Oil columns can be calculated for simple stratigraphic traps if the rock and fluid properties are known or can be estimated. Because oil migration is prevented by capillary pressure in small pores of the trap facies, direct measurements of capillary pressure allow oil columns to be calculated, but such measurements are rare. An alternative is to determine pore size from porosity and permeability data using an empirical equation, and then to compute the capillary pressure by an estimate of fluid properties.

An example of oil-column calculation is from the Milbur field, Burleson County, Texas, a lower Wilcox stratigraphic trap. Using core analysis from a nearby well, an oil column of 40-80 ft would be expected for the trap, and this estimate agrees reasonably well with an actual oil column of 60-75 ft for the field. The most important part of such calculations is the realization that the trapping facies itself can have significant porosity and permeability and yet form an effective barrier to oil migration. The result is that the best reservoir may occur downdip from dry holes with porous water-bearing sandstone and oil shows, rather than updip at the pinchout.

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