oil is known as mineral oil or crude oil, and is formed
from plant and animal matter that has broken down and been subjected to
extremes of temperature and pressure over millions of years.
The resulting mix of hydrocarbons (hydrogen and Carbon)
is a very valuable resource.
Over time, the whole chalk structure breaks down, the
space is then filled with gas that breaks out and as the pressure depletes
more, the gas expands along with additional gas from the oil to fill all the
pore space where oil was removed.-The third thing that can take the place of
the oil removed is the rock itself.-With a natural water drive or water flood,
water will displace the oil and generally double the recovery factor.
Oil is found in rocks like sandstone or limestone called
'reservoir rocks'. The oil is filling the pore space or the fractures BUT OIL IS NOT THE ONLY ONE TO DO THIS. Once
you reach a certain depth there is no
air in the pore of the rock. The pores are full with fluid, either fresh water
(usually at surface), salty water and brine (deeper), oil, gases (So when you
produce oil, the 'nearest fluid' to replace the oil should be oil or gas.
Once a reservoir is not exploited anymore, there is still
oil inside, This percentage of oil depends but it is usually between 30 to 70%.
We leave a lot of oil in the ground ! The remaining space within the pores will
be containing water or gas but it will NEVER content air.
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