Although rotary holes are taken to great depths with little or no casing, 

 cable-tool holes must be cased frequently to prevent collapse of the walls. For 

 this reason, cavings from high up the hole are more common in rotary samples. 

 On the other hand, cavings which do occur in the cable-tool hole fall to the bot- 

 tom where they are ground and thoroughly mixed with the fresh cuttings so 

 that they are more difficult to distinguish. 



The depths are more accurate in cable-tool samples because there is no lag 

 in the time between cutting and sampling. However, due to their fineness, they 

 are more difficult to interpret. 



Appraisal of Well Samples 



It has been shown that the quality of well samples varies appreciably, and 

 that the microscopist is faced at the outset of his work with the problems of 

 depths and distinguishing between representative cuttings and cavings. There 

 is no adequate substitute for experience in judging well samples. The suggestions 

 listed below should help the novice avoid certain common errors. 



1. Always be suspicious of large pieces of rock in the sample, regardless 

 of whether they are angular or well-rounded. The shape is not always indicative 

 of cavings, but the large sizes generally are. If the piece did come from the 

 bottom of the hole, then there should also be a good representation of the 

 same lithology in the finer materials. 



2. Compare the lithologies determined from the samples with the inter- 

 pretive lithologies of the electric log or radioactivity log. These logs are par- 

 ticularly helpful in sections of sands and shales or evaporites. 



3. If possible, become familiar with the lithologies of the formations in 

 the area by reviewing existing descriptions of nearby wells and surface sections. 



4. Consider the probability of caving from formations examined higher up 

 in the hole. Shales, anhydrite, gypsum, fractured limestone, friable sandstone, and 

 residual or detrital cherts are more liable to cave than well-indurated beds. 



5. When caving is suspected, review the descriptions of similar rocks 

 logged up the hole. It may be advisable to compare the cuttings with those 

 previously run. 



Preparation of Samples for Examination 



Sometimes the samples in the bags are in poor condition for microscopic 

 examination, and the microscopist must do a certain amount of reconditioning 

 before starting the microscopic work. 



Coarse and fine materials can be separated as the samples are run by a 

 process called dry panning. First, spread the sample in the scoop tray. Tilt the 

 tray slightly and, while holding it in this position, knock the edge of the tray 

 lightly against the knuckles of the other hand until the cuttings accumulate in 



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