Page 172 - Petroleum Geology

149

Ghawar with 65 X lo9 bbl may be the second largest, rather than the largest
accumulation in the world. Again, one wonders whether oil in place should
not be the basis, in which case the Athabasca tar sands with 868.7 X lo9 bbl
(reported by Hackbarth and Nastasa, 1979) would rank number one.
The account sheet therefore looks like this*:

Ultimate recoverable reserves: 1800 X lo9 bbl
Cumulative production to end-1980: 446 X lo9 bbl
Known recoverable reserves at end-1980: 649 X lo9 bbl
Reserves yet to be discovered, approximately: 700 X lo9 bbl

The disadvantage of Zipf’s law is its empirical nature, with no obvious
relation to geology or geochemistry (but perhaps to communities of organic
life?). Its advantage is that it emphasizes the giants where most of the oil is.
Whatever the true size distribution of oil accumulations may be, it seems al-
most certain that there are relatively few super giants left to be discovered,
and that most of the undiscovered oil is in large, but more modest accumula-
tions. The trends detected by Fitzgerald (1980) support this conclusion. It
will be from better understanding of petroleum geology and geochemistry,
and from improved exploration and production technology that the rest will
be discovered. If a way could be found to extract almost all of the oil, instead
of leaving about 2/3 in the ground, the recoverable reserves would be imme-
diately doubled, and our concern for the future halved.

THE OIL-RICH COUNTRIES

Just as most of the known reserves of crude oil is in relatively few fields, so
is most of the known reserves in relatively few countries. Fifteen countries
possess about 90% of the known reserves of oil recoverable with present tech-
nology and prices.
The measure of oil-wealth of a country has traditionally been her annual
oil production, usually expressed as the average daily rate of production in
barrels. Ranking of countries on the basis of production was quite satisfactory
when oil production bore some fairly constant relationship to reserves, but
by the end of the 1970s it had become misleading because political and eco-
nomic influences began to dominate, and production is now a poor indicator
of reserves. Tables 7-7-7-9 show the fifteen countries with the largest known

*The figure for cumulative production was obtained by taking Moody and Esser’s (1975)
figure of 297 x lo9 bbl at end 1973, and adding the annual figures from the Oil and Gas
Journal’s World Reports. The figure for known recoverable reserves at end-1980 is also
from the Oil and Gas Journal’s World Report.