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modify the earlier decisions. As the field becomes better known and docu-
mented, possible field extensions will become apparent. These will be investi-
gated with outstep or outpost wells. It is in the nature of such marginal areas
that some of these will be failures, either failing to find what was expected
and hoped for, or finding that the quantity or quality does not justify devel-
opment.
The nature of an oil field with a single large pool is quite different from
one with many pools and from one with many pools in a faulted structure,
and the economics also differ. A single large pool may be developed with rela-
tively few wells spaced several kilometres apart if the field permeability is
good, or a few wells if the oil column is thick and the field area relatively
small, whereas the same volume of oil in several faulted reservoirs may re-
quire wells at close spacing for each reservoir in each fault block. Part of the
development planning of such fields with multiple reservoirs involves siting
wells for multiple completion on more than one reservoir (dual completions,
even triple completions). It is in the nature of oil fields that those in trans-
gressive sequences tend to be large single-pool accumulations and those in re-
gressive sequences tend to be multiple-pool accumulations.
The extraction of oil tends to result in a gradual pressure decline in the
reservoirs. This not only impairs the productivity of the field but also leads
to surface subsidence through the compaction of the depleted reservoirs and,
in some fields, the movement of oil or water from undepeleted reservoirs to
depleted reservoirs in juxtaposition across faults. Pressure-maintenance
schemes are planned and put into operation early in the life of a field, usually
involving the injection of water at the field perimeter, or the substitution of
peripheral wells as injection wells when they go wet.

GIANT OIL FIELDS

Very large oil fields are called giants when they contain at least 100 X lo6
bbl(15.9 X lo6 m3) of recoverable oil with present technology and at present
prices; and gas fields are giants when they have at least 1 Tcf (trillion standard
cubic feet, 10’’; 28.3 X lo9 m’) of recoverable gas. Definition of a giant has
not been universally accepted, and some would have it as large as 500 X lo6
bbl and 10 Tcf, or 10’ bbl and 10 Tcf.
If we take North America as typical of a continent that has been (and is
being) intensely explored, we find that in 1968, according to Moody et al.
(1970), there were 26,250 oil fields with recoverable reserves totalling 132 X
lo9 bbl (21 X 10’ m’) for an average of 5 X lo6 bbl(755,OOO m’) each. Of
these, the 45 largest, each with more than 500 X lo6 bbl (80 X lo6 m3) re-
coverable oil, had a total of 46 X lo9 bbl(7 X lo9 m3) of recoverable oil, for
an average of 10’ bbl (159 X lo6 m3) each. Less than 0.2% of the oil fields
contained 35% of the total known North American recoverable oil. The largest