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KIRTLEY F. MATHER 
with water; hence it cannot traverse a zone of wet shale, all the 
openings in which are finer than those in which the oil occurs 
even though a coarsely porous sandstone may be only a few inches 
distant. In the second place, a considerable fraction of the oil 
will be retained in the argillaceous rocks by absorption and 
adhesion, as indicated by the experiments of Gilpin and Cram-^ 
who state ^Vhen oil is mixed with fuller’s earth and then dis- 
placed with water, about one-third of the oil remains in the 
earth.” These capillary forces would probably be most effective 
in their task of draining the shales of oil and gas and segregating 
these substances in the sandy beds if the latter were present at 
frequent intervals in the midst of a thick shale series. Recur- 
rence of sandstones is more favorable than the same amount 
of sand deposited in a single massive bed, but in any event it is 
apparent that nothing could be more effective as a seal to stop 
transverse migration of either oil or gas than a water soaked 
bed of shale capping the sands. Once driven into the sandy 
horizons the hydrocarbons are there imprisoned, unless super- 
capillary fissures traverse the shales. 
A consideration of the variability of dimensions of pore spaces 
in any sedimentary series makes it evident that migration of 
oil and gas due to capillary forces cannot in most cases extend 
over great distances. The very forces which impel segregation 
in the coarser pores make it impossible for hydrocarbons to 
traverse the average sedimentary series from bottom to top 
unless there are open fissures to serve as channels. Nor can the 
oil be gathered by lateral movement within a bed from distant 
points to a center of accumulation. Capillarity may only be 
appealed to as the force responsible for the transfer of oil or gas 
from one bed of fine-grained rock to a closely adjacent bed of 
coarser or more open texture, but in probably every oil field 
other forces have cooperated to concentrate the fluid in small 
enough compass to make an accumulation of commercial value. 
Localization of capillary segregation near surface of earth. 
Surface tension decreases with increase in temperature until 
23 J. E. Gilpin and M. P. Cram, The fractionation of crude petroleum by capil- 
lary diffusion. U. S. Geol. Surv., Bull. 365, p. 33, 1908. 
