be attained, however, if there is sufficient well control for constructing a Cam- 

 brian isopach map. 



Disconformities are frequently not recognizable in well samples or on 

 electric and radioactivity logs; therefore, so-called regional structural maps may 

 not truly represent the regional structure. The departure of the contouring from 

 that of a true structural picture is controlled by the thickness of section eroded 

 away, the relief of the unconformity, and the average rate of regional dips. 

 This discrepancy is greatest where the dips are low and the relief on the un- 

 conformity is relatively high. It is purely a matter of chance that certain wells 

 strike the horizon of the unconformity at high points and others at low. Figure 

 24-9 shows an example where four wells fail to reveal the presence of a structure 

 because the wells on the high part of the structure happened to strike locally low 

 points on the unconformity, whereas those structurally lower encountered locally 

 high points on the erosion surface. 



Sometimes a subsurface disconformity can be recognized by the types or 

 condition of the rocks in the drill cuttings; but commonly the sample work is 

 not done with sufficient care to differentiate these materials, or the diagnostic 

 materials may be absent at the particular point where the well was drilled. 

 There are two principal reasons for contouring unconformities: one is that a 

 vast amount of oil has been found in the zones of unconformities and directly 



Figure 24-7. Structure contour map on top of Jurassic. 



459 



