902 



Subsurface Geologic Methods 



100 feet. The difference in elevation between wells 2 and 1 is 500 feet; 

 therefore, five spaces are laid off along that line. Now, starting with the 

 first mark on each side of the highest well (4,600 feet), strike lines are 

 drawn as shown in the figure. These lines are contours on the fault plane, 

 and the rate of dip of the fault plane is revealed by the spacing of the con- 

 tours. This process assumes a true plane, which may not actually be 

 correct. 



When the fault-plane contours have been drawn on the map, the 

 structural contours are carefully sketched to points where they intersect 

 fault-plane contours of the same values. These intersections, as shown on 

 the map, mark the boundaries of the datum gap. 



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Apparent Dfp^ 



Figure 475. Effects of unconformities on interpretation of subsurface structure. 



Although the wells used in this illustration penetrated the fault plane 

 within the datum gap, this condition has no bearing on the solution of the 

 problem. The only requirement is three elevation points in the form of 

 a triangle on the fault plane. 



In the drawing, well 5 is on the 3,500-foot contour of the fault plane 

 and the datum elevation is 4,150 feet. Therefore, the fault will be en- 

 countered 650 feet below the datum at this location. 



In figure 477, A shows an anticline cut by a high-angle reverse fault. 

 The productive area on the upthrown block is ruled. The seven wells that 

 irilled through the fault plane and encountered the datum beneath the 

 fault are encircled. Three of these wells produce oil from the downthrown 

 block. Saturated portions of the producing formation above and below 

 the fault are shown in the cross section, C, of figure 477. The map, B, in 

 figure 477 deals with the part of the datum in the downthrown block. 

 The heavy dashed line in the central part of the area is the upper trace 

 of the thrust sheet. The heavy dashed line in the northweet quadrant is 

 the lower trace, and the area between these lines represents the horizontal 

 displacement of the datum bed. 



The fine dashed lines numbered 2,300 to 3,200 are contours on the 

 fault plane, which is shown as a true plane, since all of the contours are 

 straight lines. The solid-line contours are on the datum bed below the 

 fault plane. The upper numbers at the wells are datum elevations below the 



