ithout sufficient consideration of the
complexity and relative importance of the sundry geologic factors which
in the aggregate determine the local habit of oil occurrence.

Geographically associated fields characterized by similarity of oil
occurrence, age, and origin, are known as _petroliferous provinces_. The
factors entering into the classification of fields are so numerous that
more precise definition of a petroliferous province is hardly yet agreed
upon.

The part played by the economic geologist in oil exploration and
development is a large one for the obvious reasons given above. Probably
no other single division of economic geology now employs so large a
number of geologists. Practically no large oil company, or large piece
of oil exploration and development, is now handled without geologic
advice. Quoting from Arnold:[29]

It ought to be as obvious that exploration with the drill
should be preceded by careful geologic studies as it is that
railroad construction should be based on surveys. These
studies should include such subjects as topography,
stratigraphy, structure, and surface evidence of petroleum in
the regions to be tested. The work divides itself into two
stages--preliminary reconnaissances and detailed surveys.

The preliminary reconnaissance should consist in procuring all
the available published and hearsay evidence regarding the
occurrence of oil or gas seepages or hydrocarbon deposits in
the region; in making preliminary geologic surveys to
determine from which formations the oil is to come and the
areal distribution of these formations; in determining those
general regions in which the surface evidence is supposed to
be most favorable for the accumulation of hydrocarbons; and in
determining the best routes and methods of transportation.

The second stage includes detailed geologic surveys of those
regions where the surface evidence indicates that petroleum is
most likely to be found and the location of test holes at
favorable points. By working out the surface distribution and
structure of the formations it is usually possible to select
the areas offering the best chances of success. Geology should
always be the dominant factor in determining the location of
test holes, although modifications to meet natural conditions
must sometimes be made.

OIL SHALES

One