Subsurface Laboratory Methods 



333 



Sources of Error 



Sources of error in shale-density determinations fall into two cate- 

 gories. These are errors which may be introduced by some inherent con- 

 ditions of the sample and those acquired during the actual process of 

 weighing the sample. 



The inherent conditions of the sample, which result in variations in 

 shale density, may be attributed principally to the effects of weathering 

 and mineralization. Leaching near the surface or at unconformities will 

 result in a decrease in density. Increases in density are often brought 



Figure 136. Relationship of density to depth of burial. (After Athy. Reproduced 

 permission Am. Assoc. Petroleum Geologists.) 



about by secondary mineralization. In some cases pyritization has caused 

 significant increases in rock density. 



Several sources of error are associated with the weighing of samples. 

 Size of sample in some cases is an important factor in this respect. Minor 

 errors in weighing of a small sample can have a very important effect 

 upon the specific gravity, whereas the small errors on large samples will 

 have a relatively minor effect upon the ultimate results. Sensitivity of 

 scale used in weighing is another source of error. Water absorption by 

 porous samples can, in some cases, bring about serious error in density 

 determinations. 



The effect of the suspending cradle or fine wire and the cellulose 



