Figure 24-25 shows a well log consisting of alternating limestones, shales, 

 sandstones, and evaporites. The total thickness of this succession is 478 feet. 

 To the right of this log are four columns, each representing a lithologic class of 

 rocks. The first column contains only the sandstones transferred from the well 

 — all plotted in their correct thicknesses. The total thicknesses are 163, 160, 110, 

 and 45 feet, respectively ; of course, their sum is the thickness in the original log. 

 Now, in the column on the extreme right, these lithologic units are recombined 

 in the simplest possible manner: i.e., all of the units of one class, regardless of 

 the thickness of the individual members, are plotted as if they occurred as one 

 thick bed. Thus, the 13 members of the original log are reduced to 4 in the 

 analytic log. 



/ / n / N / w v V i 



Figure 24-26. Simplification of a complex section according to the 

 method shown in Figure 24-25 



Figure 24-26^4 is a stratigraphic cross section showing normal facies 

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

 above, the complex nature of the stratigraphy is simplified (for a specific pur- 

 pose) to the 3-unit section shown in B. 



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 24-27. 

 When the lithologic breakdown 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 this process may be used on maps in a great 

 variety of ways, a few of which are discussed briefly below. 



484 



