Unravelling the nature of deep-marine sandstones through the linkage of seismic geomorphologies to sedimentary facies; the Hermod Fan, Norwegian North Sea - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 669

also active during early differential compaction,
after sand was injected into the mound and the
time window for mound growth by sand injection
could therefore potentially be narrower. However,
injections in the Balder tuff documents a phase of
post-depositional sand mobilisation no older than
c. 54 Ma.
It appears from the above and the common
co-existence of depressions, mounds and injec-
tites, that their origins are connected. With refer-
ence to previous work on very similar mounds
(Hamberg et al ., 2007) we propose that the mounds
formed primarily by lateral movement of Hermod
sand (with possible additional contribution from
Ty sand). The faults on the steep-sided mounds
were probably initiated by the evacuation of sand
and further accentuated by mound growth and
later post-depositional compaction. The faults
juxtaposed over-pressured sand in the mounds
with normally pressured Balder tuff and the tuffs
in the hanging wall were the preferred zones of
injection (rather than breaching vertically through
the older claystones at the top of the Sele
Formation). Wild & Breidis (2010) proposed fluid
influx from the underlying structure as a probable
trigger mechanism for sand remobilisation and
formation of mounds in the Balder-Ringhorne
area. The influx of fluids could supply the neces-
sary overpressure to drive large-scale sand remo-
bilisation duranti (2007) and the presence of a
hydrocarbon focusing Jurassic structure under the
Frøy mounds could explain why the deformation
is most intense in this area. It is speculated that
the fluid phase in the present case is gas and both
biogenic and thermogenic gas are possible as
migration to the area from the adjacent source
kitchen had already begun in the Cretaceous.
The disturbances in the older Ty Member under
the Frøy mounds could be related to fluid influx
along the same routes that fed fluid to the Sele and
Balder Formations. Upward injection (Hurst et al .,
2003) of sand from the Ty Member to the Hermod
Member is also possible, as suggested by the com-
mon occurrence of disturbances in the Ty Member
under mounds in the Hermod Member and the
mounds may thus have formed by the combined
effect of vertical and lateral sand movement.
The circular depressions were probably initi-
ated as pockmarks, generated by a similar
sequence of events as forwarded by Hovland &
Judd (1988). The pockmarks probably started to
form when the Hermod reservoirs became blan-
keted by Sele Formation shale, but continued

shale deposition and Balder ash bed deposition
eventually blocked the craters. Subsequent gas
seepage along the routes established to the pock-
marks could then have provided the mechanism
for the overpressure to drive the sand remobilisa-
tion that is interpreted to have taken place after
the Balder tuffs were deposited. The mounds
where then formed by sand withdrawal from the
flank areas and re-deposition towards the mound
centres (i.e. as proposed by Brooke et al ., 1995).

THE HERMOD MEMBER BEYOND
THE SEISMIC STUDY AREA

detailed seismic imaging of sand fairways has not
been achieved for the proximal part of the Hermod
Fan (i.e. outside the seismic study area shown in
Fig. 2). The correlation of the study area to the
remaining part of the Hermod Fan has therefore
been based primarily on analysis and correlation
of well data.

Sand and shale thicknesses from
petrophysical logs

The number and thickness of sandstone and shale
layers vary significantly from well to well (Figs 4
and 13) and no strong correlation between sand
and clay content has been found at the scale of the
entire fan. The lack of correlation between total
sand and total shale content in wells in the Sele
Formation suggests a lack of 'fan-scale causation'
between these two variables. The mechanisms
that focus sand to channels, steep-sided mounds
and splays are different from the mechanisms that
spread mud out over the entire basin. The thick-
ness of mudstone intervals varies gradually and
tends to decrease eastwards in the direction of
diminishing accommodation space in the asym-
metrical Viking Graben.
The highly variable sand content within the
overall eastward thinning 3rd order fan cycle
agrees with the complex sand framework geome-
try documented by the RGB blend (e.g. Fig. 4).
The shale-sand thickness statistics underscore
the existence of discrete and localised sand fair-
ways and further document that the Hermod Fan
consists of multiple channel-splay pairs of lim-
ited lateral extent that coalesce into a basinwide
fan characterised by rather abrupt lateral changes
in both number and thickness of sandstone beds
(cf. Fig. 4).