Geology Reference
In-Depth Information
amplitude. Whilst this observation gives some justifi-
cation for interpreting net pay from seismic ampli-
tude at and below the tuning thickness, a critical
observation from
Fig. 5.30b
is that there is scatter
around the trend (i.e. there is uncertainty).
Uncertainty becomes important in field develop-
ment work where it can have a significant impact on
field development planning.
Figure 5.31
shows
another example, putting the notion of calibration
into the context of uncertainty. It shows a schematic
illustration of the reservoir (two sands with a shale
unit in between) represented by a composite ampli-
tude signature. Five wells have been drilled giving a
near linear relation between amplitude and net pay
and a high correlation coefficient. It gives the impres-
sion that the net pay error prediction is about ±/
0.05
0
-0.05
Below tuning
-0.10
-0.15
Tuning
Isolated reflector
-0.20
-0.25
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
Intercept
9 ft.
It should be remembered that wells tend to be
drilled in sweet spots and possibly are not statistically
Figure 5.27
Idealised AVO signature from the top reflection of a
low impedance wedge, modelled using a simple one-dimensional
reflectivity model.
Wedge thickness (m)
Figure 5.28
Model data showing effect
of tuning on the AVO response; (a) two
angle stacks from the model, θ ¼ 10° and
θ ¼ 30° and (b) intercept/gradient
crossplot. Note numbers are thicknesses
in metres (data from Roy White, personal
communication).
a)
5
10
15
20
25
30
35
40
θ
=10º
θ
=30º
b)
0.01
0
5
-0.01
10
25
30
-0.02
35
40
20
15
-0.03
-0.04
-0.04
-0.03
-0.02
-0.01
0
0.01
Intercept
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