436 Subsurface Geologic Methods 



carnotite and cement is then squeezed. After the cement has set and the 

 plug has been drilled, a second gamma-ray curve is run on the same 

 sensitivity as the first. The difference in gamma-ray intensity between the 

 first and second runs is caused by the carnotite. Thus, by comparison of 

 the two curves, the top and bottom of the cement travel can be determined. 

 The same technique applies to squeezing of plastic material. 



Another unusual application for radioactivity well logs is in the 

 location of permeable zones in oil-producing horizons. The combination 

 radioactivity log is run through the section to be studied. The gamma-ray 

 curve provides the local stratigraphy, and the neutron curve indicates 

 the possible fluid-bearing zones. A quantity of oil or water containing 

 a radioactive tracer is then spotted opposite the section to be studied. 

 Pump pressure is applied to force the radioactivity fluid into the produc- 

 ing horizon. A second gamma-ray curve is then run on the same sensitivity 

 as the original run. The presence of the radioactive tracer in the zone 

 increases the radioactive valuation of the zones and the relative permeabil- 

 ities of the zones can be estimated by a comparison of the two gamma-ray 

 curves. 



General Considerations 



Many varied methods for the logging of oil wells have been de- 

 veloped and practiced through the years since exploration for oil began, 

 the most common being the use of a sample log obtained by geologists, 

 who examine and identify the drill cuttings from the well and plot their 

 findings on a strip of paper with relation to depth. As the science of 

 oil exploration developed, so too did the science of oil-well logging 

 advance, until today the oil operator can call upon approximately eleven 

 general methods. Although these many methods could be used to log oil 

 wells, only a few are being used extensively at the present. The methods 

 most frequently employed commercially are the following: 



1. Optical 



(a) Examination of samples 

 (h) Fluorescence 



2. Mechanical 



(a) Drilling rate for hardness 



(h) Drilling reaction, hardness, texture 



(c) Hole calipering 



3. Radioactivity 



(a) Natural, radioactive-substance content 

 (h) Artificial, hydrogen content 



4. Electric 



(a) Natural potential 

 (h) Formation conductivity 

 (c) Mud conductivity 

 In the selection of the best method for the logging of an oil well, 



