UNDERGROUND MIGRATION OF OIL AND GAS 
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much greater importance, the size of the spaces in different 
kinds of rocks under different conditions should be determined. 
Porosity of sedimentary rocks. The total volume of pore space 
in a rock does not depend upon the size of the rock particles but 
upon their shape, assortment, arrangement, and degree of 
cementation. In figure 1, for example, the total amount of pore 
space in A and C, or B and D, is the same, but there is a pro- 
gressive decrease in the diameter of the individual pores from the 
ideal case shown in A to that in D. Variations in the dimen- 
sions of interstices in clastic rocks must in general be roughly 
proportional to the size of rock grains, other things being equal. 
It is not, however, the average size of grain which is the most 
important consideration, but the proportion of extremely fine 
particles which if present in sufficient amount would ^^sift” 
into the larger spaces between the coarser grains and leave 
openings with dimensions approximating one-sixth to one-half 
the diameter of these smallest components of the rock. An 
argillaceous sand might, therefore, have pore spaces the dimen- 
sions of which would approximate those of the interstices between 
the particles of clay in shale. The more perfect the assortment 
by size and the ^Tleaner” the sand, the larger will be the voids 
in the rock. But this neglects the effect of cementation which 
as evidenced in diagrams E, F, and G, of figure 1, greatly in- 
fluences the size of pore as well as the total amount of porosity. 
With these variables, it is readily apparent that abrupt changes 
in pore dimensions as well as in volume of openings may be 
expected in any sedimentary series when traced laterally or 
traversed vertically. 
Data are not available for more than a broad approximation 
covering the relative sizes of pores in shales and sandstones. 
It is probable however, that interstitial spaces in most shales, 
are less than 0.01 mm. in diameter, and in the finer clay shales, 
especially if considerably compressed, openings would average 
less than a micron (the thousandth part of a millimeter) in 
cross-section. Sandstones display greater variation in size of 
pores because of the limitless variations in amount of cement 
and in abundance of fine constituents. The sands used by 
