Geology Reference
In-Depth Information
a)
Water
d)
Water
Shale
Shale
PR
Porosity
PR
Porosity
Quartz
Quartz
Gas sand
AI
AI
b)
e)
+
+
Rc
Rc
Sin
2
Sin
2
Shale/brine sand
responses
-
-
Shale/gas sand
response
c)
f)
Sin
2
Sin
2
Figure 5.11
Effect of porosity and fluid fill in sandstones; (a) AI vs PR showing reference sand (blue), high porosity sand (green) and shale, (b)
AVO plot showing top sand responses of water bearing sands, (c) variation of amplitude ratio (i.e. green/blue in (b)) with respect to sin
2
θ, (d) AI
vs
plot showing the reference sand (blue) with gas (red), (e) AVO plot including the top reference sand with gas, (f) variation of amplitude
ratio (i.e. red/blue in (e)) with respect to sin
2
PR
. Note how with increasing angle the amplitude difference between the reference sand and the
high porosity brine sand decreases whereas with gas the difference increases.
θ
that most of
the gas effect occurs with the first
effectively related to the way water and gas are mixed
and this is in turn depends on the pressure and
temperature (
Chapter 8
).
Porosity is an important parameter in the mag-
nitude of expected AVO anomalies.
Figure 5.14
shows an AVO crossplot with data from two sands
of different porosity with the same overlying shale.
Both brine filled and gas filled points are shown for
each case. It is evident that the magnitude of the
separation between brine and gas points is much
less in the case of
5%
10% of added gas. In terms of the AVO response
(
Fig. 5.13d
) the low gas saturation case has a large
magnitude Class III signature. Tuning of this
response would give similar amplitudes to the 30%
water saturation response. In contrast to the gas
responses, the oil responses are more linear.
It should be noted that this
-
effect is only
significant at relatively low pressure and temperature
(Han and Batzle,
2002
). The distinctive convex down-
ward shape in the saturation vs velocity plot
'
fizz gas
'
is
the deep (compacted) 18%
66
