Subsurface Laboratory Methods 141 



exposed a foot above or below the one in the first outcrop. Only zones or 

 intervals may be correlated successfully. Outcrop samples of unweathered 

 chips, without lichen, soil, or other extraneous matter are desirable. 



Oil- or water-well cuttings and cores are the most widely used mate- 

 rials for residues. Cable-tool cuttings are the best samples, as they contain 

 a minimum amount of caved material, and each sample represents a com- 

 posite of the rock within the sampled interval. 



Rotary-tool cuttings are the most common well samples and are gen- 

 erally the only type of samples available from deep wells. They are also 

 the worst samples. Caving is very common because long sections of the 

 drill hole are frequently open. If shale beds or loosely aggregated mate- 

 rials lie above a given sample, caving may reduce the amount of indigenous 

 material of the sample to such a small percentage that an insufficient 

 amount of residue or none will be left after solution. Such samples may 

 require the use of forceps for picking out chips of the indigenous material 

 for solution. Drilling time, electric logs, and a thorough knowledge of 

 the section facilitates the identification of the indigenous material. 



Well cores must be split and a fragment taken from each inch or 

 short interval, and the whole mixed for the equivalent of a five-foot sam- 

 ple, or a shorter interval if the lithology changes. Otherwise, inconclusive 

 point-to-point correlation would be necessary. 



The observable amount of indigenous material in a sample having 

 eighty to ninety percent shale caving may be increased by placing two or 

 more unit volumes of the sample in acid, and, after solution, sieving out 

 as many unit values less one. Thus, if three units were used, two units 

 would be sieved out after solution. This will leave less than one unit 

 volume, which will contain a minimum amount of caved material but 

 several times more residue from the indigenous material. Large frag- 

 ments of chert or other insoluble material considered indigenous may be 

 picked out with forceps from the sieve and added to the residue. 



Amount of Sample 



The volume or weight of sample used to make a residue depends on 

 the purpose of the study, the type of samples used, and individual judg- 

 ment. Seven grams is an ample amount of sample for ordinary uses. This 

 weight is an average for the volume contained in a one-dram vial, 45 by 

 15 mm. The same volumes of ten different homogenous samples ranging 

 from very fine to very course fragments of limestone, shale, sand, and 

 chert were weighed, and the average of seven grams was determined. The 

 volume-weight of seven grams reduces considerably the time for the prepa- 

 ration of residues. Small samples of less-than-unit volume must be weighed 

 if percentage determinations are desired. The use of a small scoop sized 

 for a unit volume or a tip balance saves time. 



Many workers do not use percentages, but the percentages of residue 

 are valuable in many cases for correlation and identification of beds. 



