170 
KIRTLEY F. MATHER 
it is the molecular forces operating on an oil-water surface which 
are significant. Here quantitative data are not available; 
Cook’s tentative conclusion, that the minimum size of opening, 
which will allow the interchange of oil and water to occur, is 
considerably larger than the minimum which will permit the 
capillary movement of either oil or water alone, would suggest 
that the simple numerical ratio cited by Washburne does not 
directly apply.^® Nevertheless, it is evidently a fact that under 
certain conditions this interchange of oil and water does take 
place exactly as though the movements were induced by capil- 
lary action. Probably, as Cook points out, the effects of dif- 
ferential adhesion of oil and water, especially the far superior 
adhesion of water for rock, are quantitatively more important 
than those resulting from differential cohesion of these same 
liquids as expressed by their surface tension. The phenomena 
of adhesion undoubtedly play an important part in capillary 
action and may be largely responsible for the interchanges of 
oil and water, which have been observed. 
The direction of the movement resulting from such interchange 
is entirely independent of gravity; impelled by differential capil- 
lary forces, oil or gas may migrate upward or downward or 
laterally with equal facility. Migration would be from smaller 
to larger openings in the rocks, and presumably cannot reverse 
itself so long as water is present. Probably, neither oil nor gas 
can, under ordinary conditions, move into a stratum or portion 
of a stratum of rock which contains no openings of greater than 
capillary dimensions (for water) and is thoroughly saturated 
with water. 
Application to the conditions existing in the earth 
These considerations make evident at once the imperative 
necessity of data concerning the pore spaces existing in sedi- 
mentary rocks at different stages in their history. Not only 
should the total amount of pore space be known, but, of probably 
See also Skirvin, O. W., Experimental Study of the Invasion of Oil into a 
Water-Wet Sand, Econ. GeoL, vol. 17, pp. 461-469, 1922. 
