anomaly that combines with the structure to form the trap. It is in this phase, or 

 Phase II of the sequence, that half domes, open anticlines, terraces, noses, and 

 any irregularity in the regional dip become important, because such structure, 

 combined with any one of the many kinds of stratigraphic anomalies, may form 

 one of a wide variety of traps in which oil and gas pools have accumulated. 



The stratigraphic evidence for Phase II develops only as wells are drilled 

 and logs and subsurface data become available. Consequently most of the drill- 

 ing for stratigraphic traps is close to wells that have tested the crests of dome 

 folds, for it is only there that stratigraphic information is available by which 

 combination traps can be predicted. It is around the flanks of dome folds, more- 

 over, that arching or half domes occur down the pericline so that half of the 

 trap is known ; then it becomes necessary to locate only the other half — the strati- 

 graphic half — of the combination. 



For several reasons wells are sometimes drilled and pools discovered where 

 there is no local structure. One reason is that an error has been made in the 

 structural mapping, and no local fold or fault occurs where it is mapped. An- 

 other is that a permeable lens or reef is suspected. Leasing problems cause 

 many wells to be drilled without regard to structure, as for example where leases 

 are about to expire and no definite geological information can be worked out to 

 evaluate the lease even though the location is generally favorable. Large lease 

 ownerships, likewise, may justify drilling random wildcat wells to satisfy lease 

 requirements or to gain the stratigraphic information needed to proceed with 

 the exploration program. Each of such wells, even though nonproductive, adds 

 subsurface data that can be integrated with other data to work out combination 

 trap prospects or purely stratigraphic traps. 



Finally, after all structural anomalies that combine with stratigraphic fea- 

 tures to form traps have been tested, the purely stratigraphic traps remain. This 

 may be called Phase III. These traps require little or no local structural anomaly 

 to complete the trap and consist of such phenomena as sand patches and lenses; 

 shoestring, channel, and bar sands; coquina lenses; dolomitization patches; ir- 

 regularities along an up-dip wedge-out of permeability; variations in the perme- 

 ability; and a variety of permeable lenses surrounded by impermeable rocks. 

 Pools of this kind are discovered either by random drilling or by drilling based 

 on interpretations of precise subsurface stratigraphic data. 



The three phases and the kind of mapping called for may be listed as : 

 Phase I — Structural traps. Surface, subsurface, and geophysical mapping. 

 Phase II — Combination traps. Subsurface and geophysical mapping. 

 Phase III — Stratigraphic traps. Subsurface mapping. 



A sequence such as this does not have sharp time boundaries, and all three 

 phases may be operating to a varying degree in every region simultaneously. 

 Each phase, however, is seen dominating the exploration effort at some time dur- 

 ing the life of nearly all regions, and generally in the order listed above. Differ- 



10 



