384 



Subsurface Geologic Methods 



it contains. The relationship for sands with water saturations higher tlian 

 about fifteen percent is approximately 



where S = fractional water saturation. 



Ro = resistivity of sand 100-percent water-saturated. 

 R = resistivity of sand partly saturated with oil or gas. 



Since the factors Rg and R can be obtained from the electric log, a pro- 

 cedure is available for using the log as a tool in quantitative reservoir 



Figure 167. Common characteristics of an electric log in limestone. 



study. In order to facilitate the use of the relationship expressed above, 

 the equation has been placed in graphic form in figure 168. Actual quan- 

 titative log interpretations are only as accurate as the data on which they 

 are based. The resistivity values for Rg and R must be obtained practi- 

 cally unaffected by invasion of the drilling fluid, the salinity of the fluid, 

 the hole diameter, or the thickness of the bed. Such resistivities are com- 

 monly termed "true resistivities" as contrasted to "apparent resistivities," 

 a value related to true resistivity but affected by the factors noted above. 

 Experience has indicated that the long normal and lateral curves are gen- 

 erally adequate to minimize the effect of drilling-fluid invasion and hole 



