80 Subsurface Geologic Methods 



Wentworth's subdivision of grade size ^ has been widely accepted by most 

 sedimentologists. Particle-size percentages may be graphically represented 

 by histograms and frequency curves, and such values as median, coeflBc- 

 ient of sorting, skewness, and kurtosis determined. These values permit a 

 quantitative representation of particle size. 



The shape (sphericity, roundness, and flatness) of particles may be 

 determined by various methods and numerical values given. Such terms 

 as "angular" (showing very little or no abrasion) , "subangular" (show- 

 ing some effect of wear), "subrounded" (showing considerable abrasion), 

 and "rounded" (exhibiting conspicuous wear) are commonly used to 

 designate degree of angulation. Some medium-grained elastics exhibit 

 pronounced uniformity in grain angularity, whereas others show consid- 

 erable variation of angularity. 



Close examination of the elastics reveals in certain instances distinct 

 fabric pattern of the grains. Petrofabric diagrams (figs. 69 and 70) are 

 helpful for illustrating these orientation trends. 



In addition to the size, shape, and arrangement of grains, special at- 

 tention should be given the characteristics of grain surfaces, such as degree 

 of polish, smoothness, striation, and pitting. These features have definite 

 genetic significance. 



Permeability and porosity are controlled in large part by the texture 

 of the rock. Since these two factors are of primary importance to the oil 

 geologist in production problems, textural attributes of producing strata 

 should be carefully evaluated. 



Textures of carbonate and evaporite rocks range from fine to coarse 

 crystalline. Textures of chemical sediments are clearly outlined by Petti- 

 john.^ Such terms as "macrocrystalline" (granoblastic, over 0.75 mm.), 

 "mesocrystalline" (porphyroblastic, 0.20 to 0.75 mm.), "microcrystalline" 

 (0.01 to 0.20 mm.), and "cryptocrystalline" (less than 0.01 mm.) are 

 discussed. 



According to DeFord,'^ "the grade scales for clastic rock are not 

 suitable for carbonate rock, because the names of the scale units imply 

 the clastic origin of the rock." DeFord recommends the terms and size 

 limits given in figure 35. 



Structures in Sedimentary Rocks 



Structures developed in and exhibited by sedimentary strata are 

 numerous and varied. Special attention should be given these features, 

 because of their usefullness in deciphering depositional environments, 

 stratigraphic succession, and structural anatomy. Two types of structures 

 in sedimentary rocks are recognized: inorganic and organic. The former 

 includes such features as ripple marks, swash marks, current marks, pit- 



° Wentworth, C. K., Fundamental Limits to the Sizes of Clastic Grains: Science, vol, 77. pp. 633 634, 

 1933. 



^ Pettijohn. F. J., op. cit., p. 73. 



' DeFord, R. K., Grain She in Carbonate Rock: Am. Assoc. Petroleum Geologists Bull., vol. 8, pp. 

 1921-1926, 1946. 



