Subsurface Laboratory Methods 181 



Rock Classification '^^ 



One of the first aims of the petrographer is to classify the rock as to 

 type, i.e., sandstone, calcareous sandstone, argillaceous sandstone, etc. The 

 typing or naming of the rock in itself yields much information about its 

 internal makeup; and, incidentally, the thin section will usually reveal 

 sufficient information on the clastic materials of any size for classification 

 purposes. 



Cementation 



Waldschmidt,'^^ in his paper on cementing materials of sandstones in 

 the Rocky Mountain region, has worked out a sequence of deposition for the 

 various binding materials. He also shows their relation to porosity and 

 permeability in sandstones having various combinations of these cement- 

 ing minerals. His order of deposition of cementing minerals is as follows : 



Waldschmidt's conclusions are based on the study of 111 sections. 

 His paper is an excellent illustration of the use of thin sections in this 

 type of study. 



Porosity and Permeability "^^ "^^ 



Many of the principles governing porosity and permeability go be- 

 yond the realm of thin-section study; yet, much of the information as to 

 why and how certain rocks have as much or little porosity and permea- 

 bility may be learned from the study of thin sections. Grain size and ar- 

 rangement or packing, authigenic mineralization, alteration, interstitial ma- 

 terials, and cementation all exert considerable influence on the amount of 

 pore space available in a rock. 



A single thin section presents a two-dimensional view; therefore, for 



'" Pettijohn, F. J., Sedimentary Rocks, New York, Harper and Brothers, 1949. 



'^ Waldschmidt, W. A., Cementing Materials in Sandstones and Their Probable Influence on Migration 

 and Accumulation of Oil and Gas: Am. Assoc. Petroleum Geologists Bull., vol. 25, no. 10, pp. 1839-1879, 

 1941. 



" Graton, L. C, and Fraser, H. J., Systematic Packing of Spheres with Particular Relation to Po- 

 rosity and Permeability: Jour. Geology, vol. 43, pt. 1, pp. 785-909, 1935. 



" Fraser, H. J., Experimental Study of the Porosity and Permeability of Clastic Sediments : Jour. 

 Geology, vol. 43, pt. 1, pp. 910-1010, 1935. 



