184 
KIRTLEY F. MATHER 
commercial accumulations of these substances may now be 
sketched. Variations in the relative importance of the different 
factors involved may of course be expected because of the fact 
that each oil or gas field is actually unique in its composite 
features, but the general story needs only slight changes in its 
major outlines to make it apply to most fields. 
Distillation, at first dominantly biochemical and later domi- 
nantly thermochemical, begins with the entombment of organic 
matter in muds, clays and silts. With continued accumulation 
of interstratified sandy and muddy layers, the oils and gases 
thus formed are squeezed out of the finer into the coarser sedi- 
ments. Differential capillary action draws water into the finer 
openings while the volatile hydrocarbons move into the larger 
ones. Thus, transverse migration from the shale birth-place of 
petroleum to the reservoir sands and sandstones takes place. 
Circulation of subterranean fluids and movements of the earth’s 
body all contribute to this movement from one layer to an ad- 
jacent one; whenever oil or gas enters coarsely porous portions 
of the earth’s interior, completely enclosed by water-saturated 
fine-grained rocks, there it must remain. Transverse migration 
is never complete; the shales invariably retain more or less of 
the petroleum which originates therein; it is more nearly com- 
plete where there are several sandy layers interbedded at fre- 
quent intervals within the shale series than where the same 
amount of sand is concentrated into a single formation. 
Oil and gas which enters coarsely porous strata, sandstones or 
limestones, will move more or less completely to the pores above 
those occupied by water within the reservoir. If the rocks are 
saturated with water, differences in specific gravity, molecular 
forces, ground water movement, etc., will lift the hydrocarbons 
to the top of the reservoir and segregate them beneath fine- 
grained strata or portions of strata. If the reservoir roof is 
sufficiently inclined, the upward force will be resolved into 
lateral movement which in most cases will be parallel to the 
bedding of the rocks. This obliquely upward migration will be 
stopped where the oil or gas is trapped by changes in the pitch 
of the reservoir roof. Hence, a knowledge of the shape of the 
