From the lateral curve a value for R t is deduced. 

 It can be seen that 



R* = iL §A if 



a. R 3 is the resistivity of the water found in the invaded zone. It is usually 

 made up of mud and connate water mixed in such proportion that 



1 z (1 — z) . , , r i i 



— = — + , with z the percent of connate water in the total mixture 



t*-« tiw -Km 



b. Sj is the water saturation in the invaded zone. It has been found by 

 experience that S* 2 *==* S w , so that we can write R4 



s. 



Inasmuch as R* = , we obtain S„, = 



Rz/Rw 



Since Ra/R^ is dependent only on the mud resistivity and the connate- 

 water resistivity, the denominator is a function of the SP value. The upper 

 part of Figure 14-23 gives a graphical solution of this equation. The SP is 

 entered in ordinate and the ratio Rj/R* on oblique lines. The intersection 

 gives the abscissa, S w . 



The lower part of the chart permits obtaining the porosity by using the 

 water saturation, S w , just found; and in the ordinate, the value of R^/R^. 

 The intersection falls on or below oblique lines that are graduated into porosity 

 values, according to the Humble Formula. 



For a good application of this method, the small spacing signal must be 

 nearly independent of the true resistivity, thus requiring sufficient invasion 

 and flushing. The value given by the long spacing should not be too greatly 

 influenced by the invasion. Such conditions are sometimes difficult to obtain 

 in practice; and, consequently, the results will reflect the actual conditions of 

 application. This method should not be used in salt muds. 



Interpretation of Shaly Sands 



The obvious characteristic of shaly sand is its lack of character on the 

 electric log. The resistivity curves are reduced by the shale in the sand. In 

 some very shaly oil sands, one sees only a small variation between the resis- 

 tivity of the sand and that of the surrounding shales. The MicroLog does not 

 show much separation between the curves; actually, in many instances, there is 

 no separation. 



321 



