Page 263 - Petroleum Geology

238

leum provinces in which the geological evidence and the geochemical are in
conflict in the matter of source and migration of crude oil.
A striking example of conflicting geochemical and physical evidence is to
be found in the Ekofisk oil field in the Norwegian North Sea. The Paleocene/
Upper Cretaceous (Danian/Maastrichtian) chalk reservoir is overlain by abnor-
mally pressured Tertiary mudstones, and the reservoir itself is overpressured.
Byrd (1975, p. 443, fig. 2) illustrated the pressure regime in the mudstones
with the sonic transit time plotted against depth, and concluded that the
pressure drop in the mudstones to the reservoir not only reinforces the seal
but suggests that the source of the accumulated oil is in the overlying mud-
stones. Van den Bark and Thomas (1981, p. 2361, fig. 24) also feature this
pressure regime, differing from Byrd only in putting the top of abnormal
pressures in the Tertiary mudstones rather shallower, at 1036 m (3400 ft).
However, Van den Bark and Thomas (1981, p. 2353) state that the Paleo-
cene mudstones had been postulated as the source because of the similarity of
oil extracted from them and the accumulated oil; but that geochemical study
had later disqualified them from being petroleum source rocks on the grounds
of immaturity (Ro = 0.59-0.62%, at the shallow threshold of thermal matu-
rity). They state in support of this conclusion that the gas chromatography
and mass spectral data of the saturate fraction of these extracted oils, from

Fig. 11-4. Relationship between sand/shale ratio and source/non-source mudstone ratio.