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fig. 5) that these Jurassic mudstones or shales do not reach the Statfjord For-
mation subcrop, which is overlain by Cretaceous mudstones. If this is true
over the pertinent area around the accumulation, it could account for the
different quality of the Statfjord Formation crude oil.
The fact that the reservoirs are abnormally pressured requires that the
source rock not only was in a position with greater energy than the accumu-
lating oil, but almost certainly still is, and migration is still taking place. The
ultimate condition of mechanical stability is 'with normal hydrostatic pres-
sures in the water around the accumulation, so there must be potential gra-
dients in the fluids, and so a tendency for them to migrate towards the beds
with fluids with lower energy. These are either above or below, or both.
Growth faulting also influenced the development of the traps before they
were closed in Cretaceous times, so they probably did not affect the accumu-
lation of oil while they moved. The growth faulting, however, did affect the
relief of the unconformity surface (not necessarily the topography to the same
degree) and so affected the stratigraphic relationships between the Heather
and Kimmeridge Clay Formations on the one hand, and the Brent, Dunlin
and Statfjord Formations on the other. The very size of the accumulation
and the thinness of the apparent source rock over the accumulation suggests
that the main source lay outside the present accumulation, and that migra-
tion could pass through, or bypass, the faults.
The Brent oil field (Bowen, 1975) is very similar to Statfjord. It also has
two reservoirs: the Brent sandstone with 36"API oil and the Statfjord with
38.5"API oil, and the oil/water contact of the latter is about 200 m deeper.
The similarities go even further because Bowen reports Kimmeridge Clay on
the Brent, partly covering the subcrop, but Albian-Aptian marls on the Stat-
fjord. The relief on the unconformity surface is reported to be 900 m, but
much of this could be due to differential subsidence: 900 m of topographic
relief at the time is not implied. The source of the oil 1s thought to be the
deep Upper Jurassic shales in flanking troughs. The qualities of the crude oils
are not reported, but the unconformity surface is effectively sealed.
These two fields illustrate an important point in petroleum geology. The
similarities between the two are similarities of two highs, and the stratigraphic
relationships here are probably not representative of the area as a whole.
The Piper oil field in the Moray Firth basin is a folded and faulted uncon-
formity trap with crude oil in a marine sandstone of Oxfordian to early Kim-
meridgian age (Williams et al., 1975; Maher, 1980). The main field has an oil/
water contact at 2594 m (8512 ft), unaffected by faults. To the south-east,
however, there is a pool on the downthrown side of a growth fault that moved
during the Cretaceous; and this pool has a deeper oil/water contact at 2804
m (9199 ft), 210 m deeper than that in the main field. This fault accumulated
thicker Coniacian and Santonian marls (Fig. 13-9) on the downthrowing side,
and movement continued through the Campanian, marls of which cap the
subcrop in the main field area. So, by the time the main trap was closed, this