298 ANNUAL REPORT SMITHSONIAN INSTIIUTION, 1944 
the region already producing, only the upper layers of the petroleum- 
bearing rocks have been tapped by the wells so far drilled. Underlying 
the beds from which petroleum is now being withdrawn in many of 
our great oil fields there remain thousands of feet of rocks, still un- 
touched by the drill, which may very well yield petroleum when they 
are tested. 
Our total past discoveries of petroleum in the United States amount 
to about 48 billion barrels. We have explored great areas which most 
of us have agreed were of little promise. Yet our past experience has 
proved that from 1 to 2 percent of the total area in which we may 
reasonably hope to find petroleum actually produces when thoroughly 
tested. If our average experience in the area already thoroughly 
explored is valid, then thorough exploration of the entire area in the 
United States in which it is reasonable to expect to find petroleum 
should yield as much additional petroleum as we have already found. ° 
Moreover, the statement under consideration overlooks the fact that 
in addition to the 20 billion barrels of liquid petroleum reserves we 
have also in the United States proved reserves of natural gas equiva- 
lent in energy content to about 17 billion barrels of petroleum. Nat- 
ural gas is really petroleum in another form and with modern tech- 
nique is readily convertible into liquid fuels, although the cost of 
conversion is still somewhat higher. We should not overlook our 
reserves of petroleum in the form of natural gas. 
Again the statement ignores the fact that the American petroleum 
industry, operating abroad over the last 30 years, has developed addi- 
tional petroleum resources in other countries. The remaining proved 
reserves in these oil fields easily amount to another 20 billion barrels 
or more. These reserves in the hands of American nationals in other 
countries have always been available to the American consuming pub- 
lic in normal times, and they constitute an important supplementary 
proved reserve of petroleum. 
The current discussions of the amount of our petroleum reserves 
seldom touch on the facts that in the past we have usually recovered 
only about 40 percent, or less, of the total volume of petroleum origi- 
nally present in our oil fields, and that, on the basis of this past expe- 
rience, proved reserves are customarily estimated at about 40 percent 
of the total volume of petroleum in the natural reservoirs in which the 
estimates apply. Our estimates of reserves include only the petroleum 
that we know from experience will flow more or less spontaneously into 
the wells that are drilled. The sum of our estimates of proved reserves 
plus the petroleum already discovered in this country, some 48 billion 
barrels, represents, therefore, a total original volume of about 120 
billion barrels. After the estimated volume of our proved reserves 
has been completely recovered there will still remain underground in 
