948 



Subsurface Geologic Methods 



stratigraphy. From the foregoing it is clear that a set of isolithic maps 

 with their opposing thin and thick sections will tend to cancel one another 

 out in an over-all isopach map of the same stratigraphic interval. In other 

 words, the isopach map may be generally featureless, whereas the series of 

 isolithic maps reveals much of the stratigraphic information sought. 



W. C. Krumbein, of Northwestern University, describes the ratio 

 method of lithofacies mapping in the American Association of Petroleum 

 Geologists Bulletin 10, Volume 32 (1948). As mentioned earlier, the ratio 



FACI E S 



Sandstone 

 (Brown) 



Siltstone 

 (Yellow) 



m 



Limestone 

 (Blue) 



Figure 510. Lithofacies map. 



method of mapping lithofacies is essentially the same as the percentage 

 method. However, the relative spacing of contours may vary greatly. 



Figure 511 is an isopach of a group of rocks consisting of sandstones, 

 shales, limestones, and anhydrites. This isopach is used as a guide for 

 drawing the lithofacies map of figure 512. Table 32 is a summary of the 

 control points, showing the total thicknesses, thicknesses of clastic and non- 

 clastic rocks, the clastic percentages and clastic ratios. The values in the 

 last two columns are obtained from the following formulas: 



Clastic ratio = 



Clastic percentage = 



sandstones + shales 

 limestones + anhydrites 

 sandstones + shales 



sandstones + shales + limestones + anhydrites 



In figure 512 the clastic percentages are shown by solid contours. 

 The clastic ratios are shown by dashed lines, and ratio intervals are in- 

 dicated by shading. The 50-percent contour is also the ratio 1.0 contour. 



In general the two methods are very similar, the principal difference 

 being that the percentage map is contoured on a regular contour interval 



