ay be of nearly equidimensional dome form. The largest anticlines which
have been found to act as specific reservoirs are rarely more than a few
miles in extent, and in many cases only a mile or two. The "closure" of
an anticline is the difference between the height of a given stratum at
the highest point and at the edges of the structure. A considerable
number of productive anticlines are known in which the beds dip so
gently as to give a closure of 20 feet or less.

After the structural outlines of beds near the surface have been
determined, all possible information should be used in projecting these
structures downward to the oil-producing horizons. Where a number of
wells have been previously drilled in the vicinity, examination of their
records may indicate certain lateral variations in the thickness of the
beds between the horizon which has been mapped and the producing
horizon. The effect of such lateral variations may be either to
accentuate the surface structure, or to cause it to disappear entirely
and thus to indicate lack of favorable trapping conditions. The
possibility of several oil-producing beds, at different depths--a not
uncommon condition in many fields--should also be kept in mind.

As already indicated, anticlines are not always essential to make the
necessary trapping conditions. In the Beaumont field of Texas, for
instance, it has been shown that irregular primary deposition of
sediments differing in porosity both vertically and horizontally allowed
the oil to migrate upward irregularly along the porous beds and parts
of beds, and to be trapped between the more impervious portions of the
beds.

Further questions to be considered in the exploration of an area are the
content of organic matter in the sediments which may have served as a
source of oil, the presence of impervious cap-rocks or of variations in
porosity sufficient to retain the oil, the thickness of sediments and
the extent to which they have undergone differential stresses, the
amount of erosion and the possibilities that oil, if formed, has escaped
from the eroded edges of porous strata, and, where carbonaceous beds are
present, their degree of carbonization, and many other similar matters.

Each field in fact has its own "habit," determined by the interaction of
several geologic factors. This habit may be learned empirically.
Geologists have often gone wrong in applying to a new district certain
principles determined elsewhere, w