Oil and gas volumes are controlled by top-seal capillary properties, spillpoints, and trap geometry. The top-seal capillary properties and seal capacity can be estimated from the equivalent grain size (EGS) method. The EGS method uses an experimentally derived relationship between pore-throat size, porosity, and grain size to evaluate seal capacity. A pure spillpoint-limited trap is one in which the hydrocarbon column height is determined solely by the spillpoints. The observed hydrocarbon column in this trap is less than that which can be held by top-seal capacity. This trap type will be dominated by gas. In a capillary and spillpoint mixed trap, where both oil and gas can be filled down to the spillpoint, both top-seal capacity and spillpoint control relative oil and gas column heights. A pure capillary-limited trap is that where the oil and gas are not filled down to the spillpoint.

Top seal and spillpoint have been the focus of seal analyses; however, a case study for fields referred to as AN and YA in this chapter demonstrates an important relationship between trap geometry and top-seal capacity. These two fields have the same top-seal capacity, but the total column heights, as well as the relative oil and gas columns, are very different. This is explained by the different ratios of the base area to its relief in the two fields. The ratio of the area to its relief of the AN field is smaller, whereas that of the YA field is much larger. Given the same top-seal capacity, a trap with a higher area-to-relief ratio can hold a larger gas column because the oil pushed down by the migrated gas reduces its column height remarkably.

Thus, the EGS method can provide new insights into understanding hydrocarbon fill patterns in fields and prospects, including fault traps.