Geology 554 - Interpretation Project

Big Injun Sand, Trenton/Black River Plays, Central Appalachian Basin, WV

Lab Exercise- Part 3

• Fault Polygons and Faulted Horizon Grids • Map Math • Contouring time Grids • Reflection Amplitude Grids • Extraction of reflection travel time data for analysis

in PsiPlot or Excel • Summary of tasks with report guidelines

Team effort on these interpretation exercises is encouraged. This will allow you to interpret a greater number of horizons and share insights into the interpretation of the area. Wilson (2005)

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Digitizing Fault Polygons Last week you digitized the acoustic basement reflector. The basement reflectors terminate at various faults identified in your interpretation (Margin and 2nd Fault, for example). At this point double click the basement horizon in your project tree and check your map for consistency of fault locations (note the locations of fault gaps and + signs that identify the fault location. In the map below check your interpretation across the Margin Fault from Line 2 to Line 3. You may want to adjust your fault locations so that the Margin fault can pass smoothly between lines 2 and 3.

It may prove difficult to get a completely satisfactory solution to the fault plane location, but put some time into making your picks as consistent as possible.

Once you’re satisfied with your fault picks, we have to surround the faults by fault polygons so that the gridding operation will not attempt to grid across faults but only through areas between faults. To insert fault polygons, right click on your basement horizon map; click on edit fault polygons (see below).

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You may have to right click on the map once again, click on edit fault polygons and then click on digitize to activate the polygon editing tool.

Once Digitize has been selected, click on the D key. This will bring up the following window. Name the fault polygon you are going to digitize (Margin, for example),

and then digitize a fault polygon that encircles the fault gaps. The mouse arrow will be replaced by a +D. Simply click on points that enclose the fault gaps (see next page).

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You may find it helpful to zoom in on your map so that you have a more detailed view of the fault gaps. The digitization works much like it does with faults and horizons. A rubber band remains attached to the mouse tip as you pull the polygon through various parts of the map.

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I’ve incorporated some additional faults into my interpretation. The fault polygons highlighting each fault

are shown in the figure below.

Note: If your fault polygons don’t appear, go into your project tree and check the box next to unassociated faults. This should bring up your fault polygon(s). Then double click and associate the polygon with the appropriate fault in your list.

Each of you should decide what they think are potential faults and on how faults correlate from one line to the next. Feel free to work together or in groups. I have chosen not to correlate the large 2nd fault out observed on Line 6 with the large offset fault observed on the western end of Line 5. How did you decide to interpret this and why? Instead, I have interpreted additional faults, some with minor offsets that may be possible extensions of faults which have a more northerly strike and are not observed along line 6. Fault Polygon Manager Access the fault polygon management window by right clicking on your map and selecting it from the drop down list. Under display, fill color based on associated fault surface to have the fill color match the fault color. If you do not do this the fill will take on the single color you specify.

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Grid the Basement Horizon Once your fault polygons are in place, you can grid your basement surface.

Gridded basement horizon. Contours are discontinuous across faults except when offsets are small.

The fault interpretation I’ve made above is just that – an interpretation. The location of the deep blue – larger travel time – region northwest of the 2nd Fault on Line 6 and the 5th Fault on Line 5 suggest they may actually be connected. On the other hand, the intervening fault blocks could rotate down to the northeast, or there could be a northwest striking cross fault through the middle of the area across which fault blocks are decoupled. Grid Math As part of the exploration project, we would like to explore the variations in travel time between horizons and explore their meaning in terms of either syndepositional growth or movement of structures or post depositional thickening by detachment. The time maps (time grids) for the Devonian Huron Shale and Big Injun Sandstone reflection events are shown side-by-side below.

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Big Injun Time Map Huron Time Map

Note the horizon grids reveal a major synclinal trough that plunges down to the northeast. The maps shown above use the Standard KINGDOM Palette. CLB color bar. To create a map showing travel time differences, go to Tools, Calculators, Math on two Maps as shown below.

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In the following window there are several parameters to specify. You could choose to compute the differences between two time horizons or two time grids. One might consider computing the difference between horizons and then computing the grid separately for the time-difference.

In the window above, I’ve selected the Huron time grid as the first variable and the Big Injun time grid as the second. A variety of math operations and functions can be applied to the maps. Subtract the Big Injun grid from the Huron Grid by selecting the – sign from the button bar at right. You’ll also have to give a name to your map and select a color. Click Compute when finished. The time-difference grid will come up in a separate view window.

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Contouring Time Grids Generate a time-structure contour map by selecting Contour, Select (see dialog window below). Select the grid (or horizon). If you are contouring a horizon, you will have to specify the data type (amplitude or time).

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Check your contour parameters window and select an appropriate contour interval.

If you don’t check the Parameters, the Contour Parameters dialog box will automatically appear. You can also change the contour parameters by clicking on the Set Contour Parameters Icon (below) to see what the effect is.

You can check the effect of various parameter selections by leaving the contour parameters window active and selecting Apply. Your result may appear similar to that shown below.

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Time contour of the Huron grid.

Below, time contours are shown for the Big Injun-to-Huron Grid.

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Gridding Reflection Amplitudes

Increases or decreases in reflection amplitude indicate some change in the physical properties of the reservoir. Those changes could be associated with reservoir facies changes that result in velocity and density changes. The change in amplitude could also occur in response to thickening or thinning of the reservoir interval, to porosity changes, or to an increased concentration of natural gas. In the Gulf Coast and elsewhere in Plio-Pleistocene age reservoirs, high amplitude anomalies are considered an indicator of increased gas concentrations within the reservoir interval. These anomalies are often referred to as DHIs (direct hydrocarbon indicators). DHI analysis has become quite sophisticated in recent years and now incorporates evaluation of the amplitude variation with offset and angle in CMP (common midpoint) gathers. Also, note that in the present setting, the Big Injun Sandstone is an oil reservoir. The strata are much older and therefore more compacted and lithified than are the sediments deposited on active margins. DHI analysis is not common in the Appalachians Construction of an amplitude grid is identical to the construction of reflection travel time grids (see previous exercise). Extraction of reflection travel time data for analysis in PsiPlot or Excel To export time and amplitude values for digitized reflection events, click on Horizons > Export > Horizons. That will bring up the following menu.

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In the above dialog window, I have selected the output horizon (the Huron), and in the surveys list I have selected only Line 6. Specify Landmark Time as the output format. The start and end shot points could be specified, but default to the entire line. Click OK. A save window dialog will come up

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Save your file. You can then bring the file up in PsiPlot or Excel for display. The column format of the Landmark Time output format is shown below

An XY plot of the data looks like that shown below.

ShotPoint

100 150 200 250 300 350 400 450

Line6T.dat680

685690

695700

705710

715720

Tim

e (m

s)

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Consider the expanded time view of Huron travel time variations as shown below.

This utility can be used to examine travel time variations, travel time differences, amplitude variations, etc in more detail. Part 3: Assignment 1) Complete your horizon grids and isochore maps. If you are working together with others in as a team, some duplication of effort should be incorporated into individual tasks to allow for comparison and discussion of interpretations particularly where there is overlap between individual efforts (e.g. the bottom of one team member’s isochore map may be the top of another’s). 2) Explore the geologic history of the area and its possible relationship to the distribution of oil production in the Big Injun or perhaps a large gas reservoir in the deeper Trenton & Black River intervals. Familiarize yourself with the additional capabilities of the software discussed in today’s lab and take advantage of various capabilities to explore the geologic history of the

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area and its possible relationship to the distribution of oil production in the Big Injun or perhaps a large gas reservoir in the deeper Trenton & Black River intervals.

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Geol 554 Interpretation Project Summary Activities and Report

Below, is a summary of the interpretation project tasks noted at the end of the in- class lab exercises. General report guidelines follow. Part 1: Initial interpretation tasks: 1) Interpret faults in the deeper part of the section and bring your interpreted sections to class. We will discuss and compare basement fault interpretations as a group. 2) Contour the 2-way reflection times to the Big Injun reflection event at about 0.3 seconds throughout the area. Bring your contour map to class next week for discussion. 3) As noted above, oil is produced from the Big Injun sandstone. Where do you think oil production in this formation would be best? Identify a well location that you think would offer you the best chances for immediate promotion. Part 2: Assignment: 1) Individuals should complete the interpretation and picking of at least 2 prominent reflection events in addition to the Greenbrier/Big Injun and basement event picked in class today. 2) Grid each horizon and contour it. Part 3: Assignment 1) Complete your horizon grids and isochore maps (at least two). 2) Explore the geologic history of the area and its possible relationship to the distribution of oil production in the Big Injun or perhaps a large gas reservoir in the deeper Trenton & Black River intervals. In summary, you should interpret a minimum of 4 horizons, two of which include the Big Injun reflection event and the Basement reflector. You need to identify and correlate faults that offset the lower Cambrian reflection events. Identify and interpret basement faults. Construct fault polygons before you grid the Basement surface or other deeper, faulted reflection event. The isochore maps are maps of the interval travel time differences between two reflection events. Isochron has a more general usage, and can be used to describe lines of equal reflection times, equal time differences, or equal delay times. In this project, you are looking at time differences to help you gain some insight into the reactivation history of the deeper basement faults. As noted in class, the history of movement along these faults is a dynamic one. Their initial development occurred during a Cambrian episode of failed rifting, but were later reactivated during the closing of Iapetus and may represent minor deformation events associated with the Taconic, Acadian and early Alleghenian orogenies. Look at how movement on the deeper faults

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vary by constructing at least two isochore maps and examining the patterns of thickening and thinning of strata relative to underlying fault complex. With regard to exploration ideas, for this project, think in terms of structure - folds, faults, and fracture zones. There are a variety of possible plays in this area including the Big Injun oil bearing sands, the overlying Big Lime (or Greenbrier Ls.), fractured Devonian shale reservoirs in the Huron (event at 0.7 seconds identified and gridded in class Thursday), Onondaga plays ( at about 0.9 seconds, see your reference section), as well as fractured reservoirs in the deeper Trenton and Black River intervals (again, refer to the reference section). To integrate your interpretation efforts, prepare a report that discusses and illustrates your work and your interpretation of these seismic lines, to include: ABSTRACT: Provide a brief (approximately 200 word) description of the exercise and outcome. INTRODUCTION: Describe what you are trying to do and why … for example: to define the major structures in the prospect area, look for possible traps, and understand the geologic history of the area and how that may have affected things like fluid migration and reservoir development. DISCUSSION: Present your observations and how you would tie the different data sets together. Make screen captures (View> Copy Active Window) and place in figures to illustrate your interpretations SUMMARY: Briefly state the main conclusions of your analysis. REFERENCES: If you have looked into any references as a part of your work, they should be noted in the text and properly cited. I realize you all bring different backgrounds and levels of experience and perspectives to an effort such as this. My main objective is to have you demonstrate in your report that you have thoroughly analyzed the data using Kingdom Suite analysis tools as described and illustrated in the three lab exercises. Given that we will have an exam on the 11th, let’s make October 18th the deadline for this report. This will give us plenty of time to discuss any problems with the software that may arise in the next week or so.

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