Amplitude and AVO interpretation - Seismic Amplitude - page 131

a) b)

Clean wet
sand

2

Schematic
section

Shaley
hydrocarbon
sand

North

0

TWT

Possible contact

Clean
hydrocarbon
sand

-10

-2 0 1

R(0)

c)

AVO fluid
factor
projection

Inc. shaliness of sand

Fluid factor
projection
Shuey with
Sin 2 θ =0.18

Line of
section

High

Northern
hydrocarbon
accumulation

Southern
hydrocarbon
accumulation

Low

North

Figure 7.7 The effect of lithology variation on AVO; (a) schematic section with top reservoir interpretation, (b) AVO crossplot showing cluster
shapes from segments shown in (a), (c) schematic fluid factor map. Re-drawn and modified after Sams, 1998 ).

soft signature owing to the presence of the silt and the
hydrocarbon (oil) effect is simply to increase the
amplitude (i.e. the differential AVO effect between
the hydrocarbon and water cases is minimal).
When the silt is removed ( Fig. 7.10b )thetop
sand reflection becomes a weak Class I response
when the sand is wet and a Class II response when
oil bearing. The key signature effect for hydrocar-
bons in this case is a soft response at the top of the
sands on far stacks. AVO crossplots for the two
cases illustrate how the presence of the hard unit
can affect the optimal fluid projection angle. A high
angle,

The presence or absence of these hard layers could
create ambiguities for AVO interpretation. For
example, if a low angle projection (in Fig. 7.10d )
is applied when silt layers are present above sands
( Fig. 7.10c ), water wet responses will appear anom-
alousaswellasthehydrocarbonsands.

7.2.4 Class IIp hydrocarbon sands, Class
I water sands
Perhaps the most subtle and potentially ambiguous
AVO scenario occurs when hydrocarbon sands have a
Class IIp signature and water sands have a Class
I signature. A good example from Rutherford and
Williams ( 1989 ) is shown in Fig. 7.11a . The feature
has been tested by a well which found gas.

57°), is required to identify the
Class IV response at the top of the sand when the
silt is present ( Fig. 7.10c ), whereas a lower angle of
χ

χ ~ 35°( θ ¼

131

~10°( θ ¼ 25°) is required for the non-silt case.