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water-wet in the higher permeability laminae.
Capillary trapping will generally be a more impor-
tant issue to consider in more water-wet systems.
If determined to be important, the rock unit
associated with capillary trapping (e.g. in
grainfall or wind ripple strata) needs to be
defined and included as a discrete modelling
element. The impact of that element can then
either be modelled explicitly as a 3D object or
captured as part of an REV in small-scale models
used to determine effective properties (notably
effective relative permeabilities) for a larger
scale model.
contains hierarchically-organised components -
channel fill, barforms (Fig.
6.9
), point bars, lat-
eral accretion surfaces, over-bank deposits, etc.
Fluvial sandbody architecture is covered in
detail elsewhere, notably by Miall (
1985
,
1988
),
and many resources have been devoted to under-
standing fluvial sandbody architecture in outcrop
(e.g. Dreyer et al.
1993
) as
illustrated in
Fig.
6.10
.
6.2.2 Geometry
The key questions to ask when modelling fluvial
reservoirs are typically geometric:
1. What is the fluvial system - braided or
meandering - or something in between?
2. What is the channel density - channels pro-
portion well over 50 % or much less?
3. What is the channel sinuosity?
4. What are the typical channel dimensions?
5. Should we be focussing on individual
channels or multi-channel complexes?
6. What is the internal channel architecture? Is it
essentially sand rich - and therefore effec-
tively homogeneous, or is it composed of
many variable elements including muddy,
silty and sandy sub-elements?
Figure
6.11
shows examples of high-
resolution models of meandering channel
systems, illustrating typical model elements. In
one example (Fig.
6.11
, left), the focus is on
channel stacking patterns and internal channel
fill. The overbank crevasse-splay sands (green)
have been represented as simple ellipsoids,
whereas the sinuous channels have been
modelled in more detail with layers of sand (yel-
low) and silt (purple) in the channel fill, and
lateral accretion surfaces (red), all within a
muddy background (blue). Alternatively
(Fig.
6.11
, right), less effort may given to the
internal channel architecture and more attention
paid to capturing the channel types, intersections
and connectivity.
Whatever the choice of approach, the key issue
is not to attempt to model all lithofacies but to
6.2
Fluvial Reservoirs
6.2.1 Fluvial Systems
Fluvial reservoirs were one of the first reservoir
types to receive the attention of object-based
(Boolean) geological modelling efforts (e.g.
Haldorsen and MacDonald
1987
; King
1990
;
Holden et al.
1998
; Larue and Hovadik
2006
).
Finding the location of sand-rich channel objects
within a more or less muddy background is a key
issue, which lends itself to some form of proba-
bilistic modelling, and establishing the degree of
connectivity between channels is the ultimate
factor which determines hydrocarbon recovery.
However, the proportion of channel sands and
the internal character of
the channels vary
enormously.
Fluvial reservoirs fall into two broad groups:
braided and meandering. Braided channels are
formed in wide braid-plain systems (Fig.
6.9
)
with high sediment flux, whereas meandering
channels form in more mature channel systems
with overall lower sediment discharge. Braided
systems tend to have a higher density of channels,
which have lower sinuosity, whereas meandering
systems tend to have a lower density of channels,
with higher individual channel sinuosity.
Individual channels are typically grouped to
form multi-channel complexes, and when we
look at any individual channel we find it usually
