UNDERGROUND MIGRATION OF OIL AND GAS 
181 
is a biochemical one, effected largely by the action of anaerobic 
bacteria before the deposits are deeply buried or the formation 
firmly consolidated. But later, if the same beds are covered 
with thick accumulations of younger sediments, gas may be 
formed as a result of the action of heat upon the entombed or- 
ganic matter, liquid or solid. This thermochemical process^® 
is accompanied by polymerization of the oil and may be furthered 
by the increase in pressure resulting from burial. The two • 
kinds of action are mutually complementary; the first men- 
tioned dwindles to nothingness as the second increases in effect- 
tiveness; the first is long continued, but the second knows no 
time limit. Consequently, new gases are produced long after 
the petroliferous beds are more or less completely sealed by 
water-saturated ^Tover rocks.” 
To these organic gases, there may possibly be added in some 
localities a modicum of abysmal gas, inorganic in origin, slowly 
ascending from the deeper interior. The presence of argon and 
helium among the gases from the deeper sands of Kansas may 
be thus explained. But regardless of any possible inorganic 
contributions, these lighter hydrocarbons may be formed under 
all conditions of temperature and pressure, including those far 
above their critical temperatures and pressures, so that they 
may be in that state of matter where the true distinction be- 
tween gas and liquid disappears. 
Gas expansion, if permitted by local physical conditions, 
will drive fluids, petroleum or water, out of the larger openings 
in the rocks more readily than from the smaller ones where 
viscosity and capillarity tend to retain liquids. Relief of pres- 
sure at any point or along any plane may result in the migration 
of oil and water in front of the expanding gas. The result would 
frequently be scattering of oil rather than concentration, but 
upward movement would be favored, for relief of pressure 
would more commonly come from that direction. Hence, here 
20 R. H. Johnson, The role and fate of the connate water in oil and gas sands. 
Am. Inst. Min. Eng., Bull. No. 98, pp. 221-226, 1915; Trans., vol. 51, pp. 587- 
610, 1916. 
