ings. Generally the logs consist of two normal resistivity curves of different 

 electrode spacings and one lateral curve. Any of these three resistivity curves 

 can be presented in several scales to cover the full range of resistivities en- 

 countered in any particular borehole. 



All modern logging equipment can simultaneously record the spon- 

 taneous potential curve and the various resistivity curves on at least two depth 

 scales. The most commonly accepted depth scales are 2 inches — 100 feet and 

 5 inches = 100 feet. Occasionally a depth scale of 1 inch = 100 feet is 

 recorded. On some specialized services, such as the Microlog or dipmeter, an 

 expanded depth scale of 25 inches = 100 feet or 60 inches = 100 feet is 

 frequently used. In very rare cases an expanded scale of 120 inches = 100 

 feet can be recorded. 



Spontaneous-Potential Curve 



The SP log (spontaneous- or self -potential log) is a record of naturally 

 occurring potentials measured in the mud at different depths in a drill hole. 

 Usually, the SP log consists of a base line, more or less straight, having excur- 

 sions or peaks to the left (fig. 14-1). The base line corresponds in most cases 

 to shales, whereas the peaks are generally opposite permeable strata. The 

 shape and the amplitude of the peaks may be different, according to the 

 formations; but there is no definite correspondence between the magnitude 

 of the peaks and the values of permeability or porosity of the formation. 



Measurement of SP 



The recording of SP logs involves a simple technique. A measuring 

 electrode is lowered in the hole at the end of an insulated cable. The differences 

 of potential between a surface electrode which is at a fixed potential and the 

 electrode in the hole whose potential varies as it is moved along the hole are 

 observed by means of a recording galvanometer. 



The drill holes in which the SP logs are recorded are usually filled with 

 mud having a water base. The density of the mud is ordinarily such that at 

 each depth the hydrostatic pressure in the hole is greater than the formation 

 pressure. As a result, the fluid in the permeable beds cannot contaminate the 

 mud at the time measurements are made. Also, the mud has been in constant 

 circulation during the drilling operation which was prior to the logging and, 

 therefore, is homogeneous. 



Experience has shown that the deflections on the SP log correspond to 

 phenomena occurring at the contacts between the mud and the different beds 

 and also at the contacts between the beds themselves. These phenomena pro- 

 duce an electric current, called SP current, which uses the mud as its return 

 path. In so doing, it creates in the mud, by ohmic effect, potential differences 

 which can be measured and recorded. 



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