ReservoirSeismic

advertisement
5. AVO-AVA
• Introduction
• Weak-contrast, short spread
approximation for reflection coefficient
• Information content
• Classification
• Tuning effect
• Examples
Introduction
• AVO (amplitude versus offset)
• AVA (amplitude versus angle)
The link between AVO and AVA is ray
traycing in overburden.
Introduction
• Seismic lithology is the process by which rock
properties such as lithology, porosity and pore
fluid content are determined by analysis of
seismic and other data.
• Ideally, seismic data would uniquely determine
velocity, attenuation, and anisotropy for P-waves
and S-waves as functions of angle.
• AVO has the advantage of being applicable to
many conventional seismic data sets without the
requirements of prohibitive acquisition,
processing, and analysis costs.
Weak-contrast, short spread
approximation for reflection coefficient
rPP  R  0   G sin 2    K sin 4 
rPS  B sin   C sin 3 
(5.1)
P-wave: R(0) is intercept, G is gradient and K is curvature
C-wave: B is intercept and C is curvature
R  0 
1  1 

2 
2 
1 
 2  
 
G
2 2 
2
2 
  
 
1 
K
2 
  1 
B2

  2 
(5.2)
Information content
• Anomalously low Vp/Vs ratios caused by
hydrocarbons produce anomalous AVO
response
• However, Vp/Vs ratios can not be uniquely
inverted from AVO data alone
Vp versus Vs
Castagna, 1993
Vp versus density
Castagna, 1993
Vp/Vs versus Vp
Figure 5.1. Vp/Vs ratio versus Vp for different lithology
Rss versus Rpp
Figure 5.2. S to P reflection coefficients for different lithology
Gas-brine properties distributions
Figure 5.3. Seismic properties for gas/brine sands
Some conclusions
• For large negative P-wave reflection coefficients, gassand and brine-sand reflection coefficients are distinct
for all shale velocities. The lower the shale velocity, the
greater the separation
• For small P-wave reflection coefficients, gas-sand and
brine-sand reflection coefficients are well separated only
for low shale velocities and only if the shale velocity is
approximately known
• For large positive P-wave reflection coefficients, gassand and brine-sand reflection coefficients are well
separated only for the lowest shale velocities and only if
shale velocity is approximately known
AVO checklist
• Is the expected rock properties variation sufficient to produce a
detectable AVO anomaly?
• Can the same seismic response result from other earth models?
• If AVO correctly predicts the occurence of hydrocarbons, what are
the chances that the saturation will be commercial?
• Is there sufficient angular coverage for the event of interest?
• How do I know that processing has preserved and isolated the ”true”
relative AVO response?
• What is the seismic data quality?
• Overburden? Processing?
• Does the AVO anomaly conform the structure?
• Do I understand what ”red” on the AVO display really means in
physical terms
AVO misconceptions
Myth
• AVO does not work
• Gas-sand amplitude increases
with offset
• AVO can not be used to detect oil
sands
• AVO does not work in carbonates
• Land AVO is more difficult than
marine AVO
• Vp/Vs is 1.6 for brine sands, 1.8
for dolomites, 1.9 for limestones,
and 2 for shales
• Rp and Rs are readily extracted
from R(0)
Reality
• AVO does work under the right
circumstances
• Gas-sand reflection coefficients
generally become more negative
with increasing of offset.
• High GOR light oil-saturated rocks
may exibit significant AVO
anomalies
• There are some applications
• The marine short-period multiples
are still a problem
• Vp/Vs varies significantly
• Rp and Rs can be extracted from
R(0) and G if Vp/Vs is kbown
Classification
R(0)
G
+
+
+
-
-
+
-
-
Classification
Figure 5.4. For brine-saturated clastic rocks over a limited depth range in a particular locality,
there may be a well-defined relationship between the AVO intercept (A) and the AVO gradient
(B). A variety of reasonable petrophysical assumptions (such as the mudrock trend and
Gardner’s relationship) result in linear A versus B trends, all of which pass through the origin (B =
0 when A = 0). Thus, in a given time window, nonhydrocarbon-bearing clastic rocks often exhibit
a well-defined background trend; deviations from this background are indicative of hydrocarbons
or unusual lithologies.
Classification
Figure 5.5. We propose that the classification of AVO responses should be based on position of
the reflection of interest on an A versus B crossplot. First, the background trend within a given time
and space window must be defined. This can be done with well control if the seismic data are
correctly amplitude calibrated, or with the seismic data itself if care is taken to exclude prospective
hidden hydrocarbon-bearing zones. Top of gas sand reflections then should plot below the
background trend and bottom of gas sand reflections should plot above the trend. We can classify
the gas sand response according to position in the A-B plane of the top of gas sand reflections.
Classification
Tuning
Tuning
Turbidite system example
0,0
R(0)
-0,2
R(0),G,B
B
-0,4
2
RPP=R(0)+Gsin 
RPS=Bsin
-0,6
G
-0,8
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0
Net-to gross ratio
Figure 5.6. AVO attributes versus net-to-gross
Stovas, Landro and Avseth, 2005
Seismic section
Figure 5.7. Seismic section from offshore Brazil
AVO attributes
7300
2700
7400
7500
7600
7700
7800
7900
8000
2700
2800
2800
2900
2900
3000
3000
3100
3100
3200
3200
3300
2700
3300
2700
2800
2800
2900
2900
3000
3000
3100
3100
3200
3200
3300
7300
7400
7500
7600
7700
7800
CDP
Figure 5.8. AVO attributes sections
7900
3300
8000
Inversion of AVO attributes
well
Time, s
2800
A
3000
3200
Time, s
2800
G
3000
3200
0,004
A
0,000
-0,004
G
0,02
0,00
-0,02
0,6
N/G
0,4
0,2
0,0
S
0,2
0,0
Oil content
0,2
0,1
0,0
7300
7400
7500
7600
CMP
7700
7800
7900
8000
Figure 5.9. AVO attributes inversion from the top reservoir
Inversion of AVO attributes (2)
well
Time, s
2800
A
3000
3200
Time, s
2800
G
3000
3200
0,002
A
0,000
-0,002
-0,004
G
0,01
0,00
0,6
N/G
0,4
0,2
0,0
S
0,2
0,0
Oil content
0,2
0,1
0,0
7300
7400
7500
7600
CMP, m
7700
7800
7900
8000
Figure 5.10. AVO attributes inversion from the arbitrary reflection
Download
Related flashcards
Create Flashcards