Geology Reference
In-Depth Information
Several comments can be made on these plots.
a)
8
20
The various rock types have a large degree of
overlap in velocity and density (
Fig. 5.7
).In
general, carbonates tend to have higher density
and velocity than siliclastics but high-porosity
chalks overlap with sands.
16
D
o
lomite
7
12
Limestone
6
Salt
5
8
Anhydrite
4
In general there is a strong positive correlation of
velocity and density for both siliclastics and
carbonates. As velocity increases so does density.
This gives rise to the important first order
observation that there is a relationship between
acoustic impedance and porosity. As porosity
increases the acoustic impedance decreases. As
will be seen, the specific details of the relationship
for each lithology are controlled by mineralogy
and pore geometry.
Sandstone
3
Shale
2
Coal
4
1
0
1.6
1.8
2
2.2
2.4
2.6
2.8
3
Density (g/cc)
Dashed lines = AI (km/s.g/cc)
b)
5
Despite the overlapping nature of lithologies in
terms of acoustic impedance there are significant
differences in Poisson
4.5
4
s ratio characteristics
(
Fig. 5.7
) and these can be a basis for determining
lithology from seismic.
'
3.5
3
2.5
A few rock types are very distinctive, such as coal
which has very low density and high Poisson
2
'
s
1.5
ratio. Coals will cause high-amplitude soft
reflections that could potentially be confused with
gas sands. Coals tend to generate Class IV AVO
responses and this may prove a good
discrimination in cases where gas sands have Class
III AVO.
1
0.5
0
1
2345678
V
p
(km/s)
Dashed lines = Poisson's ratio
The Poisson
s ratio trends of siliclastic and
carbonate lithologies are quite different (
Fig. 5.7
).
Sands and shales exhibit steep sub-parallel trends
such that the Poisson
'
c)
0.5
Coal
s ratio of both sands and
shales decreases with decreasing porosity
(higher AI). Limestones and dolomites tend to
have fairly flat (constant Poisson
'
0.4
Limestone
0.3
Anhydrite
'
s ratio) trends on
Dolomite
the AI
PR plot. Thus, the interpreter must be
careful in applying rock physics rules of thumb
developed on siliclastic rocks to carbonate settings.
-
Salt
0.2
Shale
0.1
Sandstone
5.3.2 The role of compaction
The most important control on the elastic parameters
of sedimentary rocks is porosity. During burial, rocks
undergo both mechanical and chemical changes
which give rise to a general decrease in porosity
(and increase in density) with depth. Burial is also
an important factor in the porosity and acoustic
impedance of Chalks, but it could be argued that it
is less important for other types of carbonates, for
0
0
5
10
15
20
25
Acoustic impedance (km/s.g/cc)
Porosity
Figure 5.7
Ranges of parameters for common sedimentary rocks
(brine-bearing), based on data in Castagna et al.(
1993
): (a) density vs
P wave velocity (dashed lines ¼ AI (km/s.g/cc), (b) V
p
vs V
s
(dashed
lines ¼ Poisson's ratio), (c) acoustic impedance vs Poisson's ratio.
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