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The capacity of rock to contain petroleum is termed
porosity. The ability of the rock to permit petroleum flow is
known as permeability. The forces causing the oil migration are:
-compaction of sediments as depth of burial increases;
-diastrophism (crustal movement causing pressure
differentials and consequent subsurface fluid movement);
-capillary forces (causing oil to be expelled from fine pores
by preferential entry of water);
-gravity (promotes fluid segregation according to density
differences).
Commercial crude oil and gas reserves are found mainly in
sedimentary rocks (sands, sandstone, limestone and conglomerate).
In igneous and metamorphic rocks oil occurs rarely and as a rule,
has no commercial significance. Natural accumulations of oil and
gas in the earth’s interior are called oil and gas reservoirs. An oil
bearing reservoir (also called formation) is usually bound at the
top and the bottom by impermeable rocks (clays) that do not allow
oil and gas to penetrate into other strata.

The Reservoir
If you wanted to find a large supply of water, where would
you look? You would, of course, go out and look for a lake or a
pond. But, have you ever thought of why ponds and lakes are
located where they are?
To understand why water collects where it does, we must
consider the forces that are acting upon it. The force of gravity
makes water run downhill. Therefore, water collects in low bowl-
shaped depressions in the land. These bowl-shaped depressions
provide traps in which water collects and become lakes and ponds.
Fortunately for us, there are many such natural water traps or
reservoirs that conveniently store large amounts of water for our
use [5].
In a similar fashion, when we look for oil, we must look for
places that oil is likely to accumulate in large quantities. To
understand where oil will collect, we must consider the forces that
are acting on droplets of oil buried deep within the earth.

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