906 Subsurface Geologic Methods 



The two examples of faulted structures illustrate the importance of 

 contouring 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 on 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 re- 

 verse 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. Occasionally it is desirable to show the relation- 

 ship by contouring the "hidden" portions 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. 



A map can be contoured so that all of the technical requirements 

 just described are fully satisfied, yet fail to convey the probable structural 

 conditions. Such a map is shown in A of figure 478. There are no tech- 

 nical errors in the contouring of this map, but it fails to give a con- 

 sistent picture of structure. On the west side of the map, the strike is 

 east and west, but the dip varies from very low in the north to steep in 

 the central portion and back to low in the south. In the central part of 

 the area there is no consistency in the structural features in that contours 

 are pinched together in some places and widely spaced at others. The 

 east side shows a constantly changing dip and strike. Although it is 

 quite possible for such structural conditions to exist, it is not probable. 



In figure 478, B shows the same control points contoured in a manner 

 that reveals two plunging anticlinal noses, two synclines, and a well- 

 defined terrace. This sheet was contoured, not to tie the widely separated 

 control points together in the simplest manner, but rather to develop the 

 forms of any geologic structures that might be suggested in the variations 

 in the rate of dip or changes in strike. In other words, this map bears 

 the unmistakable marks of geologic interpretation of the data. 



A knowledge of the general character and form of structures in the 

 region aids greatly in correctly interpreting the subsurface structure where 

 the well control is sparse. When the character of folding is known, an 



