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It is very difficult to understand on the data available how the oil and gas
migrated to the reservoirs through the Kimmeridge Clay as well as more than
one kilometre of Cretaceous mudstones and marls. The pressure regime has
not been published, but HGritier et al. (1979, p. 2003; 1980, p. 64) report
“soft gumbo clays of early Oligocene to middle Eocene age” overlying the
reservoir, and “208 m of dark grey undercompacted shales of Early Cretaceous
age” underlying it, so there would appear to be abnormally pressured mud-
stones both above and below the accumulation. Again, the source must not
only have been at higher energy than the accumulation, but also there must
have been a path of continuously decreasing energy between the two. There
is the further complication in the Frigg accumulations that if Frigg and East
Frigg have the same source, the differences in their crude oils must surely
have their cause between the two.
The Niger delta is a thick dominantly regressive sequence that, in the sim-
plest terms, has been a delta since Africa and South America parted, andit
grew through the Tertiary. Intense exploration activity has discovered about
150 fields, many with multiple reservoirs. These fields are dominantly faulted
anticlines, the anticlines being generally roll-over structures on the down-
thrown side of growth faults (Fig. 11-5). The trends of the faults (as usual
with growth faults) are parallel to the depositional strike, reflecting the shape
of the growing delta (Fig. 11-6), and they shifted seaward with the regression
so that the younger faults are on the downthrown sides of the older.
The stratigraphic sequence has been divided into three formations: the
sandy, terrestrial Benin Formation on top, underlain by alternating sands
and mudstones of the Agbada Formation from which almost all the produc-
tion comes, and then the dominantly marine mudstone of the Akata Forma-

Fig. 11-6. Growth faults in the Niger delta reflect the growth of the delta, younger growth
faults being seaward of the older. (After Weber, 1971, p. 560, fig. 2a.)