692 Subsurface Geologic Methods 



terstices may be isolated; and, consequently, sometimes the terms total 

 and effective porosity are employed where differentiation between the 

 porosity based upon the interconnected pores and those that are not is 

 necessary. Total porosity obviously refers to all pores, both intercon- 

 nected and isolated, whereas effective porosity obviously refers to only 

 interconnected pores, these only being effective in fluid flow. The isolated 

 pores seldom amount to more than ten percent of the total volume of 

 pores in any sedimentary rock. 



Measurement of Porosity 



The measurement of the porosity of a rock is simple in principle, 

 but can be complicated in practice. The definition of porosity clearly 

 indicates the quantities that must be evaluated in measurement: namely, 

 the pore volume and the bulk volume. Any method of measurement that 

 will enable the reliable determination of these quantities with the degree 

 of accuracy desired may be employed. The bulk volume may be deter- 

 mined by direct measurement for simple geometric forms or by the 

 accurately observed displacement of a suitable liquid. The pore volume 

 may be measured by filling or emptying the pores of a sample with a 

 suitable fluid and measuring the amount required. If the fluid employed 

 is a gas, the accurately observed relations between pressure and volume 

 in isothermal expansion and contraction of the gas within a closed system, 

 part of which includes the pore space to be evaluated, makes possible the 

 computation of the pore volume of the sample. The porosity also is 

 calculable by comparison of the apparent specific gravity of the sample: 

 namely, the ratio of the weight of the sample to its bulk volume, with 

 the true specific gravity of the rock. 



Details of the exact procedure required in the laboratory measure- 

 ment of porosity are available in the literature.^® ^^ ^^ ^^ ^° -^ ^^ ^^ ^* ^^ 



If the pores are so small that penetration by a fluid such as mercury 

 is slight, the bulk volume is readily observed by direct displacement in a 

 suitable volumeter or is calculated from the loss of weight obtained upon 

 submersion of the sample in a liquid. Penetration of the sample by a fluid 

 can be prevented by coating the surface thinly with a lacquer, collodion, 

 or wax. The protected sample can be immersed for determination of the 

 bulk volume. The sample can be saturated with a suitable fluid such as 

 water, tetrachlorethane, or oil, and then the displacement or loss in weight 

 resulting from immersion can be observed. The familar Russel volumeter 

 is useful for this purpose ^^ (fig. 372) . 



^* Barnes, K. B., The Pennsylvania State College, Mineral Industries Experiment Station Bull. 10, 

 1931. 



" Coberly, C. J., and Stevens, A. B., AIME, Trans., vol. 103, pp. 261-269, 1930. 



" King, F. H., USGS, 10th Annual Report, pt. II, pp. 50-294, 1897-1898. 



" Melcher, A. F., AIME, Trans., vol. 65, pp. 469-497, 1921. 



=" Plummer, F. B., and Tapp, P. F., Am. Assoc. Petroleum Geologists Bull., vol. 27, pp. 64-84, 1943. 



21 Ritter, H. L., and Drake, L. C, Ind. Chem. Anal. Ed., vol. 17, pp. 782,786, 1945. 



" Russell, W. L., Am. Assoc. Petroleum Geologists Bull., vol. 16, pp. 231-254, 1932. 



23 Taliaferro, D. B., Johnson, T. W., and Dewees, E. J., U. S. Bur. Mines, R. I. 3352, 1937. 



2* Washburn, E. W., and Bunting, J.. Am. Ceramic Soc, vol. 5, pp. 48-56, 112-129, 1922. 



25 Westbrook, M. A., and Redmond, J. F., AIME, Trans., no. 165, pp. 219-222, 1946. 



2« Russell, W. L.. op. cit. 



