Method for making a probabilistic determination of total seal capacity for a hydrocarbon trap, simultaneously considering both capillary entry pressure and mechanical seal capacity, and where capillary entry pressure is estimated by relating it directly to the buoyancy pressure applied by the hydrocarbon column to the top seal. The method thus considers the substantial uncertainty associated with input parameters, which uncertainty limits the utility of such analyses for robust hydrocarbon column height and fluid contact predictions. The method disclosed for estimating seal capillary entry pressure, the requisite input parameter for capillary seal capacity analysis, by inverting trap parameters avoids the need for direct measurement by mercury injection capillary capacity tests on small pieces of rock, which test results often are not available for all desired locations nor are they necessarily representative of adjacent rocks in the seal.