BORE-HOLE INVESTIGATIONS 1051 



Soft Formations. — Figure 648 is a portion of a typical log showing the S.P. 

 curve, A ; the short normal, B ; amplified short normal, C ; long normal, D ; and lateral 

 curves, E ; for soft formations interbedded with a harder formation. 



Geological strata generally form sequences of thin sand and shale layers. The 

 relative proportion of sand and shale is variable with the depth. The intervals where 

 the proportion of sand is much greater than that of shale are considered as sand beds ; 

 sometimes the amount of shale is so small that the beds are practically clean sands. 

 Conversely, the zones where the shales are predominant are termed shale beds. 



The S.P. curve and the short normal curve give an accurate record of the boun- 

 daries of the thicker shale and sand beds, as they have just been defined; the very thin 

 layers are shovra only by minor wiggles on the curves (see, for instance, permeable 

 beds 1, 2, and 5 in Figure 648). 



Moreover, very thin hard beds in shale formations, which are rather poorly indi- 

 cated or not indicated at all by the S.P. log, appear quite clearly on the short normal 

 curve when recorded on an amplified scale, and provide, in many cases, excellent 

 markers for correlations. (See, in particular, the shale bed 4, Figure 648). 



Generally, the resistivity of the mud is much greater than that of the connate water. 

 On the Gulf Coast, for example, the mud resistivities are confined to a range from 

 about .5 to 2 ohm-meters, whereas the resistivity of the connate water is about .05 ohm- 

 meter. The resistivity of the invaded zone is therefore much higher than the true 

 resistivity of the water-bearing formation ( 10 to 20 ohm-meters for the former against 

 .5 to 1 for the latter), and sometimes the resistivity of the invaded zone may be even 

 higher than the true resistivity of oil-bearing formations (5 to 20 ohms). Therefore 

 the resistivity measured with the short normal is greatly affected by the presence of 

 the invaded zone, and peaks are generally obtained on the log in front of the permeable 

 beds, even if these beds are highly conductive.* (Note Figure 646, bed number 1.) 

 Sometimes, however, the depth of penetration of the mud into the permeable layers is 

 small enough so that the short normal curve is very little affected and gives depressions 

 in front of conductive layers (Figure 648, beds 1, 2, and 5). 



The readings on the long normal curve are little influenced by the mud and the 

 invaded zone, and wherever the thickness of the bed is great enough, the comparison 

 between the short and long normal curves determines, in general, whether the true 

 resistivity of the permeable beds is high or low. A conductive bed is indicated by a low 

 apparent resistivity on the long normal, whereas a resistive bed gives rise to a high 

 resistivity, sometimes higher than the reading obtained with the short normal. 



In Figure 646, for example, permeable beds 3 and 5 appear as being resistive, and 

 bed 1 as more conductive. Also in Figure 648, for instance, permeable beds 3 and 4 

 are resistive, whereas all the other permeable beds are conductive. These indications 

 are confirmed in the case of thick beds, by those of the lateral sonde. 



When the thickness of the bed is of the same order as the spacing of the long 

 normal, or smaller, no reliable indication can be deduced from the normal curves as far 

 as the value of the resistivity of the bed is concerned. The only indications in this 

 respect are given by the lateral sonde which gives rise to short peaks in front of resis- 

 tive beds and no peak in front of conductive beds. 



Wherever the beds are sufficiently thick clean sands, and are less resistive or of 

 approximately the same resistivity as the surrounding impervious formations, the 

 magnitude of the deflections on the S.P. log are practically equal to the static S.P. 

 When the permeable beds are more resistive than the adjacent formations, the static 

 S.P. is attained only if the bed is very thick. In the case of thin sands, or of shaly 

 sands, the magnitude of the S.P. peak is less than that of the static S.P. 



The principles of interpretation cannot be reduced to the above simplifications in 

 all cases. For instance, when the connate water is fresh, the distinction hctzvecn oil- 

 bearing and tvater -bearing beds becomes difficult, and sometimes impossible. Also, 



* The recording of the short normal on an amplified scale has often been found quite useful for 

 correlation purposes in the case of long flat shale sections, in which the recording in the regular 

 sensitivity reveals very little feature. 



