Introduction
In simple terms, conventional oil reservoirs can be described as in two (2) scenarios:
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Undersaturated Reservoirs
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Saturated Reservoirs
Undersaturated Reservoirs
An undersaturated reservoir contains pure liquid at reservoir conditions (I.e, pressure is above bubble point). As oil and dissolved gas are removed, the liquid expands to fill the void space. As soon as the reservoir pressure reaches bubble point, gas will start to evolve out of the oil in the reservoirs.
Saturated Reservoirs
A saturated reservoir contains both oil with dissolved gas and a gas cap. The oil is at its bubble point and the gas is at its dew point. Any decrease in pressure will cause gas dissolved in the oil to come out of solution.
Below is a nice illustration of reservoir with components both above and below saturation pressure