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) having high enough entry pressure to retain a commercially producible volume of hydrocarbons. Three kinds of traps may exist--structural, stratigraphic, and hydrodynamic. All 3 kinds have a reservoir bounded by a barrier, but differ in the cause of the isolated area of low potential. In classifying hydrocarbon accumulations, the conditions that determined the present location of the accumulation should be used where these conditions can be determined.

In the stratigraphic trap classification suggested here, primary emphasis is placed on usability; i.e., will the groupings help in searching 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 but rejected.

The suggested classification begins 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 either be (I) primary (depositional, and usually facies related) or (II) wholly or in part secondary (diagenetic). Those traps in contact with unconformities may either be (III) below the unconformity surface or (IV) above it. This approach uses some but not all of Levorsen's ideas and eliminates some inconsistencies in his classification. These 4 major classes (facies, diagenetic, below unconformities, and above unconformities) can be subdivided with appropriate terminology to identify the different types.

End_of_Article - Last_Page 360------------