Formation Resistivities 



The resistivities of the formations are important clues to their probable 

 lithology and fluid content. Formations conduct electric current only by means 

 of the mineralized water that they contain. The minerals that constitute the solid 

 parts of the strata are insulators when absolutely dry. The few exceptions are 

 metallic sulphides, graphite, etc., which conduct electricity like metals. In a sim- 

 ilar manner, any pure oil or gas in the formations is electrically nonconductive. 

 Other important factors in formation resistivity are the shape and the intercon- 

 nection of the pore spaces occupied by the water. These depend on the lithology 

 of the formation and, in reservoir rocks, on the presence of nonconductive oil 

 and gas. 



Unit of Resistivity 



In electric logging, the resistivity is measured. The exception is induction 

 logging, which simultaneously records a conductivity curve and its reciprocal, 

 the resistivity. The unit of resistivity is the ohmmeter. When a uniform electric 

 current is sent through a 1 -meter cube in a direction parallel to any edge, the 

 resistance in ohms is equal to the resistivity of the substance. 



Relation of Formation Resistivity to Interstitial Water Salinity and 

 Temperature 



The resistivity of a formation depends directly upon the resistivity of the 

 water in its pores. The dependence is a linear function. 



In electric logging, the resistivity of a formation water quite often has 

 to be deduced from a knowledge of its salt content. The following laws of 

 electrolytic conductance should be borne in mind. 



A. The conductivity of an electrolyte increases in proportion to the 

 amount of chemicals in solution; thus, the resistivity decreases with in- 

 creasing salinity (fig. 14-2). 



B. The resistivity of an electrolyte decreases as its temperature increases. 



Relation of Formation Resistivity to Lithology 



The ability of a formation to conduct current is directly affected by the 

 amount of water in the pores: i.e., by the porosity of the formation. 



Many experiments made on various porous formations have shown that 

 in permeable formations that do not contain shaly material the formation 

 resistivity can be related solely to the porosity through a comparatively simple 

 empirical formula. 



In order to express this relation, it is convenient to make use of a parameter 

 called formation-resistivity factor, which is equal to the ratio Kg/Kyj. R is the 

 resistivity of a given formation entirely filled with water, and R TO is the re- 



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