drilled on a sharply folded anticline. Both core dips and drilled stratigraphic 

 intervals increase with depth. Figure 24-45 shows a hole that drifts down the 

 dip on a monocline, with erroneous increases in core dips and drilled intervals 

 similar to those on the flank of the anticline. Deviation surveys used in con- 

 junction with the core dips reveal the true subsurface conditions so that the 

 formations can be correctly mapped. 



Figure 24-5^4 shows a gradual thinning of two formations in the central 

 portion of the stratigraphic succession. Core dips from the two wells would 

 show a gradual increase with depth through the converging portion of the 

 section. Dips below the thinning portion are steeper than those above, but 

 remain constant as deep as the strata are parallel. 



The two wells represented in B of Figure 24-5 would suggest a similar con- 

 vergence in the drilled intervals. There is one important difference, as shown by 

 core dips: the dips above and below the inclined part of the hole are the same. 

 Core dips from the straight hole on the right are constant throughout the appar- 

 ently converging interval. Thus, where regional conditions are well known 

 through an adequate distribution of subsurface control, it is sometimes possible 

 to infer correctly that a portion of one hole is crooked, even though a deviation 

 survey has not been made. 



It is outside the scope of this chapter to describe in detail the causes and 

 effects of crooked holes. The preceding examples are only a few of countless 



A 



Figure 24-4. A — Core dips and stratigraphic intervals increasing with depth on sharply 

 folded anticline. B — Migration of hole downdip resulting in core dips and stratigraphic 

 intervals similar to those in A. 



456 



