688 Subsurface Geologic Methods 



are typical of the majority of petroleum reservoir strata, although the 

 distribution may differ markedly from sample to sample in the same 

 reservoir. 



The configuration of the grains in a sediment determine the nature 

 of the pore spaces in the sediment. Obviously, the component grains may 

 be packed in any manner, but only an exceedingly irregularly spaced and 

 shaped pore always results no matter how regular and systematic the 

 packing may be. Although the pores of sandstone are irregular with re- 

 spect to both size and shape, the pore pattern of massive limestone and 

 dolomite is even more irregular and complicated than that of sandstone, 

 being determined largely, not by conditions of sedimentation, but by 

 secondary processes of weathering, leaching, partial solution, and sec- 

 ondary deposition of minerals within fluid passageways.^^ The behavior 

 of fluids is much easier to study in sandstones and oolitic limestones than 

 in other rocks for this reason. Although the individual pores of a rock 

 may be highly irregular in shape and size, the large number of pores 

 imparts relatively consistent mass properties to the rock as a whole, and 

 the mass physical properties do reflect the statistical distribution of size 

 and shape of the pores and do permit mathematical analysis of the gross 

 behavior of the sedimentary body with fluids as an entity. 



Porosity 



The porosity of a rock is a measure of the capacity of that rock to 

 store or hold fluids. In this sense porosity is a static quality completely 

 determined by simple geometry. Porosity is expressed quantitatively as 

 the ratio of the pore volume of the rock on either a fractional or per- 

 centage basis. For example: 



volume of pores volume of pores 



bulk volume volume of pores + rock 



and 



percentage porosity = 100/ 

 in which / is the fractional porosity (2) 



According to this definition, the porosity of porous materials could have 

 any value up to 100 percent, but the porosity of sedimentary rocks usually 

 is considerably less than 100 percent. If, for example, spheres of equal 

 diameter are arranged systematically, so that each sphere touches another, 

 the wide-packed or cubic system (fig. 371) has a porosity of 47.6 percent, 

 and the close-packed or rhombohedral system has a porosity of 26.0 

 percent. Clearly the porosity for such a system is independent of the 

 diameter of the sphere and depends solely upon the arrangement of the 

 spheres. If smaller spheres are mixed among the spheres of such a sys- 

 tem or, if small enough, are placed within the pores, clearly the ratio 

 of voids to solids becomes less and porosity is reduced. Likewise, if 



"Bulnes, A. C, and Fitting, R. V., Jr., AIME, Trans., vol. 160, pp. 179-201, 1945. 



