9. It is not unusual for gaps to occur in the sequence of well samples. 

 These gaps may be as small as three feet or as large as several hundred feet. 

 The sample man quickly acquires the habit of describing and plotting samples 

 at regular intervals, as, for example, 10 feet, without actually checking the 

 depths on each envelope. A sample gap may not be noticed for some time, 

 with the result that lithologies occurring at one depth are plotted on the strip 

 at another position higher up the hole. When logs are plotted in ink as they 

 are run, such a mistake is a serious matter, and may be one which cannot be 

 corrected entirely. One should always compare the depth of each sample with 

 the depth on the strip log before plotting the description. 



10. When a change in lithology is encountered in the samples, the material 

 should be examined closely; then an attempt should be made to trace the 

 lithology back up the hole in the samples previously examined. Where the top 

 of the formation is first drilled there may be very little of the representative rock 

 in the sample, a fact which could have been overlooked in the first examination. 

 The tops of main stratigraphic units should always be established in this way. 

 It is helpful to have briefly looked at a few samples down the hole before study- 

 ing the one to be plotted next. Some microscopists prefer to work with several 

 scoop trays simultaneously, so that principal changes in lithology will be 

 anticipated. This is a good practice. 



11. Good use should be made of electric-logs, drilling-time logs, and the like. 

 One is prone to become so interested in the problems of the actual sample 

 examination that these sources of information are neglected. However, do not 

 depend on such logs to supply information that should be obtained from the 

 well cuttings. A lithologic log should be constructed on data obtained from the 

 cuttings wherever possible, not on an interpretation of indirect methods of 

 logging. 



MISCELLANEOUS Considerable judgment and ingenuity must 



TESTING METHODS be exercised in performing the tests used in 



connection with sample examination. Tech- 

 niques employed in mineralogy are usually not applicable to this type of work 

 because of the small sizes of the particles. Therefore, many of the tests are not 

 conclusive. The following tests have been adapted to well-sample determination : 



Hardness: Unless abrasion or scratch tests for hardness are made care- 

 fully on small grains, the crushing effect is likely to be mistaken for scratching or 

 abrasion. It is impractical to employ the standard mineral scale of hardness. 

 A steel needle, a brass wire, and a small knife blade may be used as follows: 



Hold the chip firmly on the microscope stage with the forceps. While 

 viewing the operation through the microscope, lightly rub the point of the steel 

 needle back and forth across the grain being tested. Apply very light pressure. 



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