Most basins contain facies changes, unconformities with resulting truncated beds, and buried erosional or constructive surfaces such as reefs, hills, channels, barrier sand bars, and other such phenomena--which form the basic requirements for the creation of subtle traps.

If folding, normal faulting, thrusting, and the formation of salt ridges and domes are added to the picture of an evolving but continuously filling basin, the resultant structural and stratigraphic patterns become much more complex. However, no matter how complex the history, those stratigraphic relations and lithologic changes which are conducive to the formation of stratigraphic, unconformity, and paleogeomorphic traps remain.

When hydrocarbon is expelled (primary migration) by pressure and heat from sediments which contain source material into adjacent reservoir rocks, it migrates through carrier beds (secondary migration) into sealed reservoirs, or traps. As long as the conditions necessary for secondary migration of a substantial amount of petroleum exist, migration will continue along strike and updip until all migrating hydrocarbons are either trapped in the subsurface or have escaped at the surface. As the petroleum moves, it will be captured by all traps--stratigraphic, unconformity, paleogeomorphic, structural, or a combination of these--which are in the path of migration.

Because paleogeomorphic, unconformity, and stratigraphic traps are related (1) to older geologic surfaces, (2) to the location of strata on and directly below an unconformity surface, and (3) to lithologic changes within and laterally adjacent to a stratum, it is suggested that, in general, the conditions which produce most subtle traps are present before development of structural traps. If migration of hydrocarbons through a particular region were to take place before structural movements, all petroleum trapped during this early migration would be in subtle traps.

Because subtle traps generally are formed as a result of constantly recurring depositional patterns which usually precede, or may be associated with, contemporaneous structural movement, petroleum basins probably contain more subtle traps than structural traps. Although much petroleum has migrated into structural traps, possibly more has accumulated in the earlier (and contemporaneously) formed subtle traps. Because subtle traps probably contain the large undiscovered domestic reserves needed for the future, explorationists must make the purposeful search for such traps an essential and substantial part of their exploration policy.