Subsurface Logging Methods 419 



the water loss are constants which do not depend on the nature of the 

 permeable formation, and particularly on its permeability, it is logical 

 to expect that all the mud cakes in a given hole will be found to have 

 the same thickness. It is, however, possible that, even if the thickness of 

 all mud cakes should be the same in a given well, a determination of that 

 quantity would give valuable information about the behavior of the mud 

 itself in the drill-hole conditions. If further experience happens to show 

 that the mud cake thickness varies from bed to bed, these variations 

 could likely be related to some particular properties of the permeable 

 formations, which might be of interest. 



Conclusion 



A new electrical logging method, called micrologging, has been 

 discussed. This method makes use of electrodes applied to the wall of 

 the drill hole under a nonconductive pad which shields them from the 

 mud column. Two different microresistivity logs are recorded simultan- 

 eously, with two different electrode systems, both of which correspond 

 to very small electrode spacings. 



Permeable beds are, in general, clearly indicated on the micrologs 

 by a positive departure between the two microresistivity curves. Even 

 when the departure is not definite, the interpretation is easy, thanks to 

 simple rules based on the magnitude of the microresistivities and on the 

 behavior of the SP log. In all cases, the boundaries of the permeable 

 beds are determined with great accuracy. 



The microlog is, therefore, an important addition to the conventional 

 electrical log, and should contribute to a better and more accurate deter- 

 mination of the permeable beds, particularly in limestone territories. 

 Because of its accuracy in the determination of boundaries, it is, however, 

 probable that the microlog will also render important services in sand 

 and shale formations, where it could appreciably increase the accuracy 

 of the sand count. 



Acknowledgments 



The writer is indebted to the many engineers of the Schlumberger 

 Well Surveying Corporation who, at headquarters and in the field, cooper- 

 ated in the development of the method described in this paper. Acknowl- 

 edgments are also due to the oil companies for their courtesy in making 

 examples available. 



RADIOACTIVITY WELL LOGGING 

 V. J. MERCIER 

 Radioactivity well logging as practiced commercially in the United 

 States and South America is at the present time composed of two curves, 

 the gamma-ray curve and the neutron curve. 



The gamma-ray curve is a relative measurement of the natural radio- 

 activity occurring in the strata of the earth. Minute quantities of radio- 



