Subsurface Laboratory Methods 125 



B. Rough 



1. Striated 



2. Faceted 



3. Frosted 



4. Etched 



5. Pitted 



One must be careful in interpreting the meaning of surface textures. 

 If transportation has not been sufficiently long or rigorous, a pre-existing 

 surface texture may be modified only slightly and consequently may be 

 an inheritance from a pre-existing rock. Also the original surface texture 

 of detrital grains may be modified either by solution or by the addition 

 of cement of the same composition after deposition. The presence in a 

 sand of a wide range of surface textures probably means a mixed source 

 for the sand. 



Orientation 



Although it is an important property, the arrangement of sedimen- 

 tary particles in space does not lend itself to megascopic or binocular 

 examination or description in most subsurface work. The small size of 

 clay and silt particles, the small size and lack of known orientation of 

 cuttings, and the tendency of many sands to break down to individual 

 grains make orientation study of rotary and cable-tool cuttings difficult 

 or impossible. In oriented cores that contain pebbles or coarse sand the 

 orientation of the particles may give clues concerning the direction of 

 flow of the depositing currents. The imbricate or shingled arrangement of 

 flattened pebbles shows the direction of current movement. 



Dapples and Rominger ^^ have shown that the long axes of quartz 

 sand grains tend to parallel the direction of flow in streams, and that the 

 largest ends of the grains are toward the current. Similar measurements 

 should be possible along the bedding planes of some cores. Particular 

 attention should be given to elongated grains. The orientation of at least 

 25 randomly selected grains should be determined. 



Recent work on the directional permeability in sands ^^ suggests that 

 primary orientation may be important in primary production of oil, in 

 secondary-recovery operations, and in working out the paleogeography and 

 geologic history of some sands. Much basic work on deposits of known 

 environment is needed. For the finer sands and silts and for the lime- 

 stones, dolomites, and other crystalline sediments petrofabric analysis 

 will be needed. 



Composition. 



Although more than 100 minerals have been identified in sedimentary 

 rocks, only about 20 minerals or families of minerals commonly are 



Dapples, E. C, and Rominger, J. F., Orientation Analysis of Fine-Grained Clastic Sediments: 



A Report of Progress: Jour. Geology, vol. 53, no. 4, pp. 246-261, July 1945. 



' Johnson, W. E., and Hughes, R. V., Directional Permeability Measurements and Their Significance: 



Producers Monthly, vol. 13, no. 1, pp. 17-25, Nov. 194S. 



