Figure 24-40. Sandstone isolith contours with shading according to percentage 

 of sands more than 25 feet thick. 



The map is somewhat easier to read if color or shading is used between the 

 percentage contours. 



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

 ing 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 24-41. 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 adjudged 

 to be of adequate thickness are considered in tabulating values from the logs. 

 In this instance the isolith map itself is the effective reservoir map. It should 



500 



