Subsurface Logging Methods 345 



Electrical Logs and Radioactivity Logs 



Electrical-resistivity and self-potential logs have almost supplanted 

 cutting samples and cores in areas where the stratigraphic section is shale 

 and sand and where salt beds do not contaminate the drilling mud. How- 

 ever, in areas where the section is composed of several kinds of rock, 

 especially where limestone and anhydrite are abundant, electrical logs 

 must be used with sample logs since the lithologic variety and high resist- 

 ance of the rocks introduce many unknowns into the interpretation of the 

 log. Thus, it is impossible to solve the problem without a sample log 

 which will enable one to eliminate many of the possible solutions. 



In limestone reservoirs it is very difficult to recognize the fluid con- 

 tent of the reservoir rock by use of electrical logs without additional in- 

 formation from testing the well, because the high resistivity of the lime- 

 stone and the sulphur water commonly present mask the resistivity effects 

 of the other fluids in the formation. However, electrical logs are useful 

 for correlation in local limestone reservoirs where the section is well con- 

 trolled by logs from sample cuttings. One fact that should be remembered 

 in using electrical logs is that rock-salt beds cut by the bit cause a salty 

 mud with a very low electrical resistance that results in a featureless self- 

 potential curve that is useless in making correlations. 



Radioactivity logs are also useful as an auxiliary to sample logs. 

 Here, also, a wide variety of rock and fluids in the stratigraphic column 

 results in several possible interpretations for the curve, and a sample log 

 is necessary for the correct solution. Radioactivity logs are sometimes 

 very useful in logging old wells that have already been cased off or wells 

 where mechanical difficulties have prevented taking a representative set 

 of cuttings. Radioactivity neutron logs are probably more useful than 

 resistivity logs in indicating a fluid zone, but it is not possible from the 

 neutron log alone to determine whether the fluid content of the formation 

 is oil or water. 



Thus, in areas where consolidated formations are encountered, exam- 

 ination of well-cutting samples is generally the easiest and least expensive 

 method of detecting changes in the formation and determining the strati- 

 graphic section penetrated by any well. Much subsurface geologic work 

 depends on the collection of representative samples of the formation 

 penetrated, describing these samples accurately, and plotting the descrip- 

 tion so that the sections in diff"erent wells may be correlated. 



Cable-Tool Samples 



The collection of cuttings samples from cable-tool wells presents com- 

 paratively few difficulties. The samples should be collected from the first 

 bailer after each run of the bit, in a bucket hung at the end of the dump 

 box. These samples should be washed enough to carry off all mud. They 

 should be put in cloth sacks, then labeled with the name of the company. 



