1044 EXPLORATION GEOPHYSICS 



The figure shows that the static S.P. is reached only when bed thickness is equal 

 to at least 16 times the hole diameter (about 10' to 13'), and provided pt is not higher 

 than about 6 pm (for example, 6 ohm-meters in the Gulf Coast, where pm is about 

 1 ohm-meter on the average). 



It also is indicated that the S.P. curve spreads a considerable distance outside the 

 boundaries of the layer, when -^is large, and the higher pt, the greater the effect. 



From the preceding statements it can be seen that the mud resistivity has a pre- 

 dominant influence on the magnitude of the S.P. deflection: the total E.M.F.'s (static 

 S.P.) are the sum of the electrokinetic potentials (which are proportional to the mud 

 resistivity) and the electrochemical potentials (which are closely related to it). The 

 salinities of connate water are usually very high, so that, if the mud is also of high 

 salinity, the electrochemical potentials can be practically nil. Moreover, for a given 

 static S.P., the lower the mud resistivity, the smaller is the ohmic drop in the mud. All 

 these factors contribute therefore toward the same result, which is to minimize the S.P. 

 deflection. In fact, it is a common observation that the S.P. logs show very minute 

 deflections, and are even completely flat, when the bore-holes are drilled with very 

 conductive muds. In the case of comparatively fresh connate formation water, con- 

 ductive mud may give rise to reversed peaks (deflections toward the positive side) 

 provided the conductivity is not too high. The computed curves shown in Figure 644 

 are for successive beds having the same or very nearly the same values of resistivity. 

 In practice, this is not always the case, and the resistivities of the successive beds may 

 even differ widely, as for instance when shales are interbedded with oil-bearing sands. 



In such a case, the shape of the S.P. log remains approximately the same, and it 

 should be noted particularly that the boundaries between permeable and impervious beds 

 still correspond to the inflection points on the log. However, the S.P. curve is more 

 rounded off in the more resistive formation than in the other one, and the inflection 

 point is displaced toward the bottom of the peak if the peak corresponds to the more 

 resistive formation, and vice-versa. 



(d) An increase in hole diameter acts approximately like an increase in the ratio 



•^. It tends to round off to a greater extent deflections on the S.P. log and to reduce 



Pm 



the amplitude of the peaks opposite thin beds. 



(e) In general the permeable beds are invaded by mud filtrate: the boundary 

 between the mud filtrate and the liquid in the permeable formation where potentials of 

 electrochemical nature orginate is therefore somewhere inside the permeable formation, 

 at a certain distance from the wall of the hole. As a result, penetration of mud filtrate 

 into the permeable bed has an effect on the S.P. log similar to an increase in hole 

 diameter ; the S.P. peaks are wider than they would be in the case of no invasion, and 

 the amplitude of peaks corresponding to thin permeable beds located in impervious 

 formations is smaller than for no invasion. 



(f ) A combination of thin layers of sand in shale or of thin layers of shale in sand 

 constitutes what has been called a sandwich; such a combination can be considered as a 

 more or less shaly sand. One such case is illustrated by Figure 644. The following 

 points are of interest concerning the logs of sandwiches : 



(1) On thick sandwiches, the average deflection is approximately proportional to 

 the percentage of sand. 



(2) The average contour corresponding to a sandwich of finite thickness is the 

 same as for a homogeneous permeable bed of the same thickness and resistivity but 

 for which the total E.M.F. involved would be smaller. The amplitude of the ripples 

 around the average curve decreases very quickly when the thickness of the individual 

 beds is decreased, and the ripples are hardly noticeable when the individual thickness 

 of each of the sandwiched beds, both permeable and impervious, is less than one-half 

 the diameter of the hole. 



(3) The average amplitude of the peaks decreases when the resistivity of the sand 

 increases in comparison with that of the shale. 



