IMPOKTANCE OF DRAINAGE AREA 
145 
near the crest of each fold, may have been drawn is enclosed by 
a sinuous line. 
A merely desultory glance at the oversheet is sufficient to 
enable one to grasp the import of the structure. of the region so 
far as its influence upon the accumulation of oil and gas is con- 
cerned. If we assume that a suitably porous reservoir stratum 
is continuous beneath the surface of the entire region, that 
hydrocarbons were at one time disseminated uniformly through- 
out that stratum, and that they have subsequently moved up 
the dip in obedience to gravitational sorting, we may conclude 
that oil and gas will be concentrated in and near the shaded 
tracts in amounts proportional to the size of the feeding grounds 
or drainage areas. 
Such a map obviously fails to present a complete picture of 
the geologic structure of the region. Where the rocks are faulted, 
as they are in the area chosen for illustrative purposes, the struc- 
tural drainage areas may be outlined only after making certain 
unsupported assumptions as to the effect of faults upon the 
movement of the hydrocarbons. Here, for example, it was as- 
sumed that faults whose maximum throw at the earth’s surface 
was less than 30 feet would not have prevented the up-dip 
migration of oil or gas, while a fault with a throw of 50 to 70 
feet — such as the one which slices the Ducotey anticline in 
15-25-9 — is believed to have effectually halted such migration. 
Again, change of dip angle without any actual reversal of dip 
direction or closure of contour lines may be all that is needed 
to trap the migrant hydrocarbons; but it is impossible to state 
in advance how much flattening of the beds is necessary to permit 
a structural terrace to localize an oil or gas accumulation. In 
the illustrative case, it was decided to neglect all changes of 
dip angle and consider as traps only true domes or doubly plung- 
ing anticlines. The drainage outline map must therefore be 
used only in connection with the contour map on which it is 
based; it is to serve as an aid to the ready interpretation of the 
structure contours, not as an independent entity. 
Possibly the greatest value of such a map to the petroleum 
geologist is that it brings into merited prominence the factor of 
