380 



Subsurface Geologic Methods 



Figure 163. Log illustrating effect of resistive beds of various thicknesses on normal 



curves. 



amination. The latter problem has been discussed in detail by Doll,^^ and 

 reference to his paper will indicate that permeable-zone boundaries in an 

 otherwise resistive and nonpermeable formation can be determined with 

 accuracy by a careful study of the SP log in conjunction with the resistivity 

 diagram. 



The Location and Exact Depth of All Formations — ^A stratigraphic log 

 is obtained indicating the presence and depth position of all formations. 

 This practically eliminates the possibility of passing up a potentially pro- 

 ductive oil- or gas-bearing formation. 



Sand Studies — Changes in the physical characteristics of reservoirs 

 can be studied, aiding the solution of many exploration and production 

 problems. 



Determination of Thicknesses — ^The thicknesses of all formations can 

 be determined, and the net producing thickness of sand reservoirs can be 

 calculated. 



Fluid-Content Determinations — It is possible in most cases to dis- 

 tinguish between an oil or gas reservoir and a water-bearing formation; in 

 many sand reservoirs a quantitative determination of the percentage of 

 void space containing oil or gas or, conversely, the percentage of void 

 space containing interstitial water can be made. 



" Doll, H. G., op. eit. 



