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sponge and dip it in water, the sponge will soak up the water. The
water that is soaked up is absorbed into pore spaces within the
sponge. The sponge has porosity. If we now squeeze the sponge,
we will close up some of these pore spaces and water will be
forced out of the sponge.
In the same way as the sponge, certain types of sedimentary
rocks can also be porous and contain oil and gas within pore
spaces. It may not be obvious that a solid piece of rock can act like
a sponge, but if we were to take a piece of porous sandstone (a
type of sedimentary rock) and weigh it before and after we allowed
it to sit in a pail of water, we would find that it weighed more after.
This is because it would have soaked up some of the water, just as
a sponge would. We cannot squeeze the water out with our hands
because we are not strong enough, but you can imagine that if the
rock could be squeezed by some giant hands without being
crushed, some of the water would come out [7].
Porous rocks that contain oil and gas buried thousands of
metres below the earth's surface are being squeezed in this fashion
by the weight of the overlying rock and the water contained in that
rock. That is why the oil flows out of the rock and up to the
surface when the rock is penetrated by a well.

Porosity = 48% Porosity = 27 % Porosity = 14 %

Figure 3.5 – Porosity

Permeability
For oil or gas to be able to flow freely through a rock, that
rock must possess permeability as well as porosity.

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