364 Subsurface Geologic Methods 



of the regional stratigraphy. From the sections, index beds can be selected 

 for use in structural contouring. 



From the sample descriptions, volumetric estimates of ultimate well 

 yield can be made, reservoir study facilitated, and well remedial work 

 guided. 



ELECTRIC LOGGING 



E. F. STRATTON and R. D. FORD 



The electric log consists of a spontaneous potential curve and, gen- 

 erally, three resistivity curves. The specific recording practice and the 

 type and number of curves vary from one geologic province to another, 

 depending upon the nature of the formations and the problems to be 

 solved. 



Spontaneous Potential 



The spontaneous-potential (SP) log is used to distinguish between 

 permeable and nonpermeable formations, as, for example, sand and shale 

 or permeable and nonpermeable limestone. However, a quantitative 

 relationship between porosity or permeability and spontaneous potential 

 does not exist. Empirical relationships have been found, however, and 

 have been established in specific pools for particular formations. 



The spontaneous potential log of a bore hole is a record of the 

 potentials measured in the mud along the hole. In fact, the potentials 

 are measured between an electrode lowered into the hole and another 

 electrode at the surface and are related to an arbitrary constant. As the 

 SP log is generally flat in front of shales and shows positive or negative 

 anomalies opposite permeable beds, it is convenient to take the line ob- 

 tained in front of shales as the base line. 



Spontaneous potential anomalies in a bore hole are due primarily 

 to the electromotive forces generated by two different electrical phenom- 

 ena. The first of these and the more important is the electrochemical 

 cell formed between the drilling fluid, the fluid in the permeable zone, 

 and the shale surrounding the permeable section. This may be expressed 

 as: 



E = K log^ 



^ Ri (1) 



where £'=electromotive force of spontaneous potential in millivolts. 

 i?3=resistivity of drilling fluid in ohmmeters. 

 7?i=resistivity of the fluid in the permeable zone in ohmmeters. 

 ^=factor dependent upon the chemical composition of the two 

 fluids and upon the character of the shale adjacent the perme- 

 able bed. 

 The second of these electromotive forces may result from the filtra- 



