206 Subsurface Geologic Methods 



subsurface beds. In studying sandstones, for example, it would not be 

 possible to observe the actual sand grains, but it would be necessary to 

 study the cementing material and any foreign material in the sandstone. 

 Thus, in investigating the clay content versus water conductivity of oil 

 sands. Bates, Gruver, and Yuster ^^ isolated mica crystals and photo- 

 graphed them in the electron microscope for study. Sandstones bearing 

 similar mica crystals might be correlative, if other criteria attested to a 

 possible correlation. 



Correlations with limestones should involve a different set of condi- 

 tions. As a general rule limestones are compact or, if porous, contain 



Figure 87. Electron-microscope picture of Attapulgus clay (X 20,000). Note minute 

 fibers and bundles of fibers, with very few larger grains. These average less than 

 ^M- (=0.000125 mm.) in diameter and are of colloid size. Courtesy R.C.A. 

 Laboratories and Standard Oil Development Company.) 



comparatively large pores and openings. It would be difficult to impos- 

 sible to grind a thin section of limestone to a thickness allowing the 

 electrons to pass through the specimen and produce an image on the photo- 

 graphic plate. The pores of the limestone are so large as to preclude any 

 precise study of their contour or shape. For these reasons, a possible ap- 

 proach would lie in the study of the residues after the limestone had been 

 dissolved in some suitable solvent. Either the filtrates could be examined 

 for correlatable objects, or the residue, which is often largely clay, might 



^ Bates, T. F., Gruver, R. M., asd Yuster, S. T., Influence of Clay Content on Water Conductivity 

 of Oil Sands: Oil Weekly, Oct. 21, 1946. 



