Geoscience Reference
In-Depth Information
are generally much thicker and more correlative
than in the Eiriksson Fm. For the Eiriksson Fm, the
character of the fine-grained floodplain sediments
change markedly and the generally vertisol-like
palaeosols of the Raude Fm are replaced by imma-
ture or almost non-altered, mostly hydromorphic
soils. In addition, petrographic analysis show that
the content of kaolinite clay increases at the
expense of smectite clays and other mixed layer
minerals, which is interpreted as a sign of chang-
ing climatic conditions. The level of relative
minimum aggradation for this intermediate
A/S cycle is interpreted at base Eiriksson Fm. As
shown in Fig. 12, the character of this boundary in
terms of underlying higher-order cycles is non-
uniform, i.e. there is not always a complete higher-
order cycle beneath and the development of these
is in places interrupted inside the respective A/S
rise trend. This observation confirms that the
magnitude and significance of the A/S minimum
(and negative if A/S < 0) at the boundary between
the Raude-Eiriksson formations is different from
the intra-Raude A/S minimum situations (see dis-
cussion below). A certain erosive relief (at the
scale of approximately 30 metres) of the surface
between the Raude-Eiriksson formations and
resulting variable thickness of the Eiriksson Fm
(and thereby also the Raude Fm), together with
the arguments noted above, seems to support an
interpretation of this surface as a sequence bound-
ary. Correlation of this boundary away from the
erosive event reveals the existence of an interfluve
(Fig. 12, Well 5). Here, mature palaeosols are inter-
preted to be time-equivalent to erosive channel
bases in neighbouring wells, implying that the
palaeosols matured while multiple, long-lasting
erosional events and subsequent infilling occurred
in other locations.
The third order of A/S cyclicity corresponds to the
smallest correlative units that can be identified using
channel-belt stacking patterns and facies variations
as described above (Fig. 12, A/S cycle 2). Channel
belt stacking patterns potentially comprise higher
frequency cyclicity than the A/S cyclicity derived
from palaeosol maturities, because the erosive base
of the channel belt reflects a more local A/S mini-
mum and subsequent increase in A/S allows for
preservation of the channel-belt deposits. However,
for a given interpretation of channel-belt interval
cyclicity, the A/S maximum is a rather hypothetical
situation and the exact position is highly specula-
tive. One possibility is to pick the turn-around from
increasing to decreasing A/S where the variability in
sandstone facies and the proportion of fine-grained
material inside the channels is highest. Because of
the uncertainty related to the correct recognition of
its stratigraphic location, especially from core data,
the relative maximum aggradation level is not con-
sidered a good correlative marker and the relative
minimum aggradation position is generally picked
as the candidate correlation level (Fig. 12, zone
boundaries). The A/S maximum in intervals consist-
ing of mainly palaeosols is picked where the least
mature palaeosols occur (Fig. 12, well 7, zone 2,
based on core and gamma-ray log response).
In the Statfjord Group biostratigraphy cannot
provide any intra-formational time control due to
the lack of good quality recoverable biostrati-
graphic data in the deposits that can be calibrated
to absolute time needed to assess the diachronity
of subaerial unconformities and to build a reliable
reservoir zonation. Correlation of thick, mature
palaeosol intervals is assumed to give the most
robust framework for more detailed definition of
correlative marker horizons (Fig. 11A). It is these
correlative markers that are then used further in
the zonation of the reservoir (Fig. 11B).
Reservoir zonation improvements
The notion that the rate of fluvial aggradation
through time is dynamic and not static, in combina-
tion with the use of A/S concepts and a more practi-
cal definition of fluvial base level to measure A/S
change has led to the recognition of a suite of param-
eters that was used in combination to recognise
variations in the rate of A/S change and turn around
positions in core, to analyse stratigraphic architec-
ture and to define reservoir zones. Together, these
improvements in stratigraphic concept and the
zonation workflow have led to a more robust treat-
ment of the natural depositional variability and
mapping of connectivity patterns. Moreover, it has
also significantly decreased the level of uncertainty
associated with the reservoir zonation framework
and the 3D distribution of static reservoir proper-
ties. Consequently, static reservoir property trends
for each zone are better characterised which is
expected to lead to improved predictions of flow
properties and production profiles.
CONCLUSIONS
A revised definition of base level, or strati-
graphic reference level, for non-marine succes-
sions is introduced as the lower boundary of
real-time accommodation or upper boundary of
Search WWH ::
Custom Search