UNDERGROUND MIGRATION OF OIL AND GAS 
173 
King^i in measuring the observed pore space in soil and rock 
are reported to have had an “approximate effective diameter” 
which varied from less than 0.1 mm. to more than 2.5 mm., 
with the greater number of samples approximating 0.2 and 0.3 
mm. It is entirely out of the question to apply any rigid math- 
ematical formula stating the relation between pore size and sand 
grain diameter. Irregularity of outline of the sand particles 
is so great in the average sandstone that the common assumption 
of an accumulation of spheres is scarcely an approximation to 
the truth. To assume that the grains are spheres gives neither 
the maximum nor minimum total porosity or size of pore. “For 
simple sands with angular grains the pore space is much larger 
than it is for the rounded sands of the same sizes of grains, and 
in the case of crushed glass, whose grains are more angular than 
those of the crushed limestone, which have a tendency to be 
cuboidal in form, the pore space is the largest of all .”22 if 
this is true for total pore space, it must necessarily follow that 
the size of individual pores is greater in sandstones composed 
of angular grains than in those composed of rounded grains of 
similar sizes. The dimensions of occasional pores may equal the 
diameter of the sand grains if the latter are angular. The 
width of the open spaces between grains of ordinary sandstones 
may therefore vary from less than a micron to more than a 
millimeter. Probably the average pores of a poorly cemented 
sandstone would be between 0.1 and 0.5 mm. in width. 
Transverse migration of oil and gas. In any series of alter- 
nating shales and sandstones, containing sufficient water to fill 
the openings in the shales, oil and gas will therefore tend to move 
from the shales and segregate in the sandstone strata. This 
transverse movement of the hydrocarbons will not be complete. 
In the first place there will almost certainly be porous lenses in 
the shale beds, entirely surrounded by compact shale with finer 
openings. Oil once driven into these lenses cannot afterward 
return into the finer peripheral pores as long as they are filled 
F. H. King, Principles and conditions of the movements of ground water, 
U. S. Geol. Surv., 19th Ann. Rept., Pt. 2, pp. 59-294, 1899. 
22 King, loc. cit., p. 215. 
