Geology Reference
In-Depth Information
a)
b)
Class II/III
Class IIp
Ofset
Ofset
Top oil
s
a
nd
Top brine
sand
c)
d)
Class IV
Class I
Ofset
Ofset
Top
limest
o
ne
Top gas
sand
AI
Figure 5.4
only in specific
circumstances may they have a diagnostic significance. Note that all examples have the same polarity; (a) Class II/III, (b) Class IIp, (c) Class IV, (d)ClassI.
Some examples of different AVO responses. In current common usage AVO classes are simply used as descriptors
-
generalise about their significance. It is often believed
that
of a sand/shale system, reflections associated with
the base of sands and hydrocarbon contacts typically
have AVO responses with positive gradients. Base
of brine sand responses can have various styles of
response including Class IV, or a phase reversal from
negative to positive amplitude with increasing offset
or indeed a positive reflection with increasing ampli-
tude with offset (but in this case the gradient is
usually quite low) (
Fig. 5.5
).
Positive AVO with a positive intercept is generally
characteristic of modelled hydrocarbon contacts
(
Fig. 5.5
). This can be a good diagnostic, as litho-
logical changes with positive intercept usually have a
negative gradient (i.e. Class I). In real data, however,
contacts may not have strong AVO characteristics
owing to interference of the fluid boundary reflec-
tions with bedding reflections or the presence of
'
rising AVO means pay
'
, possibly because of the
descriptions in Ostrander
s(
1984
) original paper on
AVO, but this is clearly refuted by the brine sand
response in
Fig. 5.4a
. Clearly the geological context is
paramount in AVO analysis. To assess the significance
of different AVO responses the interpreter needs to
make models for different scenarios (for example by
varying fluid fill and/or porosity) and compare
the results with seismic observations. It is usually not
the class of response that is significant, but the relative
change in AVO response, for example from an up-dip
to a down-dip location or a change in a target reflector
in comparison to a background reflection. Examples of
different AVO scenarios are given in
Chapter 7
.
The AVO classes as published describe AVO
responses only for the top sand interface. In terms
'
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