Theoretically, petroleum traps in preferentially water-wet (hydrophilic) rock depend on the capillary properties of the rocks, hydrocarbons, and water involved. The boundary of a reservoir rock is a barrier to hydrocarbon migration because displacement pressure of boundary rock exceeds displacement pressure of reservoir rock. The differential (capillary) pressure between hydrocarbons and water in the reservoir rock varies directly with thickness of the hydrocarbon column; when and where capillary pressure exceeds difference in displacement pressure, migration of excess hydrocarbons results; these variables determine trapping capacity of the boundary. Fault boundaries are subject to these same variable conditions which determine the degree to which a fault is sealing or non-sealing. Hypothetical examples are discussed.