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of these, the East Texas field*, contained 5.6 X lo9 bbl (890 X lo6 m3), or
over 4% of the North American recoverable oil.
The surface area underlain by these giant fields varies from 6 km2 (1515
acres) to 1580 km2 (390,000 acres), with nearly half of them less than 100
km2 (25,000 acres). The thickness of productive zones varies from 10 to
439 m (30-1440 ft), with half having less than 30 m (100 ft). The depth of
principal production varies from 305 m to 3475 m (1000--11,400 ft), with
virtually no oil production from below 4270 m (14,000 ft). The age of the
reservoir rock ranges from Ordovician onwards, with about half the fields
younger than mid-Cretaceous (100 m.y.). Two thirds of these giants have
sandstone reservoirs, and the rest have carbonate reservoirs. The recoverable
reserves are in about the same proportion.
Similar statistics have not yet been compiled for the world, but Fitzgerald
(1980) found that of the total recoverable oil, including gas converted to oil
equivalent, discovered in the world up to the end of 1977, about 85% was
in only 288 fields. The importance of the giants remains, and the worrying
aspect of the statistics is that the volumetric rate of discovery of giants has
been declining since the 1960s, while the rate of discovery of the smaller
fields was still increasing in 1980. Overall, the rate of discovery of reserves
appears to have been declining since the 1960s.

Zipf’s law

In response to our desire to impose order on the disordered, various at-
tempts have been made at describing the size distribution of oil fields in
mathematical terms. Since Folinsbee’s stimulating presidential adress to the
Geological Society of America (Folinsbee, 1977) the use of Zipf’s law (Zipf,
1949, 1965) has become common for petroleum as well as other resources.
Zipf found that a range of social phenomena such as the frequency distribu-
tion of words used in books, the distribution of salaries, and the size distribu-
tion of cities, follows a simple “law”:. if they are ranked by size, the product
of the rank number and the size is approximately constant. Folinsbee (1977)
found that the flow rate of the world’s major rivers followed Zipf’s law. In
the context of petroleum, Zipf’s law would postulate that if all the oil ac-
cumulations could be ranked according to size, the product of rank number
and the reserves of that accumulation would be found to be constant, and
equal to the reserves of the largest accumulation. The reserves of the largest
accumulation would be twice those of the second largest, three times those
of the third largest, and so on (Fig. 7-1). In other words, the total reserves
would be distributed in accumulations in a harmonic series, 1 + 112 + 113 +

* Prudhoe Bay in the Alaskan arctic had been discovered by the time these figures had
been published. This illustrates the impossibility of accurate, up-to-date figures.