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bases of sands and hydrocarbon contacts in thick-
bedded sand settings. The classification becomes less
useful, however, when considering reservoir sections
with multiple thin beds.
The simple AVO model shown in Fig 7.1 , gener-
ated by using average values taken from logs and
input to the Aki
Richards equations, illustrates the
general trends of the amplitude changes with increas-
ing angle. Note that the model indicates decreasing
water sand reflectivity with increasing angle, whereas
the gas sand effectively brightens with increasing
angle. Thus, far stacks may be best for observing the
hydrocarbon signatures.
In this setting the identification of increasing
amplitude with angle (sometimes referred to as
-
7.2.1 Class II/III hydrocarbon sands and
Class I water sands
Figure 7.1 illustrates a full stack section from a gas
field in an ideal setting to observe hydrocarbon
effects. The hydrocarbon sands have Class II or III
responses whilst the water bearing sands have a Class
I response. Effectively this means that on full stack
sections there will be a phase reversal at the reflection
from the top of the sand as the water (red loop) gives
way to gas (black loop). Bright soft responses are
characteristic of hydrocarbon bearing sands in this
scenario. Note that the gas water contact reflection
in Fig. 7.1 is not flat. It shows
'
rising
'
'
'
AVO
; Chapter 5 ) may well be
characteristic of the presence of hydrocarbon. How-
ever, care needs to be taken not to apply this as a
general rule. Given that the optimal angle for imaging
fluids (i.e. the
or
positive AVO
) is effectively within the
range of seismically acquired angles ( Chapter 5 ) pro-
jection techniques may be of little benefit.
A popular AVO attribute combining intercept and
gradient that effectively enhances
'
fluid angle
'
owing to
the very low velocities associated with the gas sand
compared to the surrounding rocks.
'
push-down
'
(i.e.
Class III reflections and positive reflections with
'
rising AVO
'
a)
Top gas sand
Phase reversal
A I
Top brine sand
=
GWC
Migrated stack
b)
Effective Angle
+
GWC
Simple AVO
Model
Top Brine Sand
Top Gas Sand
-
127
Figure 7.1 A bright spot on migrated stack display related to the presence of gas; (a) migrated stack display (European polarity), courtesy
Rashid Petroleum Company, (b) schematic rock physics model.
 
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