Shaly Sands 



A combination of thin layers of sand in shale cr 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 example is illustrated in 

 Figure 14-7. The following points must be mentioned concerning sandwich 

 logs. 



The average contour corresponding to a sandwich is the same as for a 

 homogeneous permeable bed of the same thickness and resistivity but for which 

 the total emf 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. The ripples are hardly noticeable when the individual thick- 

 ness of each of the sandwiched beds, both permeable and impervious, is less 

 than one half of the diameter of the hole. 



The average amplitude of the peaks decreases when the resistivity of the 

 sand increases in comparison with that of the shale. 



The term shaly sand has been applied above to interbedded thin streaks of 

 sand and shale. There are also sand beds containing disseminated shales or 

 clays, and many sands contain both stratified and dispersed shales. Whether 

 the different shaly particles enclosed in a shaly sand are stratified or not, the 

 mixture behaves in substantially the same way in the SP log. The total amplitude 

 of the static SP is maximum for a clean sand, and it is reduced as the percentage 

 of shaly material increases. In a shaly sand this total amplitude is called the 

 pseudostatic SP or PSP. 



If a shaly sand is thick enough, the deflection of the SP curve is equal 

 to the pseudostatic SP. If the bed is too thin, the deflection is smaller than the 

 pseudostatic SP, as a result of the same effect of geometry and resistivities as 

 in clean sands. 



The magnitude of the pseudostatic SP is a function of the resistivities of 

 the uncontaminated zone in the shaly sand, of the invaded zone, and of the 

 shales. Accordingly, the amplitude of the SP log can be expected to be smaller 

 opposite the oil-bearing portion of a shaly sand than opposite the water-bearing 

 section. 



Because many sands are shaly, it is not surprising that a change in the 

 deflection from the shale line on the SP log occurs very often when passing 

 the oil/water contact in the sand. It is to be noted, however, that this change 

 is not a positive criterion for the detection of oil since the same effect would 

 be obtained if the salinity of the interstitial water were reduced or if the per- 

 centage of shale were increased. 



Us2 of S? Curve for Evaluation of Connate-Water Resist.'vi'.y 



It has been seen that the SP is related to R,//R ;(/ . through equation SSP = 

 — K log R m //R w . In this instance and when the mud and the connate water contain 



283 



