Geology Reference
In-Depth Information
a)
b)
Gas water contact response
Possible base of sand responses
A
I
+
Top gas sand
+
-
Gas water
contact
-
Figure 5.5
Examples of (a) hydrocarbon contact and (b) base sand AVO responses
elastic parameters of rocks and detailed styles of AVO
behaviour, but rather to develop a general discussion
to draw the interpreter into thinking about how
geology affects the seismic response. For each basin
the seismic interpreter needs to work closely with the
petrophysicist, geologist and engineer to formulate
knowledge of how the petroleum system is character-
ised and how this knowledge can be exploited in
seismic analysis.
Class V
G
+
Class VI
Class Vp
-
+
R(0)
5.3.1 Ranges of parameters for common
sedimentary rocks
The sedimentary rocks of most interest in explor-
ation are siliclastics (i.e. sands and shales) and vari-
ous forms of carbonate deposits. Further definition
of these terms will be given where relevant but it is
worth stating that the term
-
Figure 5.6
Additional AVO classes for responses with positive
gradients (such as base of sands and hydrocarbon contacts).
is used to describe
a non-reservoir rock with low permeability charac-
terised predominantly by clay minerals (usually
greater than 60% by volume) but also containing silt
and electrostatically bound water (e.g. Katahara,
2008
).
Chapter 2
highlighted the importance of compres-
sional velocity, shear velocity, bulk density and the
composite terms of acoustic impedance and Poisson
'
shale
'
amplitude decay with offset unrelated to the target
geology (see
Chapter 7
). It is possible using the AVO
crossplot to extend the descriptive scheme of AVO
classes to cover AVO responses with positive gradi-
ents (i.e. Classes Vp, V and VI) (
Fig. 5.6
).
5.3 Rock property controls on AVO
Each sedimentary basin has a particular style of
deposition, compaction, diagenesis, hydrocarbon gen-
eration and fluid flow that gives rise to seismic amp-
litude signatures. It is not the intention here to give a
detailed description of how basin processes affect the
s
ratio to the modelling and interpretation of AVO
responses.
Figure 5.7
shows some generalised cross-
plots (largely based on data from Castagna et al.,
1993
) which illustrate how these parameters vary in
different sedimentary rocks.
'
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