Subsurface Maps and Illustrations 



953 



The first step is to construct a sandstone isolith map. In tabulating 

 thicknesses from the logs, extremely thin beds are omitted. Sandstones 

 containing a great deal of clay or silt may also be ignored, particularly 

 if it is known that such characters are persistent over the region. Figure 

 513 shows a sandstone isolith prepared in this way. This isolith map in- 

 cludes sandstones as thin as five feet; but since the objective is to classify 

 sands on the basis of a 25-foot thickness, the next step is as follows: 



Figure 515. Isopach map showing reconstruction of thickness contours 

 over a young, truncated uplift. 



First, compute the percentage by total thickness of all sands that 

 occur in beds 25 feet or greater in thickness. Thus, if the total thickness 

 of sandstones in a well is 150 feet, occurring in beds of thicknesses 30, 25, 

 40, 35, 10, 5, and 5 feet, the effective reservoir sands are the first four, with 

 a combined thickness of 130 feet. This is 87 percent of the total aggregate 



