Geology Reference
In-Depth Information
+
porosity sand. Accordingly, it is potentially much
more difficult to use AVO analysis techniques with
low-porosity rocks. Inevitably there will be an AVO
floor, i.e. a point below which in practice it is not
possible to confidently use AVO techniques. It is
often the case in siliclastic environments that AVO
analysis becomes increasingly more difficult below a
porosity of around 15%.
It is generally held that AVO techniques are less
applicable in carbonate reservoirs. In carbonates min-
eralogy often has a more dominant role in controlling
elastic properties. This has the effect of limiting the
relative contribution of the fluid modulus to the over-
all rock modulus (e.g. Wang,
1997b
). The magnitude
of porosity change can also be substantially greater
than in siliclastic settings so that deciphering porosity
changes from fluid fill effects can be very difficult.
However, many carbonate AVO models display clear
differential AVO effects related to varying fluid fill
G
R(0)
-
+
Background
(Porosity)
trend
AVO Anomaly
-
Figure 5.12
An AVO anomaly defined on the AVO crossplot.
a)
2600
2400
2200
Gas
Oil
2000
1800
1600
d)
0
0.2
0.4
0.6
0.8
1
0.3
Water saturation
0.2
b)
Wet sand
30% Sw 70% gas
95% Sw 5% gas
30% Sw 70% oil
95% Sw 5% oil
5000
0.1
0
4000
Gas
Oil
-0.1
3000
-0.2
2000
0
0.2
0.4
0.6
0.8
1
-0.3
-0.3 -0.2 -0.1
Water saturation
0
0.1
0.2
0.3
c)
Intercept
0.5
0.4
0.3
Gas
Oil
0.2
0.1
0
0
0.2
0.4
0.6
0.8
1
Water saturation
Figure 5.13
The effect of gas saturation on the elastic properties of a high porosity unconsolidated sandstone, (a) S
w
vs V
p
, (b) S
w
vs AI, (c) S
w
vs PR, (d) intercept vs. gradient crossplot.
67
