Interpreting and Modelling Faults

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Interpreting and Modelling Faults

Tutorial For Minex v5.2 March 2008


Table of Contents Introduction .............................................................................................................................. 3

Requirements ........................................................................................................................... 4

Objectives ................................................................................................................................. 4

Interpreting and Modelling Faults .......................................................................................... 5

Accessing the data ................................................................................................................. 5 Using preliminary seam floor grids to interpret faults ............................................................. 6 Using Seismic sections to interpret faults .............................................................................. 8 Defining fault strings and displacements ............................................................................... 9 Generating the fault block model ......................................................................................... 12 Unfaulting boreholes ............................................................................................................ 13 Correlating and modelling unfaulted borehole seams ......................................................... 16

Summary ................................................................................................................................ 19


Introduction Faulting often complicates the seam modelling process unless the faults are interpreted well and integrated into the seam modelling. Minex accommodates both normal and reverse faults with a set of tools which allow you to model the faults and then use them to shift the borehole data to an ‘unfaulted’ position. Unfaulted borehole seams can then be correlated and modelled just like data without faults. This process covered in the “Correlating and Modelling Borehole Seams” tutorial results in all seams being defined in all holes ready for seam modelling. In this tutorial you will use options in the Unfault and Refault menu to interpret and model faults. To do this you will use digitised fault strings which can be interpreted using data from borehole fault intersections, anomalies on preliminary seam floor or roof grids and depth corrected seismic sections. Your main aim is to correlate and model unfaulted data so that you can easily create gridded seam models which can then be ‘refaulted’.

Figure 1.1 Faulted seam modelling works on the basis of fault block model where borehole data id moved within fault blocks.


Requirements For this tutorial you will need:

• To have a basic understanding of the Minex borehole database. • To understand the concept of gridded seam modelling used to model coal seams

in Minex. • To have the Minex 5 borehole database M5_Project1.B31 loaded with the split

seam intervals file M5_Project1_After_Splitting.A33. This was covered in the tutorial Correlating and Modelling Borehole Seams.

• To understand the difference between normal and reverse faults and the concepts of fault dip and throw.

Objectives In this tutorial you will learn:

• How to use preliminary seam floor grids to interpret faults. • How to display seismic sections and use them to interpret faults. • How to define fault3D strings and displacements. • How to generate a fault block model. • How to unfault boreholes using the fault block model. • How to correlate and model borehole seams using the unfaulted borehole

database.


Interpreting and Modelling Faults

Accessing the data • Borehole database M5_Project1.B31

1. Click on the M5_Project1.B31 file in the Minex 5 Explorer window to open the borehole database.

• Split interval file M5_Project1.B33s

2. Choose the unsplit seam version of the database if prompted. Expand the borehole database files in the Explorer window to check that the seam list includes D, D1 and D2.

Figure 6.2 Expand the borehole database file M5_Project1.B31 in the Explorer window to check that the seam list includes D, D1 and D2.

• Split interval ASCII file M5_Project1_After_Splitting.A33 To ensure that have the corrected seam interval file in the database load the M5_project1_corrected.A33 file via the Borehole DB > Load > Load Seam/Layer ASCII menus.

• Seismic section files P1_Faults.txt and P1_Faults.tif The file is a registration file in a Sirovision form prepared using the File > Import > Convert Seismic XYZ to Sirovision menu.

• Fault export/import file P1_Faults.xml


Using preliminary seam floor grids to interpret faults To review Create grid of D1SF as a means of looking for possible anomalies due to faults. Choose Grid > Compute

Figure 1.3 To compute a singe grid

Figure 1.4 To compute a singe grid

3. Click the Data Summary button to display in the Output window the data that will be used to create the D1SF grid.

-- Summary of Data Accessed -- ------------------------------ Data Statistics : Points Max Min Avg Random Points = 87 33.10 -97.15 -16.41 Summary Total = 87 33.10 -97.15 -16.41

4. Click Ok to save the grid in GRDFILE.GRD.


To use this grid for fault interpretation display it first as contours and look for any anomalies that might be due to faults.

Figure 1.5 Contour plan of D1SF grid showing fault strings and location of section NS-SECT2 As first look at interpreting faults you can import some fault strings which have been created already. These are imported via the SeamModel > Unfault and Refault > Fault Definition menu form. Use the Import Parameters button and select the file P1_Faults.xml. Answer Yes to all the questions and the 3 fault strings F1, F2 and F3 will be displayed in the 3D window.

Figure 1.6 Fault strings can be loaded via the Import Params button on the Fault Definition tab of the Unfaulting and Refaulting menu form.


Using Seismic sections to interpret faults Seismic reflection data is sometimes available to assist in fault interpretation. To input a seismic section and use it for fault interpretation you need a seismic section image file such as P1_Faults.tif and a registration file in Sirovision format such as P1_Faults.txt. To input this pair of files

5. Choose File > Import > Import Sirovision Model. Only the name of the coordinate registration file P1_Faults.txt is required to be entered as the image file (.tif suffix) is accessed automatically provided it has the same prefix name (P1_Faults).

Figure 1.7 Import the seismic section P1_Faults.tif as a Sirovision image using the registration file P1_Faults.txt

6. Click Ok to display the seismic section making sure you have the 3D Design window active.

Figure 1.8 Seismic section P1_Faults.tif is displayed in the 3D Design window along with fault strings and D1SF contours.


Defining fault strings and displacements Having imported the fault strings and faulting parameters from the file P1_Faults.xml you can now use the edit buttons on the Fault Definition menu form to access the dips and throws defined for the three faults F1, F2 and F3. These fault strings can be store in the geometry file as Structure Fault3D strings along with their displacement attributes such as dip and throw.

Figure 1.9 Importing the fault parameter file P1_Faults.xml results in listing F1, F2 and F3 fault3D strings in the editing window of the Fault Definition menu form. You can now access F1 displacement attributes by pressing the Edit Points button:

Figure 1.10 Edit Points dialogue.

7. Click Ok to display the attributes of the F1 string.

Note that the throws and dips only have valid values where the Disc field is ‘cont’. Where this field shows ‘n/a’ the ‘0’ values for Throw and Dip are ‘null’ and will be interpolated by the software from the known values.

Throw and Dip Conventions

-ve throw values indicate a normal fault and positive values are used for reverse faults. The sign of the dip is determined by the direction of the points in the string. Looking along the string from first point to last +ve dip values indicate dipping to the right and –ve values indicate dipping to the left.


Now display the attributes of F2 and F3 in turn to review what throws and dips have been defined.


8. Click Edit Points to display the attributes of the F2 string.


9. Click Edit Points to display the attributes of the F3 string.

Note that all three faults are shown to have negative throws are therefore interpreted as normal faults.


Settings

10. Click Settings to review the fault settings that were imported with the parameter file.

Note that the faulting is set at ‘Vertical shift only’. Now turn off the option ‘Clone Picks intersecting faults’.

Figure 1.13 Fault settings were imported with the parameter file P1_Faults.xml

11. Click Set limits to review the 3D limits that determine the extent in X Y and Z of the fault block model.

Figure 1.14 Edit Limits were imported with the parameter file P1_Faults.xml.


Generating the fault block model By pressing the Generate Fault Blocks button the settings and the displacement attributes of the 3 faults f1, F2 and f3 will be used to create a spatial model of the faults. To visualise this fault block model press the Display Fault Blocks button and the model will appear in the 3D Design window.

Figure 1.15 Display of the fault block model in the 3D Design window. Boreholes, the seismic section and the D1SF contours are also shown above.

Note that for three faults there are four fault blocks. Each block is surrounded by the 3D limit surfaces and fault surfaces which have displacement (throws) values. Having successfully generated the fault block model the next step is to shift the boreholes up or down within each fault block depending on the throws of surrounding faults. This process is called ‘unfaulting’ and results in a new borehole database and set of seam intervals which you need to correlate and model prior to creating gridded seam models.


Unfaulting boreholes

12. Choose the Unfault Boreholes tab to unfault the boreholes.

Figure 1.16 Unfaulting and Refaulting dialogue.

13. Type a name for the new borehole database and then click Unfault database. Make sure the ‘Make Unfaulted Database current’ option is ticked,

14. Type in a name for the new unfaulted database M5_Project1_unfaulted.

15. Click Unfault Database. Display the boreholes on NS_SECT2 cross section before and after unfaulting to see the shift.

Figure 1.17 Borehole cross section NS_SECT2 showing seam correlation before unfaulting.


Figure 1.18 Borehole cross section NS_SECT2 showing seam correlation after unfaulting along with the faults F1, F2 and F3.


If you now plot the unfaulted borehole in 3D you should be able to see the slight shift before and after faulting provided you had the original borehole data displayed.

Figure 1.19 The unfaulted boreholes displayed in 3D along with the fault block model D1SF contours and section NS_SECT2.


Correlating and modelling unfaulted borehole seams By shifting the boreholes up or down within each fault block controlled by the throw values on each fault, you are creating a set of borehole seams which should correlate well using the same tools used in the tutorial “Correlating and Modelling Borehole Seams”. You can refer to that tutorial when you carry out the following bore seam operations on the unfaulted borehole database.

Setting Missing Seams to exist

Figure 1.20 Setting missing seams to exist in the in the whole of each borehole (except in barren holes) in the unfaulted borehole database. Check the report on the output window to see what changes have been made to the borehole seam data.

Running the Missing Seam Interpolator

Figure 1.21 Menu to run the Missing Seam Interpolator on the unfaulted borehole database.


To check the missing seam interpolation plot the seams (Input, Estimated and Interpolated) on section NS_SECT2:

Figure 1.22 Borehole cross section plot for NS_SECT2 show unfaulted borehole seams after missing seam interpolation.

Setting interpolated seam thicknesses to zero.

Figure 1.23 Menu to run the Set Interpolated Seam thicknesses to zero below collar and above total depth in the unfaulted borehole database.


Preparing a seam summary report for the unfaulted boreholes

The Seam Summary report will show you the results to all the previous correlation and bore seam modelling activities that you have carried out. Remember that what you set out to do was define seam intervals for all seams in all holes. You can compare the number of seam intervals for the different class of seam intervals Input, Estimated and Interpolated.

Figure 1.24 Seam data Report. Use the Seam Data Report menu to prepare a seam summary report of the unfaulted seams after correlation. Set thicknesses to zero below collar and above total depth in the unfaulted borehole database. Example extract from the seam summary report: ---- Seam :E1 : Report ---- ----------------------------------------------------- Mean_Value : 68.14 68.54 0.40 Max_Value : 131.41 131.86 0.92 Min_Value : 0.83 1.21 0.00 No. Samples : 87 87 87 ----------------------------------------------------- ---- Seam :E2 : Report ---- ----------------------------------------------------- Mean_Value : 68.66 69.03 0.37 Max_Value : 132.02 132.27 0.69 Min_Value : 1.29 1.71 0.00 No. Samples : 87 87 87 ----------------------------------------------------- ----------------------------------------------------- Mean_Value : 36.33 37.72 1.38 Max_Value : 132.02 132.27 5.96 Min_Value : -69.43 -67.67 0.00 No. Samples : 869 869 869


Exporting the unfaulted seam intervals

Finally output the seam interval file containing the interpolated seam intervals by exporting to a file named M5_Project1_Unfaulted_Zero.A33. Review this file to see which intervals have zero thicknesses and an ‘ I ‘ for Interpolated or ‘E’ for Estimated. This is the database that gridding operations will function on prior to ‘refaulting’. User input (modifications) may be made if desired. An extract of this file is shown below: 1068 -47.124I -45.090I 0A1 2.034I 1068 -43.284I -42.303I 0A2 0.981I 1068 -22.662I -21.929I 0B1U 0.733I 1068 -20.799I -19.300I 0B1 1.499I 1068 -12.188I -11.753I 0C1 0.435I 1068 -7.406I -5.560I 0C2 1.846I 1068 24.690 29.517 -54D1 4.827 1068 29.517 30.800 -7D2 1.283 1068 30.990 31.400 7E1 0.410 1068 31.495 32.000 7E2 0.505 1077 -20.848I -18.993I 0A1 1.854I 1077 -15.992I -14.766I 0A2 1.225I 1077 9.427I 9.427I 0B1U 0.000E 1077 13.217I 13.217I 0B1 0.000E 1077 18.402I 18.402I 0C1 0.000E 1077 20.686I 20.686I 0C2 0.000E 1077 48.520 54.113 -54D1 5.593 1077 55.497I 55.497I -7D2 0.000E 1077-001 0.831 1.211 7E1 0.380 1077-001 1.291 1.711 7E2 0.420

Summary You have now discovered how Minex enables you to interpret and model faults which can be incorporated in the seam model. This has involved you in creating preliminary seam floor grids and displaying seismic sections to assist in interpreting faults. You now have some hands on experience with the Unfaulting process discovering:

• How to define fault3D strings and displacements. • How to generate a fault block model. • How to unfault boreholes using the fault block model and • How to correlate and model borehole seams using the unfaulted borehole

database.

The next step is to use this correlated unfaulted borehole seam interval data to create gridded seam models.

Interpreting and Modelling Faults

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