Course 6: Analyzing AVO/AVA Analysis
Use gassman’s equations to predict velocity
Objective accurately predict changes in amplitude
Open well (your fluid sub well)> save to another name :
file > Save As
Display logs in time : View > Time Model (also
make sure view depth mode is set to ft below KB)
Zoom all
If you don’t have a check shot survey you can use an estimate
of the near surface velocity 9KB elevation of working floor of
the derrik. Time shift is calculated relative to this datum. To
apply time shift>
Edit > Time Alignment Generally callibrated using
check shots. If not, then
need replacement velocity.
If not estimate from sonic
If you don’t have checkshots >
may have to estimate velocity
from sonic log to get a default
TWT time shift
TWT = (Start Sonic – KB Eleb)/(2xReplacement Velocity)
Then
insert
Well
curve
OK
OK
To make visible click Log Scales from the Edit >
Log Scales drop down
Add 3000, 10,000 to
the left and right log
scales > OK
File > Save
Select AVO/AVA Synthetics from the Synthetics drop
down on the Well Editor Window
Select your wavelet and click the save defaults
button
OK
Place your AVO
View > Aspect Ratio Not 1:1
Then View > Select Curves >
Select > OK
Turn off Formation Top Labels, CheckShots and
plotting parameter buttons;
Click on Data Area > Enter
Start and End Times
AVO Gas
Zoom
in
0-4000
4000-8000
8000-12000
File > Save As
Then close
AVO Synthetics – Oil
Select your well >
View > Time Model
Synthetics > AVO/AVA Synthetics
Wavelets tab > define wavelet > OK
Place synthetics
File > Plot > Default Synthetic Layout
View > Not 1:1
View > Select Curves
File > Plot Setup
Remove checks from
Formation Top Table
CheckShot Table and
Plotting Parameters
Click on Data Area >
Start and End Times
OK > OK
AVO Oil
Ready for Wet
Open file> select your well> OK
View > Time Mode
Synthetics > AVO/AVA Synthetics
Specify logs for wet case
Insert into present log display
Modify layout
File > plot > Default layout display
Not 1:1
Rearrange
View > Select Curves
OK
Further customize
File > Plot Setup
Turn off Fm Tops, CheckShots, Plotting
parameters
Click on Data Area button
Specify Start and End times
OK > Plot Setup > OK to Exit
Lower amplitudes with water only suggests that the
presence of hydrocarbons will produce an increase of
amplitude with offset
Close and File > Save As
Gas
Oil
Water
Use model builder to
Select well 5 times
In the Model Builder Select Edit > Default Correlations
Apply > Close
Yes to default correlations will delete any existing correlations
File Save > AVO Cross Section
OK> OK
Create Top and Base correlations
Correlation must intersect sonic and
density curves in each well
Create horizontal T1 and Base correlations
at top and bottom of cross section
Use correlation selector to select T1
Then click across the top of the log
display
Zoom all> then zoom in across base >
Select positions across the base of each log
Zoom all
Model > Create AVO/AVA model
Make sure stretch / squeeze is
selected and then > Done
Rename and enter # of
traces > select Logs to Use
Near Offsets
Select your logs
OK
Use Repeat Traces > Select all wells > and specify repeat traces
Add>
OK
Specify Datum Elevation and Velocity
Specify maximum offset and
any other parameters; then
Save Default >
Generate Model
Again, these should
define the Near Offsets
Maximize
> not 1:1
Change start and End Times and click OK
Turn off plotting parameters and click OK to close the dialog
Then save traces to SEGY
Define output file name (DOS or workstation)
Now Far Offset trace model will be generated in the Model Builder
Model > Create AVO/AVA model in
the menu bar
Leave everything the same accept for
model name and max and min offsets
(at base) >
Generate Model
Model appears >
File > Plot Setup
Data Area Dialog change
start and end times
Uncheck plotting
parameters > OK
Not 1:1
Resize the trace model windows and
arrange next to each other for
comparison
File > Save model to SEGY
File > Save Model to SEGY option not shown in the
on-line course drop down > Keep Clicking to get
Output dialog
Select a save file (far offset
… Save > OK > File close all
In this case model parameters are
not saved
Use well editor to compare traces > load SEGY Traces
Get into Geographix
Data
Modeling
Select Near Offset AVO.sgy
End trace 51 but
only 41
specified?
Insert traces
Then load the far offset traces
End trace to 51 again> OK
Very subtle I couldn’t see it.