Geology Reference
In-Depth Information
a)
b)
c)
9.6
9.6
9.6
9.4
9.4
9.4
9.2
9.2
9.2
9
9
9
8.8
8.8
8.8
8.6
8.6
8.6
8.4
8.4
8.4
8.4
8.6
8.8
9
9.2
9.4
9.6
8.4
8.6
8.8
9
9.2
9.4
9.6
8.4
8.6
8.8
9
9.2
9.4
9.6
In AI
In AI
In AI
wet sands
gas sands
shales
Figure 5.68
AI vs GI crossplots from three wells West of Shetlands showing variable lithology projection trends; (a) data from a well in
which shales have high gradient values and can be separated from the sands with a negative
χ
angle, (b) data from a well where shales
have low gradient values and sands and shales can similarly be differentiated using a negative
χ
angle, (c) data from a well where
sands and shales have similar gradient values; thus the fluid
χ
angle will also serve to differentiate lithology.
Figure 5.69
Integration of the reflection
coefficient series gives the form of the
impedance log (after Anstey,
1982
).
Lithology
V
p
AI
(V
p
)
Rc
Integrated
Rc
ρ
ρ
- +
1
3
4
5
6
low and high frequencies from the impedance log
(
Fig. 5.70
).
Generation of bandlimited impedance from seis-
mic effectively requires convolving the seismic with
a bandlimited integration operator. Waters (
1987
)
showed that this is equivalent to applying a +90°
phase rotation and high cut filter of 6 dB/octave to
a zero phase seismic dataset. This is called the
Seismic Approximate Impedance Log (SAIL) and
is similar to the
described by Anstey
(
1982
). Note that if, on inspection, it appears that
thedatawerenotinitiallyzerophase,aphaserota-
tion can be applied to the bandlimited impedance to
tie geological boundaries in wells.
From an interpretation point of view, thinking in
terms of bandlimited impedance can be very useful
'
pseudo log
'
104
