954 Subsurface Geologic Methods 



thickness of sandstones. These percentage values are posted on the isolith 

 map, and percentage contours are sketched as shown by dashed lines in 

 the figure. In the case illustrated the interval used is 25 percent. The map 

 is somewhat easier to read if color or shading is used between the percent- 

 age contours. 



A sheet of tracing paper is now placed on the map and fastened with 



Figure 516. Isopach map used in construction of spalinspastic map of figure 518. 



drafting tape. The total thickness of effective sand is computed on the 

 basis of total aggregate thickness and percentage of effective sand, and 

 these values are plotted on the overlay. The overlay is then contoured as 

 in figure 514. This map shows by contours the thickness of effective 

 reservoir sands that might be expected over the entire region. 



Approximately the same result can be attained if only those sands 



