Geology Reference
In-Depth Information
a)
b)
Base oil sand
0.2
OWC
0.1
Phi/
S
w
GR
GR V
sh
AI
PR
Synthetic
Gather
R(0)
G
0
Wet
lithologies
-0.1
Top oil sand
-0.2
Intercept
c)
0.2
y
Fluid
projection
angle (
χ
)
0.1
z
0
-0.1
AI
-0.2
Intercept
Red points = oil zone
Blue points = wet sands and shales
Figure 5.58
AVO projections using log data; (a) log section with oil bearing reservoir, calculated synthetic gather and intercept and
gradient traces, (b) AVO crossplot showing the dominant trend of non-reservoir responses, (c) AVO crossplot with calculated fluid
projection angle.
through the wet data that intersects the origin effect-
ively defines the fluid angle (
Fig. 5.58c
). The projec-
tion angle
model after defining the position of the contact.
Seismic sections have been generated using the
Zoeppritz equations at angles of incidence of 10°
and 30° (i.e. notional angles for near and far stacks)
(
Figs. 5.59a
,
b
).
Figures 5.59c
,
d
show linear projec-
tions at fluid and lithology optimised angles
respectively. The sections show that, while the far
angle stack is quite good at showing the fluid
effects, the projection is more effective at subduing
background reflectivity and highlighting the oil
sand. The lithology angle was empirically deter-
mined by inspection of a range of projection angles
for the model.
χ
can be simply calculated (
Fig. 5.58c
):
tan
1
χ ¼
ð
y
=
z
Þ :
ð
5
:
13
Þ
In this case y
¼
0.08 and z
¼
0.2, so
χ ¼
22°, or
θ ¼
39°.
To illustrate the potential impact of AVO pro-
jections in seismic interpretation the results of a 2D
model are shown in
Fig. 5.59
.The2Dmodelwas
created using well data containing a water bearing
sand with the data simply propagated parallel to the
top sand horizon. Oil was substituted into the
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