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. 



KINDS OF MAPS There is no fundamental difference between 



a surface and a subsurface structural map. 

 Structural Contour Maps Both attempt to show by means of contours 



the 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 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 24-6 illustrates an anticline that is typically 

 eroded in the surface formations. In the same figure the block is separated to 

 show a buried major nonconformity, the presence of which is suggested nowhere 

 in the surface geology. 



The structural contour map in Figure 24-7 is based on surface elevations and 

 dips in the area shown as a block diagram in Figure 24-6. The structural map in 



Figure 24-5. A — Actual convergence of section. B — Apparent convergence as a result of 

 crooked hole. 



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