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therein). However, full scale injection of the entire
tens of metres-thick bed cored in well 25/6-3 is
unlikely because of the lack of a mappable
mounded geometry and the presence of unde-
formed high GR shale on top of the sand. In situ
homogenisation is therefore a more likely expla-
nation for the lack of primary depositional struc-
tures in core from well 25/6-3.
The lack of a unique interpretation for the
massive sandstone facies that forms excellent
reservoirs and is very common throughout the
Palaeocene of the North Sea is challenging. It
appears from seismic-scale observations that this
sandstone type can have many different 3d geom-
etries and modes of formation.
only one in the study area with almost no sand
in the Sele Formation (Fig. 4). a seismic cross-
section through the well (Fig. 12) illustrates
clearly a fault in the near top Sele reflector. The
fault separates the circular depression from the
adjacent mound. a flat Intra-Balder seismic reflec-
tor is juxtaposed to the top of the Sele Formation
across the fault. The flat reflector in the Balder
Formation ties to a circa 15 m thick sand with a
blocky log response in well 25/5-1. dipmeter data
indicate that the sand is conformable with the
tuff beds lying above and below. Current correla-
tions suggest that this is the only well in the study
area with a substantial amount of sand in the
Balder Formation. The steep-sided depression
drilled by well 25/5-1 displays four key character-
istics (Fig. 10) that appear to be typical for similar
structures in the area: (1) circular to oval map
view shape; (2) contracted Sele Formation;
(3) steep-sided contact to an elevated shoulder
and (4) expanded Balder Formation with an extra
high acoustic impedance reflector. The reflector(s)
in the Balder Formation can be flat, inclined or
complex (cf. Figs 10 and 11).
The steep-sided contact to an elevated shoulder
(or mound) is in most places a fault. Some of these
faults terminate above the Balder Formation, in
the lower part of the Horda Formation.
The smallest mapped depressions are sub-
circular in map view, with a diameter of ~ 200 m
(Fig. 13). Some are isolated; some occur in clus-
ters separated by delicate ridges, others form a
trench with an adjacent ridge (Figs 10 and 11).
Larger depressions display a more irregular shape
and have an area of more than 2 km
2
(Fig. 14).
SAND REMOBILISATION COMPLEXES
Seismic geomorphologies indicative of intense
post-depositional sediment remobilisation have
been identified over the Frøy Field and the entire
Rogaland Group appears to have been affected. In
the Balder Formation, they occur as intrusive
dykes and sills, collectively referred to as injec-
tites. In the Sele Formation, the sediment remobi-
lisation structures have sharp edges that mark an
abrupt change in the following types of maps
(Figs 10 to 12): (1) the elevation of the Near Top
Sele time-structure map, (2) the isochron of the
Sele Formation and (3) the thickness of the
Hermod Member (confirmed at well locations).
discrete geometries that delineate low areas on
map types 1 to 3 are referred to as depressions
with an adjective that describes the plan-view
shape (circular, oval, irregular). discrete geome-
tries that delineate elevated areas on map types 1
to 3 are called steep-sided mounds. The underly-
ing Ty Member sandstones of the Våle Formation
display complex seismic reflector configurations
with common inclined reflectors that at various
locations extend up into the Lista Formation. The
disturbances in the Ty Member tend to occur
under mounds in the Hermod Member, as further
addressed in the interpretations below.
Steep-sided mounds
Steep-sided mounds have sub-circular to irregular
shapes and are bounded by a trench or irregular
depression on at least one side. The biggest mound
has an area of more than 5 km
2
and a crest that is
elevated more than 100 m above the deepest parts
of the adjacent depressions. The type well for the
Hermod Member (i.e. well 25/2-6) has been drilled
into a mound with an irregular and complex
geometry. Well 25/2-6 encountered 140 m of clean
sandstones with minor interbedded shale (Fig. 4)
and this makes it the well on the studied subma-
rine fan with the highest net-to-gross sandstone
ratio in the Sele Formation (out of 29 evaluated
wells; Fig. 15). Trenches adjacent to elongated
mounds can at some locations be seen to represent
Depressions
Well 25/5-1 is drilled in a small sub-circular
depression (c. 200 m wide and 40 m deep) which
is connected to a larger (>1 km wide) and more
irregularly shaped 'basin' and flanked by a steep-
sided mound (Figs 10 and 12). This well is the
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