ABSTRACT

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 (Berg, 1970), and then to compute the capillary pressure by an estimate of fluid properties.

An example of oil column calculation is the Milbur field, Burleson County, Texas, a lower Wilcox stratigraphic trap. Using core analysis for a nearby well, an oil column of 40 to 70 feet would be expected for the trap, and this estimate agrees reasonably well with actual oil columns of 60 to 75 feet for the field (Chuber, 1972). 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 oil reservoir may occur down dip from dry holes with porous water sand and oil shows, rather than up dip at the pinchout.