be pointed out, though, that this artificial classification might yield values which 

 would be difficult to contour or which would give erroneous trends. By following 

 the procedure set forth, all significant sands are dealt with in the isolith map 

 so that more reliable thickness trends are established. The overlay map is then 

 contoured according to the general grain of the sandstone-isolith map. It is 

 assumed that the thick and thin trends of the selected reservoir sands will cor- 

 respond in a general way to the pattern of distribution of the entire sand content 

 of the section. 



In order to decipher the geologic history of a region in which major uplifts 

 and severe truncation have occured, it is often necessary to reconstruct by isopach 

 contours the original thicknesses of strata that have been entirely eroded away. 



Figure 24-41. Map showing aggregate thicknesses of reservoir sands more than 25 feet thick. 



501 



