The problem of estimating the initial quantities of oil and gas in any region is much more difficult than that for coal, because oil and gas accu­mulations occupy limited regions of underground space in sedimentary basins at depths up to several miles, whereas coal occurs in stratified beds of large areal extent, and often crops out at the surface. However, in high­ly developed petroleum-bearing areas such as the United States, the com­bination of surface geology and subsurface geology determined by thou­sands of wells and miles of geophysical surveys all combine to yield pro­gressively more accurate estimates of the amount of oil and gas which still remains to be discovered. These estimates, plus the amount of oil and gas discovered already, provide estimates for the ultimate amounts to be pro­duced.

Space here does not permit the reviewing of these methods of estima­tion. However, several lines of evidence (Hubbert, 1967; 1969) converge to indicate that the United States, exclusive of Alaska, is now very near the peak in its rate of production of crude oil, with an estimated ultimate production, using present extractive technology, of about 165 billion bar­rels. Then, allowing an estimate of 25 billion barrels for Alaska (which could be too low by a factor of as much as 2), an estimate of 190 billion barrels is obtained for the whole United States.

In addition to crude oil, it is estimated that the United States, exclu­sive of Alaska, will also ultimately produce about 36 billion barrels of natural-gas liquids. When this is added to the 165 billion barrels for crude oil, 201 billion barrels (or roundly 200) is estimated ultimate production of petroleum liquids.

Similar estimates for the ultimate United States, exclusive of Alaska, production of natural gas range from about 1,050 (Hubbert, 1969) to 1,290 trillion cubic feet (Potential Gas Committee, 1967). Within recent months, the Potential Gas Committee (1969) has revised its estimate for ultimate gas production by the whole United States to 1,859 trillion cubic feet by including Alaska and additional offshore areas to a depth of 1,500 feet. Of this, however, 632 trillion cubic feet were classed as “specula­tive.” The estimates by the present author are considerably lower, about 1,200 trillion cubic feet.

Corresponding figures for the entire world are even more uncertain. However, recent estimates for the ultimate world production of crude oil range from about 1,350 to 2,100 billion barrels. Based on these two fig­ures for crude oil, the ultimate world production of natural-gas liquids and of natural gas can be estimated. These are given in the accompanying tab­ulation, which shows a range for total petroleum liquids from 1,620 to 2,520 billion barrels, and for natural gas from 8,000 to 12,000 trillion cu­bic feet.

Fossil Fuel Ultimate Production

Crude oil………………………………………. 1,350-2,100 KPbbl

Natural gas liquids…………………….. 270-420 10°bbl

Petroleum liquids…………………………… 1,620-2,520 lO’bbl

Natural gas………………………………….. 8,000-12,000 1013 ft3

Combining these approximate estimates for both the United States and the world with the corresponding production data, and then using the technique indicated in Figure 8, we can gain a good idea of about how long the resources of oil and gas can continue to supply a major part of the country’s or the world’s energy requirements.

The crude oil production of the United States, exclusive of Alaska, is shown in Figure 12; total petroleum liquids, in Figure 13; and natural gas, in Figure 14. In each, the left-hand shaded area represents the cumulative production up to the end of 1967. The right-hand shaded area represents the additional oil or gas still to be produced from fields already discov­ered, and the final unshaded area, the estimate of future discoveries. The shaded grid rectangle in the upper right-hand corner is an area scale showing the amount of fluid corresponding to each grid rectangle under the production curve.

Two other features of each of these figures may also be noted. The dashed curve at the top represents what the annual production would be if

recent rates of growth should be continued for a few more decades. The two vertical lines showing a horizontal separation of 80 per cent, show the time required to consume the middle 80 per cent of the ultimate produc­tion. Cumulative production up to the time of the first line represents the first 10 per cent of the ultimate production; that to the right of the second line, the last 10 per cent.

The significance of this is that in considering about how long a given fuel can supply a major fraction of energy requirements, the comparative­ly long periods required for the first and last 10 percentiles can largely be disregarded and attention focused upon the time required for the middle 80 per cent.

From Figure 12, which shows the crude oil production of the conter­minous United States based on an ultimate production of 165 billion bar­rels, it will be seen that the peak rate should occur near the present time. The time required to produce the first 10 per cent of the ultimate cumula­tive production was from 1860 to 1934; that for the last 10 per cent, the time after 1999; but the time required for the middle 80 per cent is only the 65-year period from 1934 to 1999. The figure also indicates that of the 165 billion barrels ultimately to be produced, about 134 billion barrels (80 per cent) has probably been discovered already.

Figure 13 is in all respects similar to Figure 12 except that it pertains to the production of total petroleum liquids in the United States. Here, also, the date of the peak rate of production is near the present, and the time required to produce the middle 80 per cent is about 64 years.

The corresponding curve for natural gas production in the conter­minous United States is shown in Figure 14. In this case, the figure indi­cates that the production of natural gas will reach its peak at about the year 1980 — or about 10 years later than that for crude oil — and that the time required to produce the middle 80 per cent will be the 65-year period from about 1950 to 2015.

It may also be pertinent to remark that since World War II, the natu­ral gas industry in the United States has been building pipelines on the as­sumption of much larger amounts of gas being available than that shown in Figure 14. As a result, the recent slowing down in the rates of discovery and of production of natural gas with respect to the requirements for these pipelines has become a source of acute stress and of some alarm within the industry.

World crude oil production is shown in Figure 15 for both a low fig-

ure of 1,350 billion barrels and a high figure of 2,100 of ultimate produc­tion. For the lower figure, the peak in the production rate is estimated to occur about the year 1990, with the middle 80 per cent of production oc­curring during the 58-year period from about 1961 to 2019. For the higher figure, the peak date is delayed only about 10 years to the year 2000, and the time span required for the middle 80 per cent is increased to only 64 years.

From these considerations it appears, therefore, that although the total span of time during which some oil and gas will be produced will probably be several centuries, the period during which the preponderance of this production will occur will be only about one human lifetime.