Figure 24-39. Maps A, B, and C of Figure 24-39 combined. 



the stratigraphic section is complex. Maps contoured on the basis of percentages, 

 ratios, or aggregate thicknesses of lithic classes do not take into account the 

 manner in which the individual units occur. As stated before, ten 5-foot units 

 have the same contouring value as one 50-foot unit. There are means of further 

 classifying a lithofacies map so that it yields the practical information desired. 

 The following example will illustrate a direct practical application of a litho- 

 facies map. 



The problem is to determine the amount of effective reservoir sand and its 

 relative worth over a broad area. It is assumed here that individual sands less 

 than 25 feet in thickness, even though several such units occur in the section, 

 are not worth exploitation. Since these thin units might coalesce to form more 

 attractive reservoirs, locally, they must be given some consideration. 



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

 nesses 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 characteristics are persistent over the region. Figure 24-40 shows an sand- 

 stone isolith prepared in this way. This isolith map includes sandstones as thin as 

 5 feet; but since the objective is to classify sands on the basis of a 25-foot thick- 

 ness, the next step is as follows: First, the percentage is computed 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, occuring 

 in beds of thicknesses 30, 25, 40, 35, 10, and 5 feet, the effective reservoir 

 sands are the first four, with a combined thickness of 130 feet. This is 87 per- 

 cent of the total aggregate thickness of sandstones. These percentage values are 

 posted on the isolith map, and percentage contours are sketched as shown by 

 dashed lines in the figure. In the case illustrated the interval used is 25 percent. 



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