AVO Attribute Volume Hampson-Russell’s Software

HRS9Houston, Texas2011

Introduction

• AVO attribute volumes allow you to measure a subset of the velocity-corrected gather to determine the zero-offset (intercept) and gradient components of the amplitude at a given time sample.

• The goal is to identify hydrocarbon anomalies in the reservoir. AVO attributes are valuable when used in conjunction with other geophysical and geological inputs for reservoir modeling and characterization.

• The AVO Attribute volumes are stored as SEG-Y files. All of the seismic volumes in Hampson-Russell are, by default, saved under seismic.dir in the project directory.

• The following slides will discuss AVO attribute theories and the workflow that the HRS-9 version of Hampson-Russell software uses.

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Δ=

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Wiggins et al. (1983) algebraically reformulated the Aki-Richards equation. They separated the equation into three reflection terms, each weaker than the previous term:

Wiggins’ Form of the Aki-Richards Equation

This is often called the AVO equation. We will refer to this as the ABC equation.

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Intercept

Gradient

Curvature

AVO Attribute

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Hampson-Russell Software has four types of analysis for AVO attributes.

1. A/B (Two Term Aki-Richards)2. A/B/C (Three Term Aki-Richards)3. Rp/Rs (Two Term Fatti)4. Rp/Rs/Rd (Three Term Fatti)

• AVO attributes extracted from well logs can be different from seismically-extracted attributes because of the offset-dependent tuning and NMO stretch, selected angle range and the approximations in the equations.

Derived Attributes

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The most popular AVO attributes are:

(1) AVO Product : A*B(2) Scaled Poisson’s Ratio Change : aA+bB (3) Scaled S-wave Reflectivity : aA-bB(4) Fluid Factor

• We discuss the Fluid Factor attribute volume in the Fluid Factor Calculation knowledgebase document.

• This attribute works very well if you have a Class 3 anomaly (nomenclature of Rutherford and Williams;1989).

• Forming the product of A and B, we get:

Top of sand : (-A)*(-B) = +ABBase of sand : (+A)*(+B) = +AB

• Based on the observation that a low impedance gas sand encased in shale will have larger negative A and B values than the associated non-pay reflectors (Castagna et al., 1998), A*B will have a positive, classical bright spots response at both top and base.

+A

-A

+B

- B

sin2θ

Derived Attributes AVO Product : A*B

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We normally display the AVO attribute volume in color. The AVO product shows a positive response at the top and base of the clastic reservoir, which can indicate the presence of hydrocarbons. This attribute works well for a class 3 AVO response.

Top

Base

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The AVO sum (A+B) shows a negative response at the top of the reservoir (a decrease in σ) and a positive response at the base (an increase in σ) indicating hydrocarbon-charged sands. This attribute works well for Class 2 and 3 AVO responses (Ross, 2002).

Top

Base

Derived Attributes Scaled Poisson’s Ratio Change : aA+bB

8• a and b from aA+bB are scale factors

The AVO difference (A-B) shows an increase in Shear Impedance at the top of the reservoir, which can be indicative of hydrocarbons. This calculation is usually done with the more accurate Fatti equation.

Top

Base

Derived Attributes Scaled S-wave Reflectivity : aA-bB

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Workflow for Generating AVO Attributes in HRS9

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RMB

• Load and display the gathers in the window. If you post the angle range, you can to determine whether to use the 2-term or the 3-term Aki-Richards equation.

• To check the range of incident angles present in the data, click the or click the right mouse button.

• Under the Color Data Volume, select Incident Angle.

Check the maximum angle of the data

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You need to provide the velocity information in order to calculate the maximum angle of the data. Choose either a Velocity table, a Single log or a SEG-Y volume.

For this example,

1. Choose Single Log.

2. Choose P-wave_gas.

3. Click OK.

[1]

[2]

[3]

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• The Color Data plot will appear showing the calculated angles. In this example, there is about 45 degrees at the zone of interest. This will be enough of an angle range where the A/B/C type of analysis can be used.

To start the Attribute Volume calculation:

1. In the Filter box at the top left hand corner, type the keyword Attribute.

2. Double-click AVO Attribute Volume.

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[1]

[2]

[3]

[4]

1. Fill in the data information.

2. Choose A/B/C (Three-Term Aki-Richards) for the Type of Analysis.

3. Enter the velocity information for angle calculation.

4. Click OK.

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Intercept (A) Gradient (B) Curvature (C)

Above are the A/B/C Attributes (intercept, gradient and curvature). These attribute volumes are created as separate SEG-Y volumes. Rarely do we use these attributes in their single form. We use them more commonly as derived attributes as shown on the following slides.

Derived Attributes : A*B

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• Product (A*B) is the default color data volume displayed when we create the AVO attribute volume.

• As expected from this Class 3 anomaly, a positive bright spot is highlighted at the zone of interest, indicating the presence of gas sands in the reservoir.

• However, if the gas sand reservoir is Class 2, A*B does not easily define the reservoir from the overlying brine sands.

Derived Attributes Scaled Poisson’s Ratio Change : aA+bB

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• Scaled Poisson’s Ratio is another useful attribute for a Class 3 AVO anomaly. It shows a decrease in Poisson’s Ratio at the top of the sand and an increase at the base. This is the expected response for a gas sand.

Converting a Derived Attribute into a Seismic Volume

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[2]

[1]

[3]

All Derived AVO attribute volumes are virtually available in the project. If you want to output the volume as a real SEG-Y volume, use the Copy Tool.

1. Type the keyword Copy under the Filter tab box.2. Double-click on the Copy Volume tool.3. Name the output volume and then select Advanced Options as the Input volume or

choose the Advanced tab.

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[1]

[2]

[3]

[4]

1. Open the Advanced Tab and select AVO Attribute Volume as the input mode.

2. Select A/B/C for the Type of Analysis.

3. Select Product (A*B) as the output. It will be a SEG-Y file.

4. Click OK.

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A*B as a Color Data Volume A*B as an output segy file

By using the Copy tool, you will have the Product (A*B) AVO derived attribute as a real SEG-Y volume on disk. By default, all seismic generated in Hampson-Russell is stored under project directory/seismic.dir. You can use this data in other applications.

Conclusion

• In the Hampson-Russell software, you can analyze several different AVO attributes. This lets you find the best attribute for our data set.

• After you have defined the best AVO Attributes, you can go farther by differentiating between brine-filled rocks and hydrocarbon-bearing formations. To help with this, use various visualization techniques such as cross plotting of AVO intercept (A) and gradient (B). Through communication between visualization tools and the seismic data set, you can draw interest to anomalous regions for further study and possible drilling opportunities.

•You can find more information of cross plotting and other visualization tools on our knowledgebase.

•Knowledgebase: http://www.cggveritas.com/knowledgebase_main.aspx?cid=861

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