900 



Subsurface Geologic Methods 



was drilled. A question likely to arise is "Why contour an unconformity 

 when so many hazards exist?" There are two principal reasons: One is 

 that a vast amount of oil has been found in the zones of unconformities 

 and directly 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 recognized 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 



Figure 473. Structure contour map on top of Jurassic. 



of the Midcontinent area, 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 carefully even minor details such as tlie exact character of 

 faulting in order to avoid drilling of unnecessary dry holes and to make 

 certain that all potentially productive locations are tested. 



The structure map in figure 476 shows a partly developed oil field 

 with a number of oil and gas wells and dry holes. A normal fault dip- 

 ping to the southwest cuts across the southwest end of the anticline. The 

 structural datum is the top of the producing horizon. 



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



