Subsurface Laboratory Methods 193 



SETTLING ANALYSIS 

 L. W. LeROY 



Stratigraphic sections involving fine-grained elastics (siltstones, clay- 

 stones, and shales) in most areas are difl&cult to subdivide and correlate, 

 particularly if paleontologic and lithologic data are inadequate. Skeeters ^^ 

 has suggested a method that is based on the rate of settling of minute par- 

 ticles, through a liquid medium, which may be of some correlative value. 

 This technique does not involve numerical determination of particle size 

 but instead considers the settling rate of the particles and resistivity char- 

 acteristics of the supernatant liquid. 



The procedure of this investigation is as follows: The argillaceous 

 sample, after thorough drying, is pulverized to — 120-mesh and placed in 

 a four-foot vertical glass tube (2.5 inches in diameter) into which com- 

 pressed air is introduced through a stopper in the base. One hundred 

 grams of material is added to 2,000 cc. of water. This mixture is then 

 air-agitated for 15 minutes, after which the height of the settling mate- 

 rial is measured at five-minute intervals and the results plotted. 



During the settling stage and at five-minute intervals, electrical-re- 

 sistance values of the supernatant turbid liquid are measured between 

 two electrodes spaced half an inch apart and suspended two feet below 

 the fluid surface. 



The settling and resistivity results of several Pierre and Fox Hills 

 shales are shown in figure 82. Skeeters ^*^ concluded that 



The height-of-settling-surface curves show the greatest promise of appli- 

 cability to correlation. The resistance curves show considerably more similarity 

 between runs on the same shale and are of sufficient variation between shales 

 to offer some promise of possible value in correlation. 



STAIN ANALYSIS 

 L. W. LeROY 



The application of stain solutions to polished surfaces and to thin 

 sections of rocks permits the rapid identification of certain minerals and 

 assists in establishing the distribution and mutual relationship of the 

 minerals. 



Stain results depend on such factors as the texture and structure of 

 the rock, the purity and relationships of the minerals, and the uniformity 

 of the applied procedure. If more exact mineralogic determinations are 

 desired, optical investigations should supplement the stain tests. 



Frequently the subsurface geologist during an examination of well 

 cuttings and cores is concerned with distinguishing between aragonite, cal- 

 cite (limestone), dolomite (dolostone), quartz, feldspar, and certain basic 

 types of clay minerals. Some of the methods applicable for rapidly de- 



*" Skeeters, W. W., unpublished research report, Colorado School of Mines, 1942. 



