A trap for hydrocarbons requires the simultaneous existence of (a) a reservoir, (b) an isolated region of low potential in the reservoir, and (c) a barrier (or seal) with high enough entry pressure to retain a commercially producible volume of hydrocarbons. Three kinds of traps exist--structural, stratigraphic, and hydro-dynamic. All three kinds have a reservoir bounded by a barrier but differ in what causes the isolated area of low potential. In classification of hydrocarbon accumulations, the conditions that determined the present location of the accumulation should be used where they can be ascertained.

In the stratigraphic-trap classification suggested here, primary emphasis has been placed on usability--i.e., will the groupings help in the search for new hydrocarbon accumulations, and is the suggested terminology simple and descriptive enough to be accepted? A classification using the time relations between barrier and reservoir was considered and rejected.

The suggested classification starts with the simple concept that stratigraphic traps are adjacent to unconformities or they are not. For traps that are not adjacent to unconformities, the reservoir and barrier may be (I) primary (depositional, usually facies-related) or (II) wholly or in part secondary (diagenetic). Those traps in contact with unconformities may be (III) below the unconformity surface or (IV) above it, or (V) both below and above it. This approach uses some of Levorsen's ideas and eliminates some inconsistencies in his classification. Subdivision of these four major classes (facies-change traps, diagenetic traps, traps below unconformities, and traps above unconformities) allows more precise description of the different types of traps.