UNDERGROUND MIGRATION OF OIL AND GAS 
169 
Theoretical considerations 
As stated above, the surface tension of crude oil is between 
one-third and one-half that of water. Capillary action must 
therefore exert a much stronger pull upon water than upon oil. 
According to Washburne/^ 
the amount of the capillary pull varies inversely as the diameter of a 
pore, hence the constant tendency of capillarity is to draw water, 
rather than oil, into the finest of openings, displacing any gas or oil 
in the latter. Gas itself is not drawn into capillaries by the action of 
surface tension, and it can leave the fine pores without any capillary 
resistance. It is therefore the most quickly and most completely 
gathered in the largest spaces available. Moreover, capillarity resists 
any movement of water from fine toward large pores more than it 
resists the movement of oil or gas in that direction. In short, water 
enters fine capillaries about three times as readily as oil, and it en- 
counters about three times as much capillary resistance in leaving them. 
The final result of this action must be the concentration of nearly 
all the gas and oil in the openings having least capillary power, namely, 
in fissures if present, and in the larger rock pores. This appears to be 
the general rule in oil fields, where the pores of the coarser-grained 
rocks contain practically all of the oil, while the pores of the adjacent 
fine-grained rocks contain practically no oil. 
Here, too, is a possible explanation of the phenomena dis- 
played in the experiments described above. Differential capil- 
lary attraction may be competent to induce the migration of 
water from larger pores into adjacent ones originally filled with 
oil and to force the oil out of these spaces into those originally 
occupied by water. 
There is, however, a possible fallacy here.^^ In comparing 
the surface tension of oil and of water, the comparison was 
between the tension on an oil-air surface and that on a water- 
air surface. In the experiments with saturated sediments, and 
presumably under most conditions obtaining within the earth, 
W. Washburne, The capillary concentration of gas and oil, Trans. Am. 
Inst. Min. Eng., vol. 50, pp. 823-858, 1915. 
Cf. Johnson, R. H., The Time Factor in the Accumulation of Oil and Gas, 
Bull. Am. Assoc. Petrol. Geologists, vol. 5, pp. 475-481, 1921. 
