352 Subsurface Geologic Methods 



eral times until the water is clear and the sample is ready for sacking and 

 labelling. Sometimes the sample is washed in a box with a fine-screen 

 bottom. This hastens the washing process, but there is some danger of 

 losing the fine sand from the sample. When drilling with oil or oil-base 

 mud, one must wash the sample entirely free of drilling fluid before it 

 is dried. This can be accomplished by using hot water, although some 

 prefer washing with kerosene or gasoline before washing with water. One 

 need have no fear of washing the indigenous oil stain and saturation 

 from the sample as any ordinary washing will only remove the drilling 

 oil and will not remove the natural oil from the formation. 



After washing, samples are commonly dried artificially. However, 

 in samples showing stain and saturation this must be done with extreme 

 caution as overheating will blacken the natural-oil stain and mask the 

 porosity as well as render it difficult to tell whether the formation fluid 

 is oil, gas, or water. In extreme cases of overheating, red shales may be 

 oxidized to a black color. If samples from wells drilled with oil-base, 

 drilling fluid are not washed clean before drying, it is impossible after- 

 ward to remove the drilling oil even by boiling the sample. In some cases 

 where samples are to be used for paleontologic examination, any shale in 

 the samples is digested with 'boiling water and soda to leave the fossils 

 free. It is probably better when there is no necessity for haste in exam- 

 ination of the samples to let them dry naturally in the air rather than to 

 dry them artificially. 



After the samples are sacked and transported to the laboratory, it is 

 commonly necessary to divide them into smaller portions for different 

 types of examination or for examination by diff^erent individuals. This 

 should not be done by merely pouring a little off the top of the sack as 

 ordinarily there will be a few large cavings on the top of the sack. These 

 should be raked off and discarded and the remainder of the sack mixed 

 so every portion will be representative. All portions or "cuts" of the sam- 

 ples should be large enough to be representative. Generally a tablespoon- 

 ful should be the minimum. 



Examination of Cuttings Samples 



If the samples are properly prepared as outlined in the preceding 

 section, they may be examined without further preparation. However, 

 samples may not be washed properly on the rig and they must be re- 

 washed before examination. This is most conveniently done in small sauce 

 pans. The samples are agitated in water and the fine mud decanted; this 

 process is repeated until the water is clear. Machines have been invented 

 to help in this operation, but they are economical only where large num- 

 bers of samples must be washed. Drying the samples should be done as 

 slowly as possible so as not to overheat and burn the sample. If a sample 

 is only slightly dusty it may be prepared for examination by shaking 

 over a screen of 100 mesh or finer. Some samples contain cavings which 

 are ordinarily larger than the cuttings. These can be scraped off the top 



