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net accommodation (lower limit to erosion). Net
accommodation lies below the maximum depth
of channel incision (upper boundary) and is
related to the lower limit to erosion and time
scale of observation that need to be sufficiently
large. Preservation in the stratigraphic record
occurs when the deposits become buried below
possible depths of channel incision and removal,
that is, below the lower boundary of real-time
accommodation, i.e. base level. A/S value can
only be applied to sediment stored in the net
accommodation 'window'. It cannot be applied
to sediments preserved in the real-time accom-
modation 'window' (defined as the accumula-
tion space at the scale of present-day fluvial
process regimes) because of the temporary nature
of sediment storage.
Two types of effective accommodation to
sediment supply value (A/S) response exist: 1)
positive and increasing A/S equivalent to
increasing rate of aggradation (fluvial base level
can be picked at subsequently higher strati-
graphic positions) and 2) positive but decreasing
A/S equivalent to decreasing rate of aggradation
(fluvial base level can be picked at subsequently
lower stratigraphic positions).
Three key fluvial stratigraphic surfaces are
defined: 1) the relative maximum aggradation
level which marks the turn-around from increas-
ing aggradation to decreasing aggradation, 2) the
relative minimum aggradation level which marks
the turn-around from decreasing aggradation to
increasing aggradation (no degradation is reached)
and 3) the subaerial unconformity surface which
marks the turn-around from aggradation to degra-
dation to aggradation (no deposition during a time
period at the studied locality). Accordingly, an
increasing A/S trend followed by a decreasing
A/S trend is defined together forming a cycle. A/S
trend bounding levels are chosen as the strati-
graphic zone boundaries. If degradation occurs
(A/S < 0), the subaerial unconformity is the zone
boundary.
It is proposed that the concept of increasing and
decreasing rates of A/S during the aggradational
part of a fluvial succession may work to divide
largely aggradational successions into zones. The
notion that the rate of fluvial aggradation through
time, as controlled by the accommodation versus
sediment supply rate, is dynamic and not static
(classified as high or low) can be applied to
spatially and temporarily restricted (limited) sub-
surface datasets at reservoir scale (0.1 km to
20 km). Facies interpretation (both fines and
sandstones) linked to analysis of temporal and
spatial changes in three-dimensional facies archi-
tecture are a key issue to detect organised changes
(trends) in preserved facies that can be used to
establish a reservoir zonation. The alternative res-
ervoir zonation approach for dividing fluvial stra-
tigraphy that is proposed herein enables the
organisation of fluvial successions in a hierarchi-
cal framework incorporating zone property trends
for, for example, sandstone body connectivity and
rock properties such as average permeability. It is
successfully applied to the uppermost Triassic to
lower Jurassic Statfjord Group (Norwegian conti-
nental shelf).
ACKNOWLEDGEMENTS
We acknowledge critical but constructive and help-
ful comments on earlier versions of the manuscript
by Frank Ethridge, Lars-Magnus Fält, Martin
Gibling, Colin North, Tobi Payenberg, Ron Steel and
Gary Weissmann. The manuscript was formally
reviewed by Cornel Olariu and George Postma.
Responsibility for remaining errors remains with
the authors. Statoil ASA is thanked for permission
to publish.
APPENDIX
Definitions for measuring change in A/S
In order to recognise changes in the rate of A/S
change in core and consequently changes in the
position of fluvial base level, a working concept
coupled with a number of definitions is required.
The argument is constructed by specifying real
time versus net accommodation. Subsequently,
fluvial base level, as used in this study, is defined
followed by the definition of change in position of
fluvial base level, which enables the recognition
of successive accumulated sediment packages.
This is considered as a measure for the change in
A/S at specific time scales.
Real time versus net accommodation
The original definition of accommodation of
Jervey (1988) included present-day and time-aver-
aged time scales. Following ideas of Blum &
Törnqvist (2000; and also Kocurek & Havholm,
1993; Kocurek, 1998), accumulation space at the
scale of present-day alluvial process regimes is
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