382 



Subsurface Geologic Methods 



content, whether it be qualitative or quantitative. Consider a sand con- 

 taining water with a salinity of 50,000 p. p.m. at 100° F. and another of 

 identical characteristics containing water with a salinity of 5,000 p. p.m. 

 at the same temperature. Reference to the chart of figure 159 shows 

 that in the first the water resistivity would be about 0.10 ohms m^/m; in 

 the second, it would be about 0.85 ohms m^/m, which, applying the for- 

 mula (1) , would mean an eight-fold increase in the resistivity of the water 

 sand. 



A decrease in porosity causes an increase in resistivity, while an in- 



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ft«tt^l-N^ 



bed 



fkwrmrtr: — l:««i 



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sc 



Figure 165. Portion of a log showing shielding effect of thin resistive beds on lateral 



curve. 



crease in porosity causes a decrease in resistivity. This factoi is not 

 especially important for small changes in porosity — less than five percent 

 — but must be given major consideration when there are marked changes 

 in percentage porosity. 



The evidence indicates clearly that fluid-content interpretation can be 

 made if the resistivity of the sand is known when it is 100-percent water- 

 saturated. These data may be obtained in one of two ways, either (1) di- 

 rect from the electric log, or (2) by laboratory resistivity measurement 

 on a core. 



The second method requires a measurement of the resistivity of a 

 core sample 100-percent water-saturated. This information permits a de- 

 termination of the formation-resistivity factor; and, thereby, knowing the 

 salinity of the water in the formation, one can calculate the resistivity of 

 the formation when it is completely water-saturated. 



It has been shown by a number of investigators that the resistivity of 

 an oil reservoir is, among other things, related to the percentage of water 



