generally greater than, or about equal to, those of the shales. This example 

 is typical of the so-called soft formations such as sand and shale series. Shaly 

 sands will be discussed later. 



Permeable Beds Adjacent to Shales (Typical Example of Soft Formations) 



Theoretical computations and field experience have shown that the ampli- 

 tude of the SP deflection is practically equal to the static SP when the permeable 

 beds are thick and when the resistivities of the formations are not too high 

 with respect to that of the mud. Moreover, the SP curves mark the location of 

 the boundaries of the bed with great accuracy. 



The amplitude of the deflection is less than the static SP for thin beds: i.e., 

 the thinner the bed, the smaller is the peak. On the other hand, when the 

 resistivity of the formations is considerably higher than that of the mud, the 

 SP curves are rounded off; the boundaries are marked less accurately. All 

 other conditions being the same, the amplitude of the peak is less than when 

 the ratio formation resistivity to mud resistivity is close to 1. 



Figure 14-7 shows theoretical data illustrating the influence of the bed 

 thickness and the resistivity of the formations. To facilitate the comparison, it 

 has been supposed that the value of the static SP is the same for all beds and 

 equal to —100 millivolts. Furthermore, for simplicity in the computations, the 

 depth of invasion in the permeable beds has been supposed shallow enough 

 to be negligible. 



The SP curve (fig. 14-7) also spreads a considerable distance outside the 

 boundaries of the layer when the ratio formation resistivity to mud resistivity is 

 large: i.e., the higher the formation resistivity, the greater is this effect. 



Figure 14-7 graphically shows the reduction of the recorded SP due to 

 bed thickness and formation resistivity. Moreover, for a given static SP, the 

 lower the mud resistivity, the smaller is the ohmic drop in the mud; and the 

 wider is the deflection above and below the permeable beds. Conversely, the 

 higher the mud resistivity, the sharper are the deflections. 



An increase in hole diameter acts approximately like an increase in the 

 ratio of formation resistivity to mud resistivity. It tends to round off the 

 deflections on the SP log and to reduce the amplitude of the peaks opposite 

 thin beds. A decrease in hole diameter has the same effect as a decrease in 

 the ratio of formation resistivity to mud resistivity. 



As shown on Figure 14-6, the permeable beds in general are invaded by 

 mud filtrate. The electrochemical emfs originate at the boundary between the 

 mud filtrate and the liquid in the permeable formations, somewhere inside the 

 permeable formation. As a result, penetration of the mud filtrate into the 

 permeable bed has an effect on the SP log similar to an increase in hole diameter : 

 i.e., the SP peaks are wider than they would be in the case of no invasion, and 

 the amplitude of peaks in thin permeable beds is smaller than for no invasion. 



282 



