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ed, and must be taken into account when reasoning back to the source rock.
Any post-unconformity folding will, of course, fold the unconformity and
the pre-unconformity sequence. Such deformation varies from the mild “bald-
headed” anticline (some of which are demonstrably due to continued diapiric
movement, as will be seen in Part 3) to more complicated deformation. But
the old saying “You don’t find much oil in steep structures’’ is as true of un-
conformity traps as of structural traps.

Prudhoe Bay oil field

The Prudhoe Bay oil field on the North Slope of Alaska is of interest not
only because it is the largest oil field in North America and situated 400 km
(250 miles) north of the Arctic Circle, with more than 600 m of permafrost,
but also because it illustrates several of the points discussed above. In its
simplest terms, it is an unconformity trap with several Palaeozoic and lower
Mesozoic reservoirs of different rock types sealed by a Lower Cretaceous
mudstone, the whole gently folded.
Prudhoe Bay oil field was discovered in 1968, but production and devel-
opment were delayed for nearly ten years by social and political controversy
over the construction of the pipeline (see Jamison et al., 1980). It was found
to contain 23.5 X lo9 bbl of oil in place, with 9.6 X lo9 bbl recoverable
(3.7 X lo9 m3 and 1.5 X lo9 m3, respectively) for a recovery factor of about

PRUDHOE BAY

-

0 140
MILES
(225 KILOMETERS)

Fig. 13-3. Major structural elements of the North Slope of Alaska. (Reproduced from
Jamison et al., 1980, p. 290, fig. 1, with permission.)