Geoscience Reference
In-Depth Information
Fig. 6.35
A variogram for
vertical porosity measured
in chalk core (Almeida and
Frykman
1994
). The
pattern is a 'hole'
variogram, showing
alternating high and low
variance between porosity
values, suggesting regular
12 m layering (Redrawn
from Almeida and
Frykman
1994
,
AAPG
1994, reprinted by
permission of the AAPG
whose permission is
required for further use)
#
objective description of the lithology, often fails to
break a reservoir system down into elements with
clear k/
touching-vug fabrics, as in the Lucia classifica-
tion scheme. The latter distinction is somewhat
semantic but is made here because of the dramat-
ically different permeability of connected
fractures and the different scale on which
connected fracture systems work (see next sec-
tion). We would argue that this imparts a dis-
tinctly different fabric on the reservoir than more
localised touching-vug fabrics, even those
including micro-fractures. Lucia's work is pri-
marily focussed on carbonate sedimentology
and petrophysics, with fractures given a reduced
role; the modified scheme proposed in Fig.
6.37
allows for the fracture classifications of Nelson
(
2001
) to be incorporated.
The typical porosity-permeability charac-
teristics of the Lucia rock fabrics are shown in
Fig.
6.37
, with a fracture group added alongside.
These classification schemes attempt to isolate
the underlying controls on rock properties, partic-
ularly permeability. It should however be
emphasized that if the origin of reservoir perme-
ability for any given case is not known and cannot
be characterised conceptually, there is little point
in embarking on a model, certainly if the end-
result is to be simulation. Simple log-based poros-
ity modelling and application of a linear trans-
form from porosity to permeability is likely to
produce a weak model (discussed further below).
relationships.
A pragmatic method of carbonate charac-
terisation at the pore scale is presented in Lucia
(
1983
) and the summaries given by Lucia (
2007
)
remain a very good starting point for
characterising carbonate reservoirs as a basis
for reservoir modelling. Lucia classifies pore
systems into two broad groups:
1. Inter-particle, in which porosity sits between
grains or crystals, and
2. Vuggy (which is everything else).
Vuggy systems divide again into separate-vug
and touching-vug fabrics. Any carbonate classi-
fication system can be mapped onto this simple
scheme, and indeed Lucia (
2007
) subdivides the
scheme to accommodate common carbonate
descriptive terms (moldic pores, fenestral pores,
breccias, etc.). The advantage of Lucia's scheme
is that it captures heterogeneity in terms of pore
fabric which lends itself to petrophysical
characterisation, and it is therefore more predic-
tive than the Dunham scheme.
With reservoir modelling in mind, a
generalisation of Lucia's scheme is shown in
Fig.
6.36
. This is set against the Dunham classi-
fication but with joint systems separated out,
rather than being treated as a special case of
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