182 
KIRTLEY F. MATHER 
is another cause tending toward the elevating of oil, if not toward 
the top of any subterranean reservoir, at least toward the sur- 
face of the earth. 
Mills has recently^! called attention to the importance of the 
escape of gas with entrained oil through fissures as a factor in 
the migration and accumulation of oil in many faulted areas. 
Both experimental and field data seem to indicate that ^^oil is 
propelled more effectively than water by the propulsive force of 
absorbed gas. Immediately upon the release of pressure, 
the absorbed gas expands and propels the oil from within. The 
comparatively high absorption capacity of oil and its tendency 
to remain entangled with the flowing and expanding gas appears 
to be largely responsible for this effective propulsion.’’ Such 
migration of gas and oil in fault or other fissures ^Tas been up- 
upward either to the surface or from one bed to another. Fis- 
suring has also facilitated the lateral migration of oil and gas 
through porous beds toward these points of escape.” 
Earth movements; folding and faulting 
The accumulation of strain within the earth’s crust may ex- 
ceed the resisting strength of the strata and necessitate adjust- 
ments by folding or faulting movement. In so far as these 
adjustments involve decrease or increase of reservoir volume, 
they may result directly in the transfer of oil or gas from place 
to place. If similar folds^^ are formed, the beds will be com- 
pressed and thinned on the limbs of anticlines and synclines, 
but thickened and expanded on the crests and in the troughs of 
the folds. The result will be a tendency toward the migration 
of interstitial fluids from the place of compression to the places 
of lesser pressure^ — the crest of an anticline or the bottom of a 
syncline. The balance between folding movements competent 
to cause such migration and those which w^ould cause regional 
d^mamic alteration sufficiently complete to volatilize all the 
R. Van A. Mills, Natural Gas as a factor in oil migration and accumulation 
in the vicinity of faults, Bull. Am. Assoc. Petroleum Geologists, vol. 7, pp. 14- 
24, 1923. 
32 C. K. Leith, Structural Geology, Henry Holt & Co., New York, 1913, p. 106. 
