associated with them. The other is that in many regions the geologic section is 

 lacking in stratigraphic horizons that can be traced continuously over wide 

 areas; and, if the formation immediately under the unconformity can be recog- 

 nized from well samples throughout large regions, it is only logical to select it as 

 a regional mapping horizon. For example, the "top" of the Mississippian is 

 commonly contoured in parts of the Midcontinent region despite the fact that it 

 is an erosion surface, because it can be easily recognized in well samples and 

 drillers' logs. 



In the construction of subsurface structural maps of oil fields that have not 

 been entirely defined, it is often of the greatest importance to work out care- 

 fully even minor details, such as the exact character of faulting, in order to 

 avoid the drilling of unnecessary dry holes and to make certain that all potential- 

 ly productive locations are tested. 



The structure map in Figure 24-10 shows a partly developed oil field with 

 a number of oil and gas wells and dry holes. A normal fault dipping to the 

 southwest cuts across the southwest end of the anticline. The structural datum 

 is the top of the producing horizon. 



Although a fault is commonly represented on maps as a single line, a 

 normal fault with a low-dipping plane invariably results in a zone where the 

 datum surface is absent. This zone is called a datum gap. The breadth of the 



■*«"-*— ' 



Figure 24-8. Structure contour map on top of Cambrian. 



460 



