exactly alike, these curves have group
characteristics which are used as a rough basis for interpreting the
future.

[Illustration: FIG. 8. The annual output of the principal oil
fields of the United States for the last twenty years. Data from U.S.
Geological Survey.]

A less reliable method is to calculate from geologic data the volume and
porosity of the oil-bearing reservoirs, and to estimate the percentage
of recovery on the basis of current practices and conditions. Complete
data for this method are often not available; but in the early years of
a field, before production curves are established, this method may serve
for a rough approximation.

[Illustration: FIG. 9. Curve showing the usual decline in oil
field production after the period of maximum output is reached. After
Ralph Arnold. The Petroleum Resources of the United States, Smithsonian
report for 1916, p. 283. Compare this theoretical curve of final
decrease with the production curve shown in Fig. 8.]

=Classes of oils.= When crude petroleum is distilled, it gives off in
succession various substances and gradually thickens until it leaves a
solid residue, which may be largely either paraffin wax or asphalt. The
two main classes of oils are determined by the nature of this solid
residual. The products given off are natural gas and then liquid
hydrocarbons of various kinds, which evaporate in the order of their
lightness. Petroleum is thus a mixture or mutual solution of different
liquids, gases, and solids. Nearly one-fifth of the domestic consumption
of crude petroleum is burned directly as fuel, and four-fifths are
refined. The several principal primary products of refinement are
gasoline, kerosene, fuel oil, and lubricating oil; but these may be
broken up into other substances, each the starting point of further
refinements, with the result that present commercial practice yields
several hundred substances of commercial value. With increasing chemical
and technical knowledge these products are being multiplied. The rapidly
increasing demand for gasoline has led to the use of processes which
extract a large proportion of this substance from the raw material, by
"cracking" or breaking up other substances; but while, under the stress
of necessity, there is possibility of slight modification of the
proportions of principal substances extracted from the crude oil, it is
not possible to change these proportions essentially. It is, therefore,
a problem to