Updated depositional and stratigraphic model of the Lower Jurassic Åre Formation, Heidrun Field, Norway - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 264

NPHI
%
RHOB

Depth
(m)

0.75 -0.15

Zone

GR

0 API 250 g/cc

1. 45 2.95

3555

3560

3565

3570

3575

3580

Fig. 8. Characteristic well-log signatures and core expression (well 6507/8-D-4BHT3) of a flooding surface within the Åre
Formation (in this case: top Åre 3 Zone). In the core the typical signature is a heterolithic laminated claystone-mudstone,
generating a wide shale separation on the NPHI/RHOB logs and a significant increase in the GR log. Note calcite cementa-
tion at 3564 m MD as indicated by the peak in the RHOB values. See Fig. 4 for abbreviations.

stratigraphic markers within the Åre Formation,
could possibly be related to concentration of
fossil fragments. This suggests that the flooding
events may have caused sediment starvation and
condensation.
Several flooding surfaces identified in this
study correspond to biostratigraphic events of
chronostratigraphic significance (Table 1). Minor
time gaps may also occur at the other defined
flooding surfaces; however, the biozone resolu-
tion is insufficient to demonstrate this. The
lateral extent of the mudstone-rich flooding sur-
faces within the Åre Formation are, in some
cases, confirmed by pressure and production
data, although complex fault segmentation often
obscures the true extent of these sedimento-
logical barriers and baffles by creating fluid
communication windows. Major changes in dep-
ositional facies at two reservoir zone boundaries

are associated with significant contrasts in acous-
tic impedance which define distinct seismic
horizons (MFS1 and TRS; Table 1).
The reservoir stratigraphy is further divided
into subzones (Fig. 4). These are bounded by
minor flooding surfaces that are interpreted to
reflect local autocyclic shifts within the coastal
system and do not correspond to larger-scale land-
ward movements of the palaeocoastline. The term
'candidate' flooding surface has been adapted for
these events.
A number of surfaces characterised by signifi-
cant erosion and marking an abrupt change into
sand-dominated channelised deposits have also
been identified as correlateable stratigraphic mark-
ers and are termed candidate sequence boundaries
(c.SB; Table 1) (sensu Sloss et al ., 1949). These sur-
faces have most likely formed in response to base
level fall. Candidate sequence boundaries do not