UNDERGROUND MIGRATION OF OIL AND GAS 
161 
outward, it will necessarily exclude ground water, oil, or gas, 
from the pores which it closes and may drive fluids and gases 
into other portions of the reservoir. The distinction between 
the sands reported by the driller as tight (and dry) and those 
stated to be loose (and water, gas, or oil-bearing) is probably 
in most cases a result of differential cementation. 
At present, however, it is impossible to say whether a tight 
sand is devoid of oil because the cementation of the sand drives 
out its fluid content or because the presence of cement filling the 
pores prevented the immigration of valued hydrocarbons; 
probably the latter is more often the case. In general, it should 
be observed that the presence of oil in a rock pore will itself 
hinder if not entirely prevent the filling of that pore by cement 
carried in aqueous solutions. Between the encroaching cement 
and the retreating oil there must always be a buffer of water. 
It is doubtful whether progressive cementation of sediments 
has even been an important cause of the migration of oil or gas. 
Capillary action 
Molecular forces are by no means completely understood. 
The exact nature of both inter-molecular and intra-molecular 
attractions and repulsions are more or less of a mystery The 
best that can be done in the present state of knowledge is to 
give names to the more obvious phenomena resulting from these 
forces, and to postpone the enquiry as to their fundamental 
causes until the physicist and the chemist have completed their 
research concerning them. 
Of the phenomena attributable to molecular action, the two 
most important in this connection are capillarity and surface 
tension. Surface tension is the property ‘Vhich exists in the 
surface film of all liquids and tends to bring the contained volume 
into a form having the least superficial area.” It is this prop- 
erty which causes small quantities of oil, water, mercury, or 
other fluids to form into spherical “drops” when scattered over 
the surface of a solid or sprayed into the air. It apparently 
results from cohesion, the attraction existing between closely 
adjacent molecules of the same substance, a force which is “quite 
