The pressure contrast between the hydrocarbon gradient and the water gradient at a given depth in the reservoir (height above free water) is the buoyancy pressure (P<sub>b</sub>). The longer the column, the higher the P<sub>b</sub> at the top of the column. | The pressure contrast between the hydrocarbon gradient and the water gradient at a given depth in the reservoir (height above free water) is the buoyancy pressure (P<sub>b</sub>). The longer the column, the higher the P<sub>b</sub> at the top of the column. |