896 



Subsurface Geologic Methods 



mapping. The core dips should always be adjusted by means of hole 

 deviation or directional surveys when available. When straight-hole de- 

 terminations have not been made, core dips should be used with caution. 

 In figure 470, A shows a straight hole drilled on a sharply folded anti- 

 cline. Both core dips and drilled stratigraphic intervals increase with 

 depth. In figure 470, B shows a hole that drifts down the dip on a mono- 

 cline, with erroneous increases in core dips and drilled intervals similar 

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

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

 formations can be correctly mapped. 



Figure 469. Effects of crooked holes on datum elevations and interpreted dips. 



The two wells represented in B of figure 471 would suggest a sim- 

 ilar convergence in the drilled intervals. There is one important dif- 

 ference, 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 apparently 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 por- 

 tion of one hole is crooked, even though a deviation survey has not been 

 made. 



Now examining figure 471 further, A 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. 



