Geology Reference
In-Depth Information
Data from three wells are shown in
Fig. 5.68
, with
varying disposition of sands and shales. Both (a) and
(b) have possible lithology trends at negative
90º
57º
Vp/Vs
G
51º
angles,
whereas in (c) the fluid angle will also be the lithology
angle.
χ
σ
22º
13º
λ
5.5.3 Bandlimited impedance
The starting point for considering seismic inversion
(in terms of the derivation of impedance from seis-
mic traces) is the relationship between reflectivity
and impedance. Integration of the reflection coeffi-
cient series gives a scaled version of the acoustic
impedance log (
Fig. 5.69
). A practical problem for
seismic inversion is that seismic reflectivity is ban-
dlimited (
Chapter 3
), meaning that an integration
of a seismic trace will give only a smoothed form of
the impedance. This is equivalent then to the
impedance log being convolved with a (zero phase)
wavelet or, alternatively, preferentially removing
Κ
AI
0º
R(0)
SI
313º or -47º
302º or -58º
μ
Figure 5.66
AVO crossplot showing projection axes which
typically correlate strongly with particular elastic parameters (after
Connolly,
2010
).
a)
b)
Correlation plot
12000
120
11000
100
10000
80
9000
60
8000
40
7000
20
6000
5000
90
60
30
0
-30
-60
-90
4000
6000
8000
10000
χ
EEI(0) (AI)
c)
d)
140
45
40
120
35
100
30
80
25
60
20
15
40
10
20
5
0
0
4000
6000
8000
10000
4000
6000
8000
10000
EEI(25)
EEI(-51)
103
Figure 5.67
Theoretical fluid and lithological discrimination using EEI with facies determined from logs (purple ¼ shales, blue ¼ water sands
and green ¼ oil sands).
