The term reservoir implies storage. Reservoir rock, therefore, is that rock in which the
hydrocarbon can be stored and from which it can be produced. The fluids of the
subsurface migrate according to density with the dominant fluids in hydrocarbon regions
being hydrocarbon gas, hydrocarbon liquids and salt water. Since the hydrocarbons are
the less dense of these fluids, they will tend to migrate upward, displacing the heavier salt
water down elevation. Hydrocarbons may be forced from their source rock during
lithification, and migrate into the reservoir rock in which they are stored. The fluids
present will separate according to density as migration occurs.
