BORE-HOLE INVESTIGATIONS 1045 



The expression shaly sand has been applied above, in a general way, to interbedded 

 thin streaks of sand and shale. There are, however, sand beds containing certain 

 unstratified shaly material, which therefore must also come under the category of shaly 

 sands. Whether the different shaly particles enclosed in a shaly sand are stratified or 

 not, the material affects the S.P. curve in substantially the same way, as long as the 

 shaly sand remains permeable ; the amplitude of the S.P. log is maximum for a clean 

 sand and it is reduced with an increased percentage of shaly material, but this amplitude 

 does not depend on the distribution of the shaly material, provided, of course, that the 

 average distribution is uniform. 



The presence of oil in a shaly sand will increase the resistance of the permeable 

 part of the medium. Accordingly, the amplitude of the deflection on the S.P. log can be 

 expected to be smaller opposite an oil-bearing section than opposite a water-bearing 

 section. Such a change in the magnitude of the peaks can be observed only for shaly 

 sands or for clean sands of small thickness ; it will not be observable for very thick 

 clean sands. 



Since many sands are shaly, it is not surprising that a change in the deflection on 

 the S.P. log occurs when passing the oil-water contact in a sand. It is to be noted 

 however, that this change is not necessarily diagnostic of oil, since the same effect would 

 be obtained if the salinity of the interstitial water were reduced, or if the percentage 

 of shale were increased. 



Base Line Shifts. — Usually the S.P. is the same in front of all thick shale beds, or, 

 in other words, there is a straight "shale line" or "base line" on the S.P. log. Experi- 

 ence has shown however that in certain fields there is a systematic shift of this shale 

 line which occurs always at the same location in the geological column. In fact, in 

 certain cases, such base line shifts constitute excellent markers. 



The shift is generally due to a difference in the nature of the shales above and 

 below the shift level. There can be a shift in the base line, however, even though the 

 shales above and below the shift level are of the same nature. This occurs when there 

 is a dissymmetrical sequence of formations in the ground, constituting one or more 

 three-link chain E.M.F.'s which do not cancel out. 



The S.P. Log in Hard Formations (Limestone Fields) 



The case of limestone fields, or more generally the case of permeable beds in com- 

 pact and highly resistive formations, is of particular interest. 



The S.P. log in hard formations has a character which is often difficult to under- 

 stand. It must be remembered that such formations are highly resistive except for 

 conductive zones of two types: (1) permeable zones, whether oil-bearing or water- 

 bearing, which are usually conductive because of their connate water of generally high 

 salinity; and (2) shales and other conductive beds, which are of an impervious nature 

 but contain highly conductive connate waters. 



According to the nature of the electromotive forces present, the S.P. current tends 

 to flow from the hole into the permeable zones, and from the shales into the hole. If 

 the beds adjacent to these conductive zones were also conductive, they would furnish 

 the return path for the S.P. currents. If the adjacent beds are highly resistive, the 

 S.P. currents tend to be confined to the hole opposite the resistive beds, flowing via the 

 hole between the permeable beds and the nearest conductive, impervious (shale) beds 

 and completing the circuit, after deep penetration into the conductive beds, by flowing 

 through a very large cross-section of the highly resistive beds. 



This means that S.P. currents are flowing in the hole along the entire thickness of 

 the resistive beds, and in so doing they produce potential differences by ohmic effect in 

 the mud. The result is that the peaks corresponding to the permeable zones spread 

 above and below these zones in an apparently abnormal manner, and it is impossible to 

 determine with accuracy from the S.P. log the boundaries of the permeable zones. 



