184 Subsurface Geologic Methods 



SIZE ANALYSIS 

 L. W. LeROY 



Size analysis permits comparison of grain-size similarities and 

 dissimilarities of sands and dissaggregatable sandstones. With regard to 

 results obtained by size analysis, Twenhofel and Tyler ''^^ comment: 



Statistical analyses certainly permit rapid and easy comparison of large 

 numbers of sediments and render it simple to point out similarities and differ- 

 ences. The best that may be stated is that the significances of the studies 

 are not apparent. Statistical studies certainly permit extensive use of mathe- 

 matical formulae which are of interest to those mathematically inclined. The 

 writers have found these formulae of great interest but not particularly useful 

 so far as interpretation of the sediments is concerned. 



According to Pettijohn "^^ the purposes and significance of size anal- 

 yses are as follows: 



(1) The improvement of classification and the precision of nomencla- 

 ture of clastic sediments, (2) the study of the influence of grain-size dis- 

 tribution on porosity and permeability, (3) the study of relations between 

 the dynamics of stream flow and the transportation of particulate materials, 



(4) quantitative studies of facies changes and correlation problems, and 



(5) identification of the agent or environment responsible for the origin 

 of the sediment. 



In the Lake Maraciabo Basin of western Venezuela, size-analysis data 

 locally reflect the contact between the El Milagro (Pleistocene) and Onia 

 (Pliocene) formations and between the Onia and La Villa (Miocene) 

 (fig. 76). The Lyons (Permian) and Fountain (Pennsylvanian) contact 

 east o*f the Front Range of Colorado may be locally differentiated (fig. 

 77) . The classification of soils has been based on the grade-size principle. 

 The procedure is utilized as a basis for the computation of the most effi- 

 cient size of casing perforations in petroleum-production problems and 

 for the selection of gravel-packing installations in water wells. 



Fine-grained elastics (particles less than 0.088 mm. in diameter) may 

 be graded by decantation or by elutriation methods. Pipette and hydro- 

 meter techniques have also been employed for fractionating fine materials. 



The Wentworth size classification given in table 4 (p. 187) has been 

 widely adopted for defining grain-size fractions of clastic sediments. 



Preparation of Sample 



A 300-gram sample is disaggregated by carefully crushing the sample 

 in a mortar with a rubberized pestle. Grinding should be minimized to 

 prevent excessive grain breakage. Checking the aggregate periodically 

 by microscope is essential to insure normal and complete disaggregation. 



The material is placed in a nest of U. S. or Tyler sieves. The sieve 

 series may be hand-shaken or placed on the "Ro-Tap," a mechanical vibra- 



''* Twenhofel, W. H., and Tyler, S. A., Methods of Study of Sediments, p. 120, New York, McGraw- 

 Hill Book Co., Inc., 1941. 



'^ Pettijohn, F. S., Sedimentary Rocks, p. 30, New York, Harper & Brothers, 1949. 



