Page 319 - Petroleum Geology

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There is no difficulty in accepting a post-unconformity source for the
Prudhoe Bay oil. Compaction of the Barremian mudstones would have ex-
pelled fluids downwards into any subcropping permeable formation in which
the water was at lower potential, but the position of the source rocks is re-
stricted to the areas of subcrop. If evidence of intercommunication between
reservoirs is obtained, the requirement is reduced to source rocks' being in
the area of one subcropping formation, or overlying one of the reservoir rock
units.

The North Sea

The first exploration well was drilled in the North Sea in 1964, the first
discovery was made in 1965 (British Petroleum's West Sole gas field), and
within 15 years the North Sea has become a major petroleum province. Many
of the very large accumulations are in stratigraphic traps, and unconformity
traps are typical of the northern accumulations.
The Statfiord oil field lies mainly on the Norwegian side of their boundary
with the United Kingdom, in the Viking graben (Fig. 13-6). Its essential fea-
tures are shown in Fig. 13-7. It is the largest single oil field in the North Sea,
with recoverable reserves of 3 X lo9 bbl (477 X lo6 m3) of 38"/4l0API oil.
The volume of oil in place is estimated at 4.8 X lo9 bbl(763 X lo6 m3). Most
of the oil is found in the Brent (sandstone) Formation, some in the Statfjord
(sandstone) Formation, and a small pool is in a sandstone in a member of the
intervening Dunlin Formation. There are several points of interest.
The Brent Formation (which is the main producing horizon of the Viking
graben) is unconformably overlain by transgressive Upper and Middle Jurassic
shales that are less than 50 m thick (Kirk, 1980). These thin to zero on the
eastern flank where the later Jurassic and early Cretaceous unconformity
cuts down the reservoir sequence. Above this second unconformity lie trans-
gressive Cretaceous mudstones and marls, with some limestones (Fig. 13-8).
The crude oil in the Statfjord Formation is of poorer quality than that in
the Brent Formation, and it is a smaller accumulation with an oil/water con-
tact 220 m (725 ft) deeper than that in the Brent. It is therefore reasonable
to conclude that there are two sources. The small pool in the Dunlin Forma-
tion has the same oil/water contact as the Brent Formation accumulation:
there is therefore evidence of communication between the two, probably at
the unconformity surface. The reworked Jurassic sand found in places at the
subcrop is the obvious connexion.
Geochemical source rock evaluation indicates the Kimmeridge Clay For-
mation (Upper Jurassic) as the main source rock and its position overlying
the Brent Formation and its subcrop, with the intervening mudstone of the
Heather Formation, lends geological support to this (but the reservoirs are
over-pressured, and detailed study of the pressure regime in the area would
also be required). It appears from published information (Kirk, 1980, p. 100,