UNDERGROUND MIGRATION OF OIL AND GAS 
177 
enough lifting force of water displacement to overcome adhesion 
and inertia, and to float it freely upward. All three of these 
phenomena are active in the interstices of the rocks. Circula- 
tion of ground water may simulate the first; crustal movement, 
tidal kneading, and earthquake vibrations are the second; 
and molecular forces are probably identical beneath the surface 
of the earth to those in the laboratory. But, in the petrolif- 
erous rocks, if an oil drop starts to obey the impulse of gravita- 
tive displacement, its upward progress is hindered by the fric- 
tion of the rock surfaces past which it must move. The retard- 
ing effect of friction cannot be quantitatively determined from 
the data now available; it is entirely possible that it may in 
most instances prevent effective gravitative assortment of oil 
and water, although this is by no means certain. 
Gravitational sorting of gas and water. The displacement of 
gas bubbles in water is quite another matter. Gas rises through 
water-saturated sandstone several hundred times as readily as 
oil. Differences ' in specific gravity are probably quite compe- 
tent to concentrate gas above the water body in a porous rock 
mass. It is probable that soon after the formation of a bubble of 
gas, as a result of chemical action, change of temperature, or 
increase in pressure, the bubble would rise through the water 
in any supercapillary fissure or pore until ascension was stopped 
by a barrier of water-filled spaces, none of which were of more 
than capillary dimensions. 
Effect of gas migration in transporting oil. But if while being 
forced upward the bubble of gas touches a droplet of oil clinging 
to the wall of Assure or pore, it will be surrounded by a thin 
film of oil, for oil has an extraordinary capacity of spreading 
along any surface between water and gas.^^ Each bubble of 
gas rising from its place of origin or later moving upward through 
rock cavities may carry with it a pellicle of oil and thus accom- 
plish gravitational sorting of oil as well as of gas. When the 
gas bubbles with their films of oil unite in the space to which 
they are driven by the lifting force of water displacement, the 
25 Johnson, R. H,, The accumulation of oil and gas in sandstone, Science, 
vol.,35, pp. 458-9, 1912. 
