Geology Reference
In-Depth Information
Amplitude and AVO interpretation
Chapter
7
7.1 Introduction
The purpose of seismic amplitude interpretation and
AVO analysis is to explain changes in seismic signa-
tures in terms of fluid and rock variations. In hydro-
carbon exploration, the interpreter looks particularly
for seismic amplitude changes that may be related to a
change in fluid (i.e. from water to hydrocarbon).
These seismic effects are often called direct hydrocar-
bon indicators (
reservoir porosity, mineralogy and stiffness
characteristics,
relative compressibility of hydrocarbon and water,
non-reservoir properties,
seismic bandwidth (resolution) and data quality,
AVO angle,
reservoir architecture
·
layering characteristics (e.g. thick or thin
isolated reservoir units vs thick sequences of
thin-bedded reservoirs)
).
The search for DHIs and the analysis of AVO data
go hand in hand to develop a view of the prospectivity
of an area. There are a variety of interpretation tech-
niques that can be used to exploit AVO phenomena
and aid interpretation. Commonly used techniques
include:
'
DHIs
'
lateral extent and morphology.
·
Clearly, well calibration is invaluable for understand-
ing the seismic response. The preceding chapters have
emphasised how seismic modelling with well data is a
critical tool in guiding the interpreter in what to
expect from local rock and fluid effects. In areas
without well calibration detailed interpretations may
not be possible but AVO might be used as a scanning
procedure to highlight anomalies and rank different
areas.
The following discussion focusses on the problem
of hydrocarbon detection and examples will be
presented of how various techniques might be used
in a variety of different geological settings. Given the
large variety of expressions of AVO behaviour this
discussion is intended to get the interpreter thinking
rather than as an exhaustive account of all published
approaches to AVO analysis.
partial offset and angle stacks (e.g. near, mid, far)
(
Chapter 2
),
AVO crossplot colour coding (e.g. Verm and
Hilterman,
1995
),
AVO hodograms (e.g. Keho et al.,
2001
),
AVO projections (e.g. Whitcombe et al.,
2002
; and
Chapter 5
),
target/reference
'
relative AVO
'
method (e.g.
Chiburis,
1993
).
It should be remembered that confidence in ampli-
tude interpretation is to a large extent determined by
the geological and spatial context. DHIs are most
commonly, but not exclusively, found in relatively
shallow unconsolidated and partially consolidated
sand/shale sequences, where fluid compressibility
can have a significant effect on the whole rock
compressibility. In well consolidated sandstone and
carbonate situations the fluid tends to make a smaller
contribution to whole rock compressibility (
Chapters
5
and
8
) and consequently DHI and AVO effects are
usually more subtle.
Some of the key factors for the interpreter to
consider are:
7.2 AVO and amplitude scenarios
The following discussion focuses on common ampli-
tude and AVO scenarios encountered in siliciclastic
sands and shales, carbonates and fractured reservoirs,
highlighting the use of various interpretation tech-
niques and illustrating potential variability. A useful
starting point is the AVO classification presented in
Chapter 5
, particularly for the description of tops and
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