sistivity of the water in its pores. For all formations that do not contain shaly 

 material, the formation factor is a constant, whatever the resistivity of the 

 water. 



m 



U) CD 



5 10 

 POROSITY 



6 8 10 

 % WATER 



20 30 40 60 100 



SATURATION S v 



Figure 14-3. Porosity 

 tor graph. 



formation fac- 



Figure 14-4. Water Saturation — re- 

 sistivity index graph. 



It has been found experimentally that the formation factor can be express- 

 ed with reasonable accuracy in terms of porosity by the equation 



F = a/<j> m (1) 



F = formation factor 



$ = porosity 



m ■=. an exponent called cementation factor, which varies with the 



lithology 

 a = coefficient 



Several values have been proposed for a and m , but the resulting curves 

 showing F versus (f> do not differ much. 



In the construction of numerous graphs for log interpretation, the formula 

 proposed by W. 0. Winsauer and others (1952) has been used: 



F = 0.62/^ 2.15 (fig. 14-3) (2) 



Limestones often contain fissures and vugs caused by tectonic stress and 

 leaching circulating waters. It is often advisable for these limestones to use 

 relationships determined by local observations instead of the above formula. 



270 



