Subsurface Maps and Illustrations 



939 



s 



s 



Total 

 Footage 



M7S 



163 



160 



ZEE 



LithoIog,fc Units 

 Re-combined 



Figure 500. Lithologic breakdown of a well log into four main rock types. 



changes from the edges toward the center of the basin. By the method 

 described above, the complex nature of the stratigraphy is simphfied (for 

 a specific purpose) to the three-unit section shown in B of figure 501. 



To digress momentarily, it is not necessary to replot a log in the 

 simpler form. The thicknesses of all the individual beds of limestone, for 

 example, are tabulated and then totaled for use on maps. A convenient 

 form for this purpose is shown in figure 502. When the lithologic break- 

 down of the well logs or surface sections has been made, the results are 

 recorded in the appropriate column as aggregate thicknesses, ratios, or 

 percentages, depending upon the mapping units to be used. Such a table 

 provides a permanent record of all computations of lithologic proportions. 



The lithologic values obtained by the process just described may be 

 used on n^aps in a great variety of ways, a few of which are discussed 

 briefly below. 



Ratio Maps 



The ratio contour map shows the ratio of the aggregate thickness 

 of one lithologic class to that of the remaining classes that go to make 

 up the complete section. For example, a sandstone ratio map shows the 

 ratio of sandstones to all other rock types. Thus the ratio value of sand- 



