through the fault plane and encountered the datum beneath the fault are en- 

 circled. 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 24-11. The map, B, in Figure 24-11 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 northwest quadrant is the lower trace, and the area between these 

 lines represents the horizontal displacement of the datum bed. 



The fine dashed lines numbered 2300 to 3200 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 fault, and the lower numbers 

 (in parentheses) are elevations of the fault plane. As in A of Figure 24-11, the 

 productive area is ruled. 



The curving of the datum contours beneath the thrust sheet, northwest of 

 the hidden syncline, suggests that some folding had taken place prior to the 

 faulting; therefore, the possibility of accumulation of oil in the upper edge of 

 the faulted flank beneath the fault might be anticipated, if the upturned edge of 

 the reservoir had been adequately sealed by the fault. 



The two examples of faulted structures illustrate the importance of contour- 

 ing the fault "planes" cutting productive structures. As stated previously, at 

 least three elevation points on the "plane" are required, and with only three 

 points for control, it is necessary to contour this surface as a true plane. If a 

 larger number of wells penetrate the fault, it may be possible to contour the 

 irregularities and undulations of the plane. 



Suggestions for Contouring 



There are a few rules for contouring a group of numbers on a map: 



1. Each contour line of given value must everywhere pass between those 

 points whose numerical values are higher and lower, respectively, than that of the 

 contour. For example, two points having elevations of 110 feet and 95 feet must 

 not lie on the same side of the 100-foot contour. 



2. No contour can cross over itself or any other contour. There are two 

 exceptions to this rule: overturned or recumbent anticlines, and reverse faults. 

 In practice the underside of a recumbent anticline and that part of the datum 

 lying below a thrust sheet are ordinarily omitted on a contour map because of 

 the confusion of lines that would result if these surfaces were contoured. Oc- 

 casionally it is desirable to show the relationship by contouring the "hidden" por- 

 tions with dotted or dashed lines. 



3. Two or more contours may merge into a single line only where the 

 datum is vertical or where faulting has displaced the datum along the strike by 

 an amount equal to or exceeding the contour interval. 



463 



