Shales and clays are porous and usually impregnated with mineralized 

 water. They have, therefore, an appreciable conductivity. On the other hand, 

 the size of the pores is so small that practically no movement of fluid is possible. 

 Accordingly, the shales, whether deposited in thin laminations or dispersed in 

 the interstices of the sand grains, contribute to the conductivity of the formation, 

 without contributing to its effective porosity. The relation between formation 

 resistivity and porosity becomes more complex than that for clean formations. 



The argillaceous materials present within permeable formations such as 

 sands, are often designated under the descriptive expression of conductive solids. 



Because of the additional conductance due to the presence of interstitial 

 shale, the ratio of formation resistivity to water resistivity (i.e., the formation 

 factor) changes when the resistivity of the water changes. Nevertheless, if the 

 shale content is not too great, it has been observed experimentally that this 

 ratio is almost constant for low values of water resistivities. Accordingly, the 

 formation factor measured when the pores contain highly mineralized water 

 can be considered, at least to a first approximation, as a characteristic parameter 

 of the formation that is related to the effective porosity through equation 2, as 

 in clean sands. 



Evaluation of Formation Resistivity to Fluid Saturation 



When a part of the pore space is occupied by an insulating material such 

 as hydrocarbon, the resistivity of the rock increases with respect to the value 

 it would have if it were 100 percent water bearing. The resistivity of such rock 

 is a function of the relative proportion of hydrocarbons and connate water in 

 the pores. The parameter generally used in well-logging interpretation is the 

 water saturation, S w , i.e., the fraction of pore volume occupied by water. 



The water saturation in a reservoir rock in turn depends on many factors: 



A. Characteristics of the rock {porosity, permeability, surface areas of 

 grains, etc. ) . 



B. Characteristics of the fluids present {viscosity, density, etc.). 



C. Height above the water table of the level under study. 



For substantially clean formations, the relation between formation resis- 

 tivity and fluid saturation can be expressed by the following equation: 



S„ = {R /R t ) lln (fig. 14-4) (3) 



S w = water saturation (proportion of pore space occupied by water) 

 R ( — resistivity of the formation (containing hydrocarbons and 



water, with water saturation, S w ) 

 R„ = resistivity of the same formation when entirely saturated with 



the same water (S„, — 1) 



272 



