Geology Reference
In-Depth Information
Seismic amplitude applications
Chapter
10
10.1 Introduction
This chapter focuses on the specific role of amplitude
interpretation for the purpose of reservoir evaluation.
A number of techniques based on seismic amplitude
have been described in previous chapters, including
AVO analysis and various inversion techniques, and
the interpreter
published. Inevitably, pitfalls or drawbacks in the
various techniques are not always laid bare. The
reader is urged toward scepticism but with an open
and enquiring mind. There are no silver bullets with
seismic amplitude technologies and there is no sub-
stitute for personal experience.
s choice will depend to a large extent
on the quality and type of data available as well as the
problem at hand. In general, relative techniques, such
as AVO analysis for defining fluid related anomalies,
are appropriate in the exploration phase whereas in
development projects more sophisticated techniques
requiring significant well control, such as determinis-
tic and stochastic inversion, are warranted. At each
stage the interpreter can attempt to use seismic amp-
litude to describe the critical aspects of the reservoir
as well as defining limits of uncertainty. It should be
noted that to be consistent with the aim of the topic in
terms of the physical interpretation of seismic ampli-
tude, purely statistical or
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10.2 Litho/fluid-facies from seismic
A key tool for the interpreter, at least in the explor-
ation stage, is AVO analysis (
Chapter 7
). Whether the
approach is simply to look for anomalous signatures
on partial stack data, identify
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data
clusters on the AVO crossplot, or generation of AVO
projections (
Chapter 5
), the primary idea behind
AVO analysis is to identify anomalous reflectivity that
may be related to the presence of hydrocarbons.
These signatures need to be put into a clear geological
context. In some areas, lithological projections can
assist in establishing the sedimentary framework,
though this is not always the case. AVO projection
techniques effectively provide a means of scanning
the data in search of anomalies and their geological
context.
In areas where there is well control it is possible
to evaluate the detailed facies context of AVO signa-
tures. Well log analysis that incorporates the vari-
ability of elastic parameters within each facies
inevitably shows that AVO signatures (for example
defined by intercept and gradient) are to some extent
non-unique. If the well log statistics are considered
representative then it is possible to model the
probabilities for each facies for given AVO signa-
tures (
Fig. 10.1
). Following a non-trivial calibration
step (
Chapter 6
) the statistical model can then be
used in conjunction with techniques such as fuzzy
logic (Cuddy,
1998
), neural networks (e.g. Trappe
and Hellmich,
2000
) or Bayesian classification (e.g.
Mukerji et al.,
2001
) to predict litho-facies from
down to the left
type tech-
niques have not been included in the discussion.
In field development, seismic interpretation is an
integral part of the field geological model; key reser-
voir parameters that can be characterised include
geological facies, reservoir properties (including por-
osity, net-to-gross and water saturation) and reservoir
geometry and connectivity. In addition to the static
element of the reservoir, time-lapse seismic offers the
chance to evaluate dynamic changes. Seismic ampli-
tudes also have a central role in the risk evaluation of
prospects characterised by DHIs. In addition, modern
amplitude technologies can produce results with rea-
sonable certainty such that they can be incorporated
into the process of reserves determination.
To a large extent the following discussion uses
published examples for illustration of the range of
applications of seismic amplitude. This has the disad-
vantage that the authors are not in a position to make
critical comment, as not all relevant information is
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data mining
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