176 
KIRTLEY F. MATHER 
Differences in specific gravity 
Although the direct influence of gravitation is probably not 
generally important in causing underground migration of oil 
or gas, its indirect effect through differences in specific gravity 
of fluids and gases is unquestionably of prime importance. Most 
crude oils are quite appreciably lighter than water, especially 
the heavy salt water which is commonly associated with oil. 
The excess in specific gravity of water varies from more than 
35 per cent, in the case of a very light oil compared with a heavy 
brine, to less than 5 per cent, if heavy oils be compared with 
dilute brines. An ordinary crude oil, with a specific gravity 
of 32° or 33°Baume, is about 20 per cent lighter than the ordi- 
nary salt water with which it is commonly associated. If, 
then, petroleum and water are associated together in openings 
of super-capillary size, the difference in specific gravity will 
result in a tendency for the oil to rise until its upward progress 
is barred by a rock mass in which the openings are all of capillary 
size and are filled with water, or until it reaches the surface of 
the water body which displaces it. 
Gravitational sorting of oil and water. The tendency of water 
thus to drive oil upward in coarsely porous strata is unques- 
tioned; the actual movement of oil due solely to differences in 
specific gravity is by no means certain. If drops of oil are 
squeezed from a fountain pen filler against the bottom and 
sides of a basin filled with water, it may be observed that ^^most 
of the drops cling persistently to the vessel and do not rise 
through the water.’’ The slight difference in weight between 
the small drop of oil and an equal volume of water is not suf- 
ficient in itself to overcome adhesion between the liquid and the 
adjacent solid, although the area of solid contact is very much 
less than that which would be operative if the same drop of oil 
were spread out in the pores of the ordinary sandstone. The 
drops of oil in the basin of water may be dislodged by stirring 
the water or by tapping the vessel; or if separate drops are so 
closely adjacent that cohesion and surface tension unite them 
into a single large drop, its size may be sufficient to induce 
