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 If you don’t have checkshots > may have to estimate velocity from sonic log to get a default TWT time shift Generally callibrated using check shots. If not, then need replacement velocity. If not estimate from sonic

TWT = (Start Sonic – KB Eleb)/(2xReplacement Velocity) Then insert Well curve

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

Zoom in AVO Gas

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

Water Gas Oil

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.