in; that certain
important oil-bearing sands of the younger Appalachian formations were
laid down in waters which are believed to have been only slightly
saline; that natural gas is present in fresh water basins; and that it
has not been demonstrated that salt in appreciable amounts is necessary
for the geologic, any more than for the artificial, distillation of oil.
Most of the great oil fields have been in regions of marine or other
saline water deposits, but it has not been proved that this is a
necessary condition. White[27] says: "At the present stage of our
knowledge, fresh-water basins appearing otherwise to meet the
requirements should be wildcatted without prejudice."

The principal oil-bearing horizons in any locality are comparatively
few, and it is ordinarily easy to determine by stratigraphic methods the
presence or absence of a favorable geologic horizon. By knowing the
succession and thicknesses of the beds in a given region it is possible
to infer from surface outcrops the approximate depth below the surface
at which the desired horizon can be found. To do this, however, the
conditions of sedimentation, the initial irregularities of the beds, the
structural conditions, including unconformities, and other factors must
be studied.

In exploration for oil the determination of the existence and location
of the proper horizon is but an initial step. For instance, the oil of
the Midcontinent field of the United States is in the beds of the
Pennsylvanian, which are known to occupy an enormous area extending from
Illinois and Wyoming south to the Gulf of Mexico. This information is
clearly not sufficiently specific to limit the location of drill holes.
Sometimes seepages of oil or showings of gas near the surface are
sufficient basis for localizing the drill holes.[28] Commonly, however,
it is necessary to find some structural feature in the nature of a dome
or anticline which suggests proper trapping conditions for an oil pool.
This is accomplished by geologic and topographic mapping of the surface.
Levels and contours are run and outcrops are platted. As the outcrops
are usually of different geologic horizons, it is necessary to select
some one or more identifiable beds as horizon markers, and to map their
elevations at different points as a means of determining the structural
contours of the beds. When several key horizons are thus used, their
elevations must be reduced to the elevations of one common horizo