Closure in stratigraphic traps and subtle combination traps is created by stratigraphic, lithologic or hydrodynamic variations in the sedimentary section. Both tend to be poly-seal traps, which rely on low horizontal permeability in updip seals for entrapment. High-permeability interbeds can compromise seals. Closure in structural traps is created by folding and faulting. Many are one-seal traps, in which top seals with low vertical permeability control entrapment. Hence, it has often been claimed that structural traps are capable of supporting larger hydrocarbon columns than stratigraphic/subtle traps and that large stratigraphic/subtle traps must be located in areas with gentle dip, where small hydrocarbon columns can produce areally extensive accumulations. Although these concepts are entrenched in the literature, they have never been rigorously tested.

To test these hypotheses, key geologic parameters for 204 stratigraphic/subtle traps were compared to those for 558 structural traps. Although this evaluation confirmed that stratigraphic/subtle traps tend to occur in areas with gentler structural dip and have lower net pay and larger productive areas than structural traps, there was little discernible difference in the size distribution of hydrocarbon columns between the two trap types. The stratigraphic/subtle traps were then subdivided into two groups, those with small hydrocarbon columns (<500 ft) and those with large hydrocarbon columns (>500 ft). The two groups were evaluated to determine the key factors controlling hydrocarbon column height. Differences in seal lithology and thickness, main trapping mechanism, and structural setting account for the differences in hydrocarbon column height between the two groups.