Reservoir Model Types - Reservoir Model Design - page 191

Fig. 6.24 Summary of the
effect of confinement
on deep-water reservoir
architecture

Unconfined Confined

Confinement

Non-
amalgamated

Amalgamated

Architecture

Low

High

Kv / Kh

or subsequent flows through the same conduit.
The amalgamation of sand-rich units results in
permeability architectures with favourable k v /k h
ratios.
In unconfined systems, flows are more deple-
tive, less erosive and prone to the generation of a
more layer-cake architecture (Fig. 6.24 ). Sand
amalgamation is limited and the finer-grained,
lower-permeability units separating the sands
tend to remain continuous. The resulting k v /k h
ratio is low.
The degree of confinement and the resulting
amalgamation ratio therefore link directly to per-
meability architecture (Stephen et al. 2001 ) and
an understanding of the reservoir in terms of
confinement is an essential aspect of the concep-
tual sketch which we have argued underpins any
good modelling exercise (Chap. 2 ) .
Figure 6.25 illustrates the degree of confine-
ment in terms of two controlling factors: the size
of the gravity flow depositing the sediment and
the size of the container it flows in to. Similar
geometries may result from large flows entering
a large basin as those from small flows entering a
smaller space (e.g. Stanbrook and Clark 2004 ).
Confinement is thus a relative issue, and a series
of flows making up one gross reservoir interval
may be a combination of confined and uncon-
fined components. Variations in confinement, via

changes in amalgamation ratio, lead to variations
in permeability architecture, k v /k h ratio and

recovery efficiency.

6.5.2 Seismic Limits

Once a reservoir prospect has been identified
from seismic attributes and proven by drilling,
the reservoir often becomes clearly 'visible on
seismic' leading to three common tendencies in
reservoir modelling:
1. To limit the field description to the observed
seismic attributes,
2. To treat the reservoir as largely connected
within-attribute, and
3. If high N/G sands are encountered initially, to
assume the field is relatively tank-like within
the seismically-constrained envelope.
These simplifying tendencies may occasion-
ally work (the rare 'sand tank' reservoir) but sub-
seismic heterogeneities usually emerge during
the producing life of a field.
A distinctive feature of deep water systems is
the predominance of stratigraphic traps. Unlike
structurally closed fields, where there is a geo-
metric limit to how much volume can be
contained in a defined closure, the addition of
previously undetected connected HCIIP beyond