Subsurface Logging Methods 353 



after agitation, and the residue of the sample examined. The same process 

 helps to remove gel flake and other plugging materials added to the mud 

 to control lost circulation. Iron particles in the samples can be removed 

 by a magnet. 



Samples may be examined while covered with water. This method 

 brings out by diff"erential refraction some of the qualities of the sample 

 such as oolitic structures and anhydrite crystals which may be overlooked 

 in dry samples. When examining the sample wet, it is not necessary to 

 have them so clean as when examining them dry since a certain amount 

 of washing may be done in the water in which the sample is examined. 

 Dry examination is more convenient since it is not necessary to have a 

 number of petri trays or watch glasses with which to examine the samples 

 in water. 



Cuttings samples ordinarily are examined under a binocular micro- 

 scope of low power. The exact power used varies with the individual 

 geologist and with the nature of the samples. The power should be high 

 enough to see the essentia) structure and texture of the cuttings. Porosity 

 coarse enough to have a permeability sufficient to make commercial oil 

 production should be seen easily. Oolites, inclusions, and other sedimen- 

 tary features should be clearly visible also. On the other hand, the micro- 

 scope power should be low enough to permit examination of large num- 

 bers of samples without eye strain and should permit a field of view 

 large enough for estimating percentages of the various constituents accu- 

 rately with only a few changes in position of the sample. The powers 

 that meet these requirements vary between 12 and 24. Higher powers 

 are used for special purposes such as the description of minor features 

 of microscopic fossils. For general use, however, in the oil industry it is 

 not desirable to have a power higher than 24, since a very high power may 

 give false impressions about the permeability and porosity of cuttings. 



The illumination of the microscope field is important. In the past, 

 small incandescent lights which may be focused to give intense illumina- 

 tion have been used widely. However, with the development of the fluores- 

 cent lighting tube, it has been found that the white light given by this 

 tube is much superior to the yellowish light of the incandescent bulb. 

 This is especially true when searching for oil stain in the cuttings, since 

 the yellowish incandescent light commonly masks the light-brown oil stain. 

 Usually it is not practicable to focus the fluorescent tube so as to give 

 intense light for high-power work. However, for the usual low-power 

 examination, the fluorescent tube is very satisfactory. In examining sam- 

 ples for oil stain and saturation it may be desirable to use an ultraviolet 

 light. 



In examining well cuttings samples the geologist should endeavor to 

 estimate as closely as possible the proportion of each rock type in the 

 sample and to describe the lithologic characteristics of the rock that are 

 essential to the correlation of the beds. In commercial work it is not 



