Geology Reference
In-Depth Information
a)
0.05
0
-0.05
c)
-0.1
χ
=14º,
θ
=30º
χ
=45º
-0.15
Top reservoir
-0.2
0.0
0.1
0.2
0.3
0.4
0.5
Well 3
Sin
2
θ
shale/brine sand
shale/gas sand
GWC
b)
0.15
χ
=45º
0.1
Well 5
-0.05
0
-0.05
-0.1
-0.15
-0.2
-0.1
0
0.1
0.2
Intercept
shale/brine sand
shale/gas sand
Figure 7.9
Simple model of a reservoir with a Class IV AVO response; (a) AVO plot showing wet and hydrocarbon sand responses,
(b) AVO crossplot, (c) seismic models from two pseudo-wells, showing the use of AVO projections and bandlimited impedance in highlighting
the gas water contact (after Simm,
2009
).
7.2.5 Class I hydrocarbon sands, Class
I water sands
When both water sands and hydrocarbon sands have
Class I responses the effect of hydrocarbon is to dim
the hard response at the top of the sand. On full stack
sections the effect can be quite subtle, as is shown by
Fig. 7.16
. In some cases there may be a slight phase
reversal at the top hydrocarbon sand that may be
diagnostic.
There is likely to be interpretation ambiguity if
only full stack sections are used in this scenario. For
example, it may not be possible to differentiate a
dimming due to an increase in porosity from a hydro-
carbon effect.
Figure 7.17
shows a model based on
well log data in which a high porosity sand is overlain
by a low impedance shale. The near angle model
(
Fig. 7.17a
) shows a subtle dimming of the top sand
response, whilst the far angle model (
Fig. 7.17b
)
shows a more dramatic change with a dim spot at
the top of the hydrocarbon sand. Note that picking
from the top water sand across the hydrocarbon con-
tact is a possibility. The termination of the red trough
on the far stack associated with the base of the sand is
a useful diagnostic of the presence of a contact.
AVO projections (
Chapter 5
) may be useful for
discriminating hydrocarbon sand in Class
I situations.
Figure 7.17
shows a modelled example.
Amplitude changes are subtle on near and far angles
(
Figs. 7.17a
,
b
; however, a fluid projection (
Figs.
7.17c
,
d
) emphasises the presence of hydrocarbon.
With real data the result may not be so clear owing
to the limited differential AVO effect and the noise-
prone nature of the gradient estimation, but it is a
technique worth trying.
One pitfall in Class I AVO analysis is the misin-
terpretation of critical angle energy (
Chapter 2
). The
processor needs to be careful to remove all traces of
critical angle effects in order that angle stacks and
gradient estimations are valid for interpretation.
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