418 Subsurface Geologic Methods 



In figures 185 and 186, sequences of sands, sandstones, limestones, 

 and shales are exhibited. At the upper part of figure 186, a sharp depres- 

 sion in the microlog brings the microresistivity down to approximately the 

 resistivity R^ of the mud, a fact which is almost a certain indication of 

 caving. This interpretation is confirmed by the section-gauge log. 



Figure 187 shows the behavior of the microlog in the case of a thick 

 limestone formation (Ellenburger) , composed of compact zones and of 

 fissured zones with vugular porosity. 



Figures 188 and 189 illustrate the features of the microlog in se- 

 quences of non-consolidated sands and shales, such as those commonly en- 

 countered in the Gulf Coast or in analogous geological provinces. In these 

 regions, the conventional SP and resistivity logs differentiate very well the 

 different sections where sands or shales are respectively predominant, but 

 they are not capable of delineating each separate sand or shale streak, if 

 these are thin. A very detailed record of the individual permeable and im- 

 pervious beds is obtained from the microlog, as shown on the figures. This 

 result is of interest for an accurate determination of the proportion of 

 shale and sand in shaly sands, or, in colloquial terms, for the evaluation 

 of the sand count. 



Possibilities of Quantitative Interpretation 



To date, the microlog has been used primarily for a qualitative 

 determination of the permeable beds and an accurate determination of 

 their boundaries. It must be kept in mind, however, that the microresis- 

 tivities measured for permeable beds are largely dependent on the resis- 

 tivity and thickness of the mud cake, and on the resistivity Rxf, of the 

 formation immediately behind the mud cake. It is possible that the de- 

 velopments presently under Avay could bring the micrologging equipment 

 to a point where R^o and the thickness of the mud cake could be deter- 

 mined quantitatively. 



When the first few inches of the permeable bed immediately behind 

 the mud cake are practically saturated with mud filtrate, and when the 

 mud cake is entirely built outside of the formation, and not partly within 

 its pores, R=ro is equal to F Rmf, and, therefore, gives a direct measure of 

 the formation factor F if the resistivity R„if of the mud filtrate at the 

 corresponding temperature is known. The possibility of determining the 

 formation factor in situ and continuously would obviously be of great 

 interest because of the close relationship of that factor with the porosity, at 

 least when the permeable material is reasonably free from conductive 

 solids such as clay. 



It is more difficult to predict whether useful information could be 

 derived from the thickness of the mud cake. Since laboratory experi- 

 ments -^ have shown that, for a given mud, the thickness of the cake and 



-2 Byck. H. T., The Effect of Formation Pemeability on the Plastering Behavior of Mud Fluids: API 

 Drilling and Production Practice, p. 40, 1940. 



