Subsurface Logging Methods 399 



THE MICROLOG 

 H. G. DOLL 



In conventional electrical logging, the spontaneous potential (SP) 

 log is used to delineate the permeable beds, and the resistivity logs are 

 used primarily to provide indications concerning the fluid content of the 

 beds. 



When the formations are much more resistive than the mud, as 

 happens, for example, in limestone fields, the SP currents are short-cir- 

 cuited by the more conductive mud column, with the result that the SP 

 log is quite rounded. In that case, the SP log generally gives the ap- 

 proximate location of the permeable formations, but it cannot be used 

 for an accurate determination of the boundaries of each permeable bed.^^ 



Solutions for the problem of obtaining a better determination of 

 the permeable beds in limestone fields were developed from two angles. 

 One approach consisted in improvements of the logging of the SP, as 

 given by Selective SP logging and Static SP logging. ^^ These new methods, 

 which have been described in an earlier paper, give good results when the 

 mud is not too salty; but they are still in a somewhat experimental stage, 

 mostly because the development efforts have lately been concentrated on 

 another approach to the problem, i.e. the microlog. 



The mJcrolog, which is the subject of the present paper, has been 

 developed primarily as a means for the accurate determination of the 

 permeable beds, where the SP log alone does not give a satisfactory 

 answer. For that reason, this new development has found its first field 

 of application in limestone areas, where the usefulness of micrologging 

 is most obvious. The microlog is, however, also of importance in sand 

 and shale formations, if only for a more precise determination of the 

 boundaries between successive beds, and for a better evaluation of the 

 sand count. 



It is emphasized that the present paper is not intended to give an 

 exhaustive and definitive description of the subject. In fact, the application 

 of the new method has had a partly experimental character up to the 

 present time, and several features of the corresponding technique are 

 still being improved. It is possible that some of the improvements now 

 under way will modify, to a certain extent, the response of the micrologs 

 and the procedure of interpretation. These differences, however, should 

 not bring about any fundamental changes in the principle of this method, 

 and should rather make its application easier and more reliable. 



Principle of Micrologging 



A microlog is a resistivity log recorded with electrodes spaced at 

 short distances from each other in an insulating pad which is pressed 



''Doll, H. G., The SP Log: Theoretical Analysis and Principles of Interpretation: Petroleum Tech- 

 nology, vol. II, or Transac'ions AIME. Petroleum Branch, vol. 179, p. 146, Sept. 1948. 



*° Doll, H. G., Selective SP Logging: Paper presented at the AIME Columbus, Ohio Meeting Sept. 

 25-28, 1949, and at the San Antonio, Texas meeting, Oct. 5-7, 1949. 



