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appreciable change in the relative position of oil and water. 
But with a little motion that oil will gradually be distributed 
above the water along the upper side of the tube. It does not 
appear to be necessary to have very extensive movement, but 
some degree of movement is essential. 
Or, again, the slow circulation of ground water may permit 
surface tension phenomena to play a part which would other- 
wise be denied them. If a body of gas has been segregated 
above the water in porous sands, circulation or migration of 
the water may bring globules of oil to the surface of contact 
between gas and water. If so, these will be held on the surface 
of the water by its tension. The oil will first spread as a film 
along the gas-water surface and any oil which is brought in 
contact with this film must join it. In this way oil may ac- 
cumulate above water in any subterranean reservoir in spite of 
the slight difference in specific gravity between the two fluids. 
^^Any movement of the water in the sand, which would in time 
bring the various parts of the water in contact with its upper 
surface, would carry bodies of oil of all sizes to the water-gas 
surface, where surface tension would retain them permanently.’’ 
Rich believes that the 
principal cause of the migration of oil and gas is the movement of 
underground water which carries with it minute globules of oil and 
bubbles of gas, possibly as fast as they are formed. Accumulation 
results from the selective segregation of oil and gas, which, on account 
of their buoyancy, always tend to work their way upward as they are 
carried along and are caught and retained in anticlinal or other suitable 
traps.^^ 
Gas expansion 
Destructive distillation of organic matter, animal or vegetable, 
under conditions such as those commonly occurring within the 
body of the earth where free access of air is precluded, produces 
hydrocarbon gases as well as liquid hydrocarbons. The process 
2 9 John L. Rich, Moving underground water as a primary cause of the migra- 
tion and accumulation of oil and gas, Econ. Geol., vol. 16, pp. 347-371, 1921. 
