Geology Reference
In-Depth Information
a)
b)
Top sand
-100
-50
0.8
0
0.4
50
Base sand
0
100
150
Frequency (Hz)
Amplitude
c)
d)
e)
1.2
-0.0006
-12000
0.8
-0.0004
-8000
0.4
-0.0002
-4000
0
0
0
0 0 0 0 0
0 0 0 0 0
0 0 0 0 0
Apparent thickness (ms)
Apparent thickness (ms)
Apparent thickness (ms)
Figure 10.14
Low impedance wedge model; (a) impedance model convolved with bandpass wavelet, (b) bandpass wavelet (output from
the coloured inversion process), (c) tuning curve, (d) seismic N:G, (e) scalar (after Simm,
2009
).
s(
2007
)methodis
particularly appropriate for deep sea turbidite
sequences.
Figure 10.17
illustrates an example from
Offshore West Africa. A bandlimited impedance far-
stack seismic cube is the basis for the net pay interpret-
ation of the lower reservoir unit shown in
Fig. 10.17a
.
Apparent thickness and average bandlimited imped-
ance are calculated between the zero crossings picked
on the seismic (
Figs. 10.17b,c
). Seismic N:G is deter-
mined from the bandlimited impedancemapmultiplied
by the scaler obtained from a simple wedge model using
the bandpass wavelet implicit in the coloured inversion.
Net pay is simply calculated from the product of the
seismic N:G and apparent thickness maps. Note how
the sweet spots on the final net pay map are located to
the north of the main area of bright impedance.
The successful
The application of Connolly
'
(3) there is no interference between the reservoir and
events above or below;
(4) the zero-crossing picks are clear;
(5) there is a single hydrocarbon phase (variable
errors will be expected around contacts: Simm,
2009
);
(6) the thickness of the reservoir is no greater than a
half cycle of the lowest frequency component, e.g.,
60 ms at 8 Hz low cut frequency (Connolly,
2007
).
Estimates of net pay based on Connolly
s(
2007
) tech-
nique are likely to be an underestimate in areas where
there are thin sands below seismic resolution. This
may be significant in reservoirs with subtle dip where
hydrocarbons extend away from the visible edge
of the amplitude anomaly. A rough estimate of the
distance can be made from the smallest thickness
calculated from the visible amplitude anomaly and
the dip of the top of the reservoir. For low-relief
structures, there may be a significant contribution to
the volume of hydrocarbons in place hidden in this
invisible pay zone.
The squared term inherent in Connolly
'
s
(
2007
) technique will in practice depend on a number
of geological and data factors, the prime requirements
being:
implementation of Connolly
'
(1) good lateral and vertical fidelity of seismic
amplitude;
(2) Bi-polar geology in which sands have one
impedance and shales have a higher impedance;
s approach
means that the accuracy of the thickness measurement
is critical. In situations where there is difficulty in
'
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