Geology Reference
In-Depth Information
a)
500
0
-500
Figure 7.2 The AVO product indicator (intercept × gradient) (or
'AB' or 'PG' stack) highlighting Class III gas sands with increasing
amplitude with offset (red) in contrast to Class I water sands with
decreasing amplitude with offset (blue) (after Bacon et al., 2003 ).
-200
-100
0
Intercept (A)
100
200
b)
600
650
Figure 7.4
An example of colour coded crossplots highlighting
'
Class III signatures associated with the presence of gas:
(a) AVO crossplot, (b) intercept section (wiggle) with data from
yellow and blue zones highlighted. Courtesy Hampson-Russell.
anomalous
'
Figure 7.3 Gas accumulation with Class III AVO identified with the
ERG
(enhanced restricted gradient) attribute. Red colours represent
positive AVO and blue colours represent negative AVO.
7.2.2 Class III hydrocarbon and water sands
A more difficult situation for interpretation is where
both hydrocarbon sands and water bearing sands
have Class III AVO signatures. In this instance a soft
response with a negative gradient is not necessarily
diagnostic of hydrocarbons. Both hydrocarbon sands
and water sands have soft responses but hydrocarbon
sands will have higher amplitudes than water bearing
sands (assuming they have similar porosities).
In this situation the interpreter needs to assess
whether a response is bright enough to be due to the
presence of hydrocarbon. In some basins the relative
amplitude difference between hydrocarbon and water
bearing sands is exploited by calculating the AB ratio
(not to be confused with the AB AVO attribute). The
AB ratio is the ratio of the target amplitude to selected
'
positive gradients) is the intercept × gradient attribute
(also called the
attribute). An AB attri-
bute section showing two thin gas sands is displayed
in Fig. 7.2 . The red signature indicates
'
PG
'
or
'
AB
'
,
in this case associated with thin gas sands, whereas the
blue signature indicates
'
positive AVO
'
'
negative AVO
'
associated
with water bearing sands.
A useful reconnaissance attribute that tends to
highlight Class III (positive AVO) responses and
which is relatively insensitive to residual NMO is
the enhanced restricted gradient (ERG) (Barton and
Gullette, 1996 ). It is defined on the basis of the amp-
litude envelope complex attribute:
ERG
¼ð
far envelope
near envelope
Þ
×far envelope
:
Figure 7.3 shows a gas accumulation with the ERG
attribute. The hydrocarbon is identified by the
mottled red/blue signature (red values represent posi-
tive AVO) whereas the water sands have a broad blue
signature typical of a Class I AVO signature.
background
'
amplitudes. High values of the AB ratio
(e.g. 5
6) would tend to indicate hydrocarbon pres-
ence. In basins where the AB ratios are much smaller
(e.g. 2
-
128
-
3) and the reservoir units are relatively thin it
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