562 busch. SUBSURFACE TECHNIQUES [Ch. 31 



the formations penetrated are indicated on the sample log in Fig. 2. 

 Such graphic records, when laid side by side, serve as an excellent 

 means of identifying formations and key horizons, of noting changes 

 in type of rock, changes of thickness of intervals, etc. Printed log 

 forms for the noting of pertinent data are of many types. The top por- 

 tion of a typical log form is shown in Fig. 3. A convenient and 

 customary vertical scale is 100 feet to 1 inch. Such log forms may be 

 purchased with graduated depths of 4,000 feet and 6,000 feet. 



ELECTRIC LOGS 



Electric logging has become the most universal method of logging 

 oil and gas wells, principally because of its rapidity, accuracy with 

 respect to depth, facility in correlation work, and relatively low cost. 

 The use of electric logs has to some extent supplanted the examination 

 of cuttings and cores. Although they have definite and severe limita- 

 tions, electrical logs are probably the most important and useful tool 

 available in subsurface geological investigations. 



Conrad and Marcel Schlumberger, French geophysicists, discovered 

 that in a borehole the electrical potential, with respect to a fixed 

 reference, is different opposite a bed of shale from that opposite a bed 

 of sandstone, and they developed a technique for recording these differ- 

 ences. They also adopted the methods for determining earth resistivity 

 to use in a borehole. Their technique for electrical logging was used 

 in Russia, Romania, and Venezuela before 1930 but did not come into 

 general use in the United States until 1935. A typical electric log is 

 shown in Fig. 2. 



The potential curve, also called self-potential, spontaneous potential, 

 or S.P. curve, is a curve of natural electrical potential in the hole 

 plotted against depth. It is always recorded on the left-hand side of 

 the log strip, with depth increasing downward and positive potential 

 increasing from left to right. It is obtained by measuring the difference 

 in direct-current potential between a grounded electrode placed near 

 the wellhead at the surface and that of another that is lowered slowly 

 on a cable into the well. The record customarily is made by a re- 

 cording galvanometer on photographic film that is driven by a geared 

 coupling attached to the sheave over which the cable passes at the well- 

 head. By changing the gears the film can be made to travel 1 inch 

 for each 20, 50, or 100 feet of movement of the traveling electrode. 

 The changes in potential difference range from 1 to 250 millivolts, and 



