Subsurface Maps and Illustrations 



897 



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 conditions that test the ingenuity of the subsurface geologist. 

 These examples will serve to show that all data obtained from wells, such 

 as formation tops, dips, thicknesses, and others, are subject to critical 

 examination and balancing, one against the other, before they may be 

 used with confidence in mapping. 



Structural Contour Map 



There is no fundamental difference between a surface structural map 

 and a subsurface one. Both attempt to show by means of contours the 



Figure 470. A — Core dips and stratigraphic intervals increasing with depth on 

 sharply folded anticline. B — Migration of hole downdip resulting in erroneous 

 core dips and stratigraphic intervals. 



configuration of a selected continuous stratigraphic horizon, commonly 

 called the "datum," or "datum horizon." As stated earlier, the principal 

 difference is in the kinds of data used in their construction. 



The subsurface structural map is almost or quite dependent on wells 

 for the necessary control. Ordinarily, the elevations on which the contours 

 are drawn are obtained by the simple process of subtracting the depth 

 to the datum horizon from the surface elevation of the well, the latter 

 being established at the point from which depth measurements are made. 

 This point in most cases is the rotary table or the rotary bushing of the 

 drilling rig. 



The block diagram, A, in figure 472 illustrates an anticline that is 



