The low resistivity of the shale or of the colloids in the sand is responsible 

 for the low resistivity measured. Such low resistivity would seem to infer a 

 very large water saturation even in sands saturated with oil. The resistivity 

 of the flushed zone (R x0 ) is also affected in the same manner; and, if we used 

 its value to obtain the formation-resistivity factor, a too low value for F 

 results. (Thus, too high porosity is derived.) This is a well-known phenomenon 

 in the laboratory where it is found that the formation resistivity factor in a 

 shaly sand is not a constant, but decreases as the resistivity of the flushing water 

 increases. 



It is thus obvious that the standard interpretation technique, if applied, 

 will give too high values of porosity (<£) and of water saturation (S w ). In 

 fact, many shaly sands do not exhibit sufficient resistivity to catch the eye of 

 the interpreter unless he remembers that a smaller SP in a given section is 

 sometimes indicative of hydrocarbon saturation in a shaly sand. 



A large amount of work has been done in the last few years in order to 

 understand the electric log in shaly sands. For the last two years, a technique 

 has proved quite satisfactory; and we will confine ourselves to the necessary 

 steps to be taken for a good interpretation. This technique applies in sand- 

 shale laminations and also in sands with disseminated colloids. 



In a shaly sand it is necessary to use the SP at the level of the formation 

 under study (PSP) ; and we also need the SSP: i.e., the SP that would have 

 been found had the sand been clean. This can usually be determined by observ- 

 ing the larger SP found near the shaly sand. If this SSP cannot be obtained, 

 the knowledge of R^, the formation-water resistivity, must be obtained from 

 other sources. Figure 14-24 is used, with the PSP as abscissa and R^o/R^ 

 as ordinate. This gives us the apparent saturation. To find the true saturation, 

 a line is drawn through the origin and the point representing the apparent 

 saturation; and this line is extended until it intercepts the SSP or the cor- 

 responding value, Rotz/Rm,. 



It must be remembered that in sand-shale laminations the value of S w 

 is the water saturation of the sand itself. In disseminated shaly sands, it is the 

 water bound by the quartz grains and does not include the water held by the 

 colloids. 



If the SSP is not known and R w unobtainable, it is not possible to determine 

 the water saturation, S w . Nevertheless, plotting the ratio R x0 /R t versus PSP on 

 Figure 14-24 will show whether or not the sand falls on the 100 percent water- 

 saturation line. If it falls some distance away from this line, there is a chance 

 for production. 



323 



