UNDERGROUND MIGRATION OF OIL AND GAS 
175 
at the critical temperature of the substance it vanishes. The 
tables prepared by Johnston and Adams^^ indicate that with 
a temperature gradient of 1°C. per 100 feet the total pressure 
upon a water surface inside pores having diameters of 0.01 
microns at a depth of approximately 1 mile would be no greater 
than the pressure outside the pores due to overlying rock. 
^^Alor cover, the surface tensions of all but the lightest hydro- 
carbons decrease much less rapidly than that of water for each 
increment of temperature, so that the surface tension of water 
does not have such great excess over that of oiF^ at depths of a 
mile or two as it does in the zone of lower temperature close 
to the earth’s surface. It is therefore probable that capillary 
segregation of oil and gas must all be effected within two or three 
miles of the ground surface. At greater depths oil must remain 
disseminated throughout the shales, if that were its original dis- 
tribution, unless it had migrated into the sandy strata during 
the time when the beds concerned were closer to the surface. 
Gravitation 
Migration of oil is caused directly and simply by the force of 
gravity only under certain restricted conditions. To permit 
oil to flow “down hill” in obedience to the attraction of gravity, 
the oil drops must be in openings not filled with water, and of 
super-capillary (for crude oil) dimensions. If the openings are 
tubular, their diameter must be greater than 0.25 mm.; if fissures, 
they must be more then 0.12 mm. wide. Pores and crevices of 
greater size than this are probably present in many sedimentary 
rocks near the surface of the earth, but more commonly they are 
filled with water. If water is absent from such a coarsely porous 
rock into which oil has moved, the oil will flow to. the lowest 
portion of the porous stratum and may there collect in com- 
mercial quantities. Or if only sufficient water is present partly 
to fill the open spaces in the bed, the downward passage of the 
oil will be arrested when it reaches the water-bearing portion 
of the porous rock. 
24 J. Johnston and L. H. Adams, Observations on the Daubree experiment and 
capillarity in relation to certain geological speculations. Jour. Geol., vol. 22, 
p. 13. 1914. 
