Geology Reference
In-Depth Information
,
with which shale parameters can be estimated from
the log data and the relevance of the effective param-
eters to the estimation of moveable hydrocarbon. In
terms of the porosity equation given above, the effect-
ive porosity of shaley sandstones is calculated when
the mineral density is derived for a mixture of quartz
and shale. Calculation of total porosity is often diffi-
cult because of the uncertainty in dry clay density.
The following equations provide a link between
effective and total porosity measurements:
1
ρ
cl
ρ
sh
ρ
cl
ρ
w
ϕ
t
¼
ϕ
e
+V
sh
1
ð
ϕ
e
Þ
ð
8
:
33
Þ
where S
we
and S
wt
are effective and total water satur-
ation,
ϕ
t
are effective and total porosity, V
sh
is
the volume fraction of shale, and
ϕ
e
and
ρ
w
are
the densities of clay, shale and formation water. An
example of water saturation relationship to porosity
and shale content is shown in
Fig. 8.26
. The water
saturation increases as the shale content increases and
the porosity decreases.
From the perspective of using rock physics models
to predict velocities (e.g. such as the Raymer
ρ
cl
,
ρ
sh
and
ð
S
we
Þ
ϕ
e
¼
ð
1
S
wt
Þ
ϕ
t
,
ð
8
:
31
Þ
ð
1
S
we
Þ
ϕ
e
-
Hunt
S
wt
¼
1
,
ð
8
:
32
Þ
model or any other velocity model) it is the
'
porosity that is used. However, in terms of fluid
substitution, and owing to a number of practical
issues (see
Section 8.5
), it can be argued that Gass-
mann might be parameterised either in terms of total
or effective porosity (
Section 8.5
).
'
total
ϕ
t
Total porosity
Effective
porosity
Clay
bound
water
Quartz
Clay & Silt
8.2.5 Dry rock relations
The dry rock values inverted from Gassmann
Hyd
Water
s equa-
tion are important parameters for quality control.
They are also important
'
Shale
in rock characterisation
Figure 8.25
Schematic representation of the components of a
shaley sandstone.
studies.
a)
b)
Phi
e
1.
0.5
1.
0.45
0.9
0.8
0.4
0.8
0.35
0.7
0.6
0.3
0.6
0.25
0.5
0.4
0.2
0.4
0.15
0.3
0.2
0.1
0.2
0.1
0.05
0.
0.
0.
0.
0.05
0.1
0.15
0.2
0.
0.2
0.4
0.6
0.8
1.
Phi
e
S
w
169
Figure 8.26
Results from a petrophysical analysis of an oil-bearing shaley sandstone; (a) effective porosity vs water saturation, (b) water
saturation vs shale content.
