logic units in the section is determined for each well and these numbers are 

 plotted on a base map at the correct locations and contoured. Now when this 

 sheet is registered over an isopach map of the same stratigraphic interval, the 

 average thickness of the lithologic units can easily be determined at any loca- 

 tion. A modification and more practical application of the alternation map is 

 outlined below. 



The alternation rate or frequency is based on some predetermined strati- 

 graphic interval, as, for instance, 100, 500, or 1000 feet. The number plotted on 

 the map would then be the number of changes in lithology per 100, 500, or 1000 

 feet, as the case might be. The advantage in this method is that the rate of 



Figure 24-36. Isopach map used as a base for clastic-ratio map. 



495 



