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or diagenetic features, often (perhaps incorrectly)
excluded from descriptions of 'facies'.
Modelling elements are defined here as:
three-dimensional rock bodies which are
petrophysically and/or geometrically distinct
from each other in the specific context of the res-
ervoir fluid system.
Model Elements
Having established a structural/stratigraphicmodel
framework, we can now return to the model con-
cept and consider how to fill the framework to
create an optimal architectural representation.
The fluid-fill factor is important as it
highlights the fact that different levels of hetero-
geneity are important for different types of fluid,
e.g. gas reservoirs behave more homogeneously
than oil reservoirs for a given reservoir type.
The identification of 'model elements' has
some parallels with discussions of 'hydraulic
units' although such discussions tend to be in
the context of layer-based well performance.
Our focus is on the building blocks for 3D reser-
voir architecture, including parts of a field
remote from well and production data. It should
be spatially predictive.
2.4.1 Reservoir Models Not Geological
The rich and detailed geological story that can
be extracted from days or weeks of analysis of
the rock record from the core store need not be
incorporated directly into the reservoir model,
and this is a good thing. There is a natural ten-
dency to 'include all the detail' just in case
something minor turns out to be important.
Models therefore have a tendency to be over-
complex from the outset, particularly for novice
modellers. The amount of detail required in the
model can, to a large extent, be anticipated.
There is also a tendency for modellers to seize
the opportunity to build 'real 3D geological
pictures' of the subsurface and to therefore make
these as complex as the geology is believed to be.
This is a hopeless objective as the subsurface is
considerably more complex in detail than we are
capable of modelling explicitly and, thankfully,
much of that detail is irrelevant to economic or
engineering decisions. We are building reservoir
models - reasonable representations of
2.4.3 Model Element Types
Having stepped beyond a traditional use of
depositional facies to define rock bodies for
modelling, a broader spectrum of elements can
be considered for use, i.e. making the sketch of
the reservoir as it is intended to be modelled. Six
types of model element are considered below. Lithofacies Types
This is sedimentologically-driven and is the tra-
ditional way of defining the components of a rock
model. Typical lithofacies elements may be
coarse sandstones, mudstones or grainstones,
and will generally be defined from core and or
log data (e.g. Fig. 2.7 ).
detailed geology - not geological models.
2.4.2 Building Blocks
Hence the view of the components of a reservoir
model as model elements - the fundamental
building blocks of the 3D architecture. The use
of this term distinguishes model elements from
geological terms such as 'facies', 'lithofacies',
'facies associations' and 'genetic units'. These
geological terms are required to capture the rich-
ness of the geological story, but do not necessar-
ily describe the things we need to put into
reservoir models. Moreover, key elements of
the reservoir model may be small-scale structural Genetic Elements
In reservoir modelling, genetic elements are a
component of a sedimentary sequence which
are related by a depositional process. These
include the rock bodies which typical modelling
packages are most readily designed to incorpo-
rate, such as channels, sheet sands or
heterolithics. These usually comprise several
lithofacies, for example, a fluvial channel might
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