Subsurface Laboratory Methods 331 



body, the pores of which are filled with air. Hov/ever, the apparent specific 

 gravity of dried core samples, as used in Refshauge's experiments, does 

 not signify oven-dried samples, but those naturally dried in an arid 

 climate. Consequently, they may retain small amounts of water. Density 

 of such samples will, in most cases, average slightly higher than Hedberg's 

 bulk density, but less than his natural density. This term is essentially 

 the same as "dry density," a term commonly applied to naturally dried 

 core samples. 



Compaction is expressed as the percentage of reduction of rock vol- 

 ume. 



Method of Determination of Densities 



The most commonly used method of shale-density determination in 

 petroleum work is to weigh the sample in air and then in water. The 

 density is equivalent to the weight in air divided by the difference between 

 the weight in air and the weight in water. The most satisfactory results 

 can be obtained by weighing core samples immediately after extraction 

 from the core barrel. In this manner results approaching the "natural 

 density" of the sediment are obtained. Samples which readily absorb 

 water or show evidence of disintegration from contact with water must be 

 coated before weighing. Ambroid cement or collodion have proved to be 

 satisfactory coatings. However, old X-ray or photographic films dissolved 

 in acetone form an effective coating which may be applied by dipping the 

 samples in the solution. Samples chosen should weigh at least 200 grams 

 in order to obtain the most accurate results. If large-size samples are used, 

 an ordinary laboratory balance may be employed. A very fine wire is 

 attached to one side of the balance for the purpose of suspending the 

 sample in air and water. A Westphall balance can be used for smaller 

 samples. 



It is very important that "pure shale" samples be used for density 

 analysis. Every effort should be made to select massive shale free from 

 sand, lignite, secondary mineralization, or other material which might 

 abnormally affect the increase in density brought about by compaction. 

 Experience has shown that very satisfactory results can be obtained if 

 large numbers of samples are carefully selected by simple inspection. 



Samples dried in an arid climate and which have been in storage 

 in a laboratory for considerable time will generally require coating be- 

 fore immersion in water. However, successful results have, in many cases, 

 been realized on large, naturally dried samples if the operation is carried 

 out very rapidly. The results obtained are those falling within the range 

 of apparent specific gravity, mentioned above. A series of laboratory 

 checks on samples have shown that they lose their natural fluid content 

 with time so that results obtained by direct weighing of samples in labora- 

 tories will approach bulk density. (See fig. 135.) 



