Geology Reference
In-Depth Information
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a)
50ft reservoir with variable shale
18ms tuning thickness
b)
12000
10000
9.5m
sand
8m
sand
16.5m
shaley sand
8000
6000
Figure 5.29
The non-uniqueness of thickness and impedance
below tuning.
4000
2000
representative. Various models were created by
changing the thicknesses of the sands and shale unit
and the results are shown in
Fig. 5.31
. Assuming that
the modelled variability is geologically reasonable it
is evident
0
0
5 0 5 0 5 0
net sand (msecs)
Figure 5.30
Sand/shale model with constant (tuning) thickness; a)
form of the model with synthetic responses, after Meckel and Nath
(
1977
; AAPG © 1977, reprinted by permission of the AAPG whose
permission is required for further use) b) relationship between
composite amplitude (i.e sum of absolute amplitudes of trough and
peak) and net sand.
based on the wells
(
Fig. 5.31a
) is not an adequate description of the
uncertainty. An appropriate approach for deriving
estimates of uncertainty in this instance would be to
use Monte Carlo simulation with probability density
functions describing the distribution of elastic prop-
erties and thicknesses of sand and shale as well as
incorporating the effect of seismic noise (e.g. Mukerji
and Mavko,
2008
; Saussus and Sams,
2012
). This
that
'
calibration
'
5.3.6 The effects of pressure
Pressure is an important factor in the acoustic prop-
erties of rocks, and seismic reflectivity and AVO
responses may vary from those expected simply
through the effects of pressure. There are two aspects
of interest: (1) the effect of pressure variations with
depth and, in particular, the differences between
seismic signatures in normally pressured sections
compared to those from over-pressured regions, and
(2) the effect of pressure variations due to reservoir
production.
5.3.5.3 Transitional boundaries
Amplitudes respond differently to gradual versus
sharp boundary changes (e.g. Anstey,
1980
).
Figures
5.32a
c
illustrate how the thickness of a transition
zone can have a direct impact on the amplitude and
signature. Increasing the thickness of the transition
zone results in a decrease of amplitude and imparts a
lower-frequency look to the response. When the tran-
sitional zone is associated with a thin bed, for example
with coarsening or fining upward sands the presence
of the transitional zone will impart asymmetry to the
composite response (
Figs. 5.32d
-
5.3.6.1 Rock velocity and effective pressure
Laboratory studies give valuable information on the
response of sandstones and other rocks to pressure
variations. Experiments are performed on core plugs
-
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