AVO and Seismic Attribute Analysis and Pitfalls

Course instructor:   Mangat R. Thapar, Ph.D

 Who should attend?

Interpreters, geophysicists, geologists, technical support personnel, seismic processors, exploration and data processing managers, and data acquisition managers.

Prerequisites:

It is recommended that participants already have an understanding of Seismic Survey Design, Acquisition and Processing. This background can be obtained from SCA’s course on Seismic Survey Design, Acquisition and Processing or equivalent courses.

Course Description:

The understanding and application of AVO and attribute analysis is vital to any seismically driven exploration or development program. Learn from one of the industry’s top geophysicist; Dr. Mangat Thapar. This course is a MUST for all geoscience interpreters. Using Microsoft’s Excel, Mathcad, and VISTA software, Dr. Thapar will demonstrate some of these principles & pitfalls related to AVO and seismic attributes. This course brings out the Do’s and Dont’s, and is beneficial to explorationists using AVO and attributes in all areas.

This course covers the fundamental principles of AVO/AVA and seismic attributes. These principles are: suitable seismic data, preserve amplitude, properly model AVO/AVA and attribute response and match the predicted response with seismic data, and avoid pitfalls. These principles control the success or failure of AVO and seismic attributes and with it the potential success of an exploration program.

(1)           Detailed discussion on how to select appropriate acquisition parameters and processing flow to properly record and preserve AVO and seismic attribute anomalies

(2)           Demonstration and discussion of blocking logs using zonation, mean and variance methods to develop AVO/AVA and attribute models. How to extract interval velocities from stacking velocities through ray-tracing in the presence of dip. Discussion on how to select appropriate wavelet for modeling.

(3)           Prediction of AVO/AVA and attribute responses using properly generated models and comparison with AVO/AVA extracted from seismic data

(4)           Extensive discussion of pitfalls in AVO and seismic attributes and how to avoid them throughout this presentation.

(5)           These concepts are reinforced with a number of real world exercises.

   Exercises are conducted throughout the course to enhance understanding of principles presented, and some of these exercises are:

(a)           Exercises on the tuning thickness effect on AVO, extraction of rock properties (λ μ ρ), AVO interpretation and cross plotting techniques, thin bed thickness estimation from amplitudes and related pitfalls, absorption due to thick reservoir, and many other topics.

(b)           Group discussion and selection of acquisition parameters and processing flow for AVO and attributes.

(c)           Group discussion to develop a risk analysis spreadsheet for AVO which encompasses all factors in acquisition, processing, modeling, and interpretation.

(d)           Group discussion and exercise on how to select appropriate attributes for analysis over a prospect.

Course Content

·                     AVO Concept & related factors

·                     AVO pitfalls and assumptions

·                     Petrophysics and AVO

·                     Rock properties and wave propagation

·                     Hydrocarbon detection using AVO

·                     AVO modeling and processing

·                     Impact of processing on AVO analysis

·                     Techniques for interpretation of AVO

·                     AVO Cross-plotting techniques

·                     Case studies of AVO

·                     Seismic attributes and wave factors

·                     Decon and scaling effect on attributes

·                     Time and Frequency domain attributes

·                     Thickness using amplitude

·                     Thin bed thickness from frequency

·                     Applications of amplitude, energy, and frequency related attributes

·                     Frequency decomposition

·                     Continuous wavelet transforms and short window transform

·                     Seismic inversion techniques, examples and pitfalls

·                     Pitfalls of attributes

Participants will learn how to:

·                     Analyze and apply AVO and seismic attributes correctly and appropriately

·                     Recognize pros and cons of AVO

·                     Determine when AVO is applicable using risk analysis exercise

·                     Select interpretation relevant attributes

·                     Resolve the effect of processing on AVO and seismic attributes

·                     Avoid AVO and attribute pitfalls

Participants are encouraged to bring data examples related to this course.

AVO & Seismic Attributes

Chapter1: Introduction and Overview

Chapter 1 : Introduction and Overview  1-1

Factors Affecting AVO.. 1-2

Plane Waves. 1-2

Partition and Conversion of Incident Energy. 1-2

Acoustic Impedance and Reflection Coefficient 1-4

Elastic Impedance: 1-4

Reflecting Interfaces. 1-5

Reflection and Transmission of P&S Waves. 1-6

Velocities and Reflection Coefficients: 1-8

Liquid/Solid Interface. 1-10

Amplitude Variation with Offset 1-10

AVO Modeling Related to Vp and σ Changes. 1-13

Extracting AVO Response from Seismic Data. 1-17

Petrophysics and AVO.. 1-18

Seismic Data Processing Techniques for AVO.. 1-18

Difficulties in the Application of Rock Physics Principles. 1-19

Limitation of AVO Modeling. 1-19

AVO Analysis. 1-19

Weighted Stacks. 1-19

Weighted Stack Processing: 1-20

AVO Interpretation. 1-22

Gas Detection Using AVO.. 1-22

Monitoring Enhanced Oil Recovery with AVO.. 1-23

Oil Detection with AVO.. 1-23

Lithology Classification with AVO.. 1-23

Porosity Classification with AVO.. 1-24

Land and Marine AVO.. 1-24

Techniques for Interpretation of AVO.. 1-24

Common Offset Stacks. 1-24

V_P/V_S from Elastic Inversion of Seismic Data. 1-25

Multi-Component AVO.. 1-27

AVO Analysis – An Aid for Seismic Interpretation. 1-27

AVO and Seismic Data Processing. 1-27

Future Considerations for AVO.. 1-27

Chapter2: Application of Petrophysics

Chapter 2 : Application of Petrophysics  2-1

Application of Petrophysics to AVO Analysis. 2-2

Introduction. 2-2

Elastic Wave Propagation Parameters. 2-2

Effect of Temperature and Pressure on V_p / V_s 2-4

V_p vs V_S Relations in Different Rocks. 2-4

Full Wave Sonic. 2-7

Vertical Seismic Profiling VSP for S-Waves. 2-8

Velocity and Density Relationships in Different Rocks. 2-9

Velocity in Mixed Rock Types. 2-11

Effect of Pressure on Velocity and Density of Different Lithologies: 2-12

Elastic Parameters of Pore Fluids. 2-12

Oil 2-12

Brine. 2-12

Gas/Brine Sand Coefficient Separation. 2-12

Empirical Formulae for P-Wave and S-Wave Velocities. 2-13

Gassmann’s Equations for Velocities in Porous Media. 2-14

The Assumptions in Gassman’s Equations Are: 2-14

Vp and Vs in Coal 2-15

Attenuation or Absorption. 2-16

Vp and Vs Anisotropy in Rocks. 2-16

Petrophysical Analysis. 2-18

Sand – Shale Vp, Vs and ρ. 2-19

Reflection Amplitudes are Dependent on the Following Key Factors. 2-19

Sands-Shales: P-Wave Velocities and Densities. 2-20

Velocity – Porosity – Shale Content of Sand. 2-20

Chapter 3: Seismic Modeling

Chapter 3 : Seismic Modeling  3-1

(a) Subsurface model:............................................................................................................... 3-2

(i) Cross-section................................................................................................................. 3-2

Block Log. 3-3

VSP Data: 3-3

(ii) Interpreted maps............................................................................................................ 3-3

(b) Data acquisition parameters.................................................................................................. 3-11

(c) Vp, Vs, and ρ...................................................................................................................... 3-11

RMS and Interval Velocities. 3-16

RMS and Stacking Velocities. 3-17

Factors affecting AVO.. 3-20

Reflectivity Variation with Incidence Angle. 3-20

Normal Incidence. 3-20

Approximations to Knott & Zoeppritz Equations. 3-22

Interval Velocity and Angles of Incidence Using Improved Dix’s Method. 3-22

Amplitude Corrections for AVO.. 3-23

Surface Boundary – Reflected PP and PS Waves. 3-23

Spherical Divergence. 3-23

Array Response and Source-Receiver Directivity. 3-25

Seismic Attenuation or Absorption. 3-27

Effects of Transmission Coefficient on AVO.. 3-27

Anisotropic Effects on AVO.. 3-27

Integrated AVO Analysis: 3-29

Amplitude Variations Due to Wave Propagation Effects. 3-32

Transmission Losses in the Overburden and Offset Synthetic Seismogram.. 3-33

Waveform.. 3-35

AVO of a Gas Sand Example. 3-36

AVO Responses Dependent Upon Velocity and Density Relationship. 3-36

Chapter 4: AVO Processing and Pitfalls

Chapter 4 : AVO Processing and Pitfalls  4-1

Data Processing Flow. 4-2

Noise Analysis and Removal 4-2

4-D Amplitude Display. 4-6

Statistical or Data Derived Corrections for Seismic Amplitudes: 4-6

Amplitude Variations Due to Instrument Generated/Related Noise. 4-6

Processing Steps. 4-9

Geometric Spreading Correction. 4-10

F-K Filtering. 4-10

Surface Consistent Deconvolution. 4-11

Surface Consistent Statics. 4-11

Dip Moveout (DMO) Correction. 4-11

NMO Correction. 4-11

Amplitude Smoothing. 4-11

Amplitude with Offset Display (AVO) 4-11

Amplitude with Angle Display (AVA) 4-11

Offsets and Angle of Incidences. 4-11

Processing Flows. 4-19

AVO Analysis. 4-21

Q-Compensation for AVO Slope. 4-21

Approximations for AVO Response. 4-22

AVO and Seismic Modeling: 4-22

Example of AVO: Producing and Non-Producing Bright Spot: 4-25

Pitfalls, Precautions in AVO Processing and Analysis. 4-26

Data Processing. 4-26

Data Acquisition. 4-27

AVO Modeling. 4-27

Chapter 5: Analysis and Interpretation of AVO Data

Chapter 5 : Analysis and Interpretation of AVO Data  5-1

Introduction. 5-2

Pitfalls and Problems in AVO Data Analysis and Interpretation. 5-2

Loss Due to Geometrical Spreading or Spherical Divergence: 5-3

Errors Due to Normal Moveout Correction: 5-3

AVO Modeling. 5-4

Complex Structures and Curvature of the Reflecting Horizon: 5-6

Frequency and Tuning Effects: 5-8

Effects of Frequency, Velocity, Thickness on AVO.. 5-9

AVO –Response  5-9

Gas-Sand  5-9

Water-Gas-Sand  5-9

Water-Sand  5-9

Tight-Sand  5-9

Non-AVO –Response  5-9

Changing Reservoir (Gas-Sand to Tight-Sand) 5-9

Change in Thickness  5-9

Gas-Sand  5-9

Water-Gas-Sand  5-10

Water-Sand  5-10

Tight-Sand  5-10

V_P and V_S Distribution in the Overburden Layers above the Target Horizon: 5-19

AVO Analysis. 5-21

Seismic Modeling. 5-21

Develop 1-D Seismic Models for P and S Waves. 5-21

2-D Modeling. 5-21

AVO Modeling of Gas-Oil-Brine Saturated Rocks. 5-22

Viscoelastic Modeling. 5-22

Interpreted Seismic Events. 5-24

Amplitude Ratio between Target and Reference Layer 5-24

AVO Results. 5-27

Observations and Conclusions. 5-29

Interpretation and Displays of AVO Analysis. 5-29

AVO Maps. 5-29

Bright Spots - AVO.. 5-29

Crossplotting Normal Incidence Reflectivity vs Far Offset Reflectivity. 5-31

AVO Procedure NI vs PR. 5-31

Rotating NI * PR Attribute to Change Class 2 to Class 3 AVO Anomaly. 5-32

Gulf Coast Example of NI vs PR Crossplotting. 5-34

Crossplotting AVO Intercept A and Gradient B. 5-36

AVO Crossplotting and Fluid Factor Calculation. 5-38

Statistical Correlation or AVO Background Trend. 5-38

Crossplotting λ*ρ, μ*ρ and k*ρ. 5-40

Crossplotting Δλ/(λ+2 μ) vs Δμ /(λ+2 μ) 5-43

Important Steps in AVO Analysis. 5-43

Chapter 6: Case Studies of AVO Analysis

Chapter 6 : Case Studies of AVO Analysis  6-1

Failures and Pitfalls of AVO.. 6-2

Case History #1. 6-2

Conclusions: 6-2

Case History #2. 6-2

Conclusions: 6-3

Case History #3. 6-3

Conclusions: 6-3

Case History #4. 6-3

Conclusions: 6-3

Case History #5. 6-5

Conclusions: 6-5

Successful Case Studies of AVO Analysis and Interpretation. 6-5

Case History #1. 6-5

Case History #2. 6-11

Major Observations and Conclusions: 6-13

Case History #3. 6-13

Conclusions and Observations: 6-13

Case History #4. 6-17

Conclusions and Observations: 6-17

Case History #5. 6-19

Practical Lessons in AVO Application. 6-19

Geologic Setting and Prospecting Strategy. 6-19

AVO Model 6-19

Application of AVO Modeling in Exploration. 6-21

Case History #6. 6-23

AVO and Petrophysical Properties. 6-23

Seismic Data Processing for AVO.. 6-25

VSP. 6-25

Interpretation: 6-25

Case History #7. 6-27

Case History #8. 6-28

Case History #9. 6-28

Case History #10. 6-29

Chapter 7: Seismic Attributes

Chapter 7 : Seismic Attributes  7-1

Introduction. 7-2

Seismic wavelet 7-2

Amplitude of Reflections. 7-3

Factors Affecting Amplitude. 7-3

Factors Affecting Velocity of Seismic Waves: 7-5

Density and Pressure. 7-5

Porosity and Saturation. 7-5

Depth, Age, and Lithology. 7-5

Velocity – Depth Curves. 7-5

Seismic Processing Techniques. 7-5

Resulting Amplitude Variation. 7-9

Frequency of Reflections. 7-9

Factors Affecting Frequency. 7-9

Effect of Processing Techniques. 7-11

Resulting Frequency Variation. 7-11

Phase or polarity of the wavelet 7-12

Phases. 7-12

Effect of Processing Techniques. 7-12

Resulting Phase Variation. 7-12

Factors Affecting Phase. 7-15

Types of Seismic Attributes. 7-17

Wavelet Attributes. 7-17

Window Based Attributes. 7-17

Multi-Channel (multi-trace) Attributes. 7-18

Use of Seismic Attributes. 7-19

Seismic modeling. 7-19

Seismic Data Processing. 7-19

Comparison. 7-19

Attribute Case Histories. 7-20

Application of Seismic Attributes. 7-21

Chapter 8: Classification of Seismic Attributes

Chapter 8 : Classification of Seismic Attributes  8-1

Seismic Attributes and Reservoir Rock Properties. 8-2

Amplitude Based Attributes. 8-2

Energy Based Attributes (Time Domain) 8-2

Polarity Based Attributes. 8-2

Frequency Based Attributes. 8-2

Energy Based Attributes (Frequency Domain) 8-2

Phase Based Attributes. 8-3

Absorption Based Attributes. 8-3

Coherency Based Attributes. 8-3

Seismic Attributes Classification for Reservoir Rock Properties. 8-5

Definitions of Seismic Attributes. 8-6

Amplitude Related Attributes. 8-7

Frequency Related Attributes. 8-9

Amplitude Attribute - Window. 8-11

Frequency Attribute - Window. 8-16

Phase Related Attributes. 8-22

Multi-Trace Attributes. 8-22

Chapter 9: Practical Seismic Attributes

Chapter 9 : Practical Seismic Attributes  9-1

Relative True Amplitude (B&W) 9-2

Structure Section (B&W) 9-2

Relative True Amplitude. 9-2

Structure Section and Relative True Amplitude. 9-2

Interval Amplitude between Picked Horizons. 9-2

Frequency. 9-6

Interval Band - Limited Energy. 9-7

Factors Affecting Frequency. 9-8

Frequency Anomalies and Hydrocarbon Saturation. 9-9

Dominant Frequency. 9-9

Frequency Spectra Plots. 9-9

Spectral Energy Ratios. 9-11

Balancing Frequency Ratios. 9-11

Energy. 9-14

Energy Frequency. 9-15

Absorption of Seismic Energy. 9-16

Seismic Inversion. 9-20

Inversion Techniques. 9-20

Quality of Inversion Depends on: 9-21

Observations on Inversion. 9-22

RC Integration to Velocity. 9-30

Acceleration from Seismic Inversion. 9-33

Phase. 9-36

Factors Affecting Phase. 9-36

Wavelet Analysis. 9-36

Chapter 10: Application of Seismic Attributes

Chapter 10 : Application of Seismic Attributes  10-1

Application of Seismic Attributes in Interpretation. 10-2

Amplitude. 10-2

Phase. 10-2

Polarity. 10-3

Frequency. 10-3

Structural Interpretation and Seismic Attributes. 10-5

Stratigraphic Interpretation. 10-8

Oil Production. 10-8

Gas Production. 10-8

3-D Interpretation. 10-13

Thickness Estimation of Thin Beds Using Amplitude. 10-17

Procedure for Thickness Estimation. 10-17

Wavelet Processing. 10-20

Absolute Amplitude and Δt Values. 10-22

Calculation Thicknesses Using Amplitude and Δt 10-23

Calculating Amplitude Response Curve. 10-25

Thickness Estimation Using Dominant Frequency. 10-27

Frequency. 10-28

Dominant Frequency and Dominant Amplitude. 10-28

Discrete Frequency Components. 10-28

Thickness Calculation from Frequency Components. 10-29

Wavelet Character Variation. 10-32

Chapter 11: Data Processing and Pitfalls

Chapter 11 : Data Processing and Pitfalls  11-1

Effect of Processing Techniques on Seismic Attributes. 11-2

Amplitude Recovery. 11-2

Amplitude Scaling (AGC) 11-2

Wavelet Processing. 11-6

Deconvolution. 11-10

Amplitude. 11-10

Frequency. 11-11

Amplitude and Frequency Differences Using Autocorrelation. 11-14

Phase. 11-21

2-D Filtering. 11-21

Static Correction. 11-21

NMO.. 11-21

Low Frequency on Far Offsets. 11-21

DMO and Migration. 11-21

Pitfalls in Using Seismic Attributes. 11-24

Uncertainties in Correlation of Seismic Attribute with Reservoir Properties. 11-24

Chance or Spurious Correlations of Seismic Attributes to Reservoir Properties. 11-24

Number of Well Correlations with Seismic Data. 11-24

Potential Risk Based on Seismic Attributes. 11-24

Seismic Attributes as Purely Empirical Evidence. 11-24

Important Steps in Attribute Analysis. 11-25

Seismic Data Processing Induced/Removed Attribute Anomalies. 11-27

Parameters Affected by Processing. 11-31

Processing Bias. 11-31

Bias to Expected Results. 11-31

Chapter 12: Case Histories

Chapter 12 : Case Histories  12-1

Case Histories. 12-2

Offshore Gulf of Mexico. 12-2

California Oil Field. 12-3

California Gas Field. 12-5

Offshore Louisiana. 12-6

Anadarko Basin Modeling and Seismic Attributes. 12-7

Amplitude Modeling. 12-7

Frequency Modeling. 12-8

Overplotting Attributes. 12-9

Offshore Gulf of Mexico. 12-9

South Texas On-Shore. 12-9

Anadarko Basin. 12-9

California Gas Field. 12-9

NE Wyoming. 12-9

Attributes for Gas. 12-15

Attributes for Gas and Coal 12-15

GOM Example. 12-15

Exercises on AVO and Seismic Attributes: Principles and Applications

AVO and Seismic Attributes: Principles and Applications  3

Exercise 1: Critical Angles. 3

Exercise 2: Raypaths. 3

Exercise 3: Poisson’s Ratio and V_p/V_s 3

Exercise 4: Liquid/Solid Interface. 4

Exercise 5: Pitfalls in AVO.. 4

Exercise 6: Tuning Thickness - Water Sand and Gas Sand. 4

Exercise 7: Seismic Data Processing Techniques for AVO.. 16

Exercise 8: Reflectivity with Offset 16

Exercise 9: Amplitude Corrections for AVO Analysis. 16

Exercise 10: Integrated AVO Analysis and Interpretation: 16

Exercise 11: Noise. 16

Exercise 12: Amplitude Trends. 16

Exercise 13: Rock Properties. 16

Exercise 14: Reflection Amplitudes. 16

Exercise 15: Full Wave Sonic. 16

Exercise 16: Estimating Vs from Vp. 16

Exercise 17: Rock Porosity and Density Relationships. 18

Exercise 18: Porosity – Velocity Relationship. 18

Exercise 19: Density – Velocity Relationship. 18

Exercise 20: λ μ k ρ Extraction from Vp,Vs and ρ. 19

Exercise 21: 2-D Filtering (F-K or Velocity) 19

Exercise 22: Processing and AVO Response. 19

Exercise 23: AVA. 19

Exercise 24: Q-Compensation. 19

Exercise 25: AVO Processing. 19

Exercise 26: Pitfalls. 20

Exercise 27: Normalized Amplitudes. 20

Exercise 28: Target and Reference Horizons. 20

Exercise 29: AVO Response with Depth. 20

Model 1. 20

Model 2. 21

Model 3. 21

Model 4. 21

Exercise 30: Crossplotting Intercept (A) vs Gradient (B) 22

Exercise 31: Rotating NI vs Pr Plots. 22

Exercise 32: Forward Modeling. 22

Exercise 33: Bright and Dim Spots. 22

Exercise 34: Rock Properties. 22

Exercise 35: Thin Beds. 22

Exercise 36: AVO Risk Analysis and Management 22

Exercise 37: Seismic Attributes. 25

Exercise 38: Amplitude Correction. 25

Geometric Spreading. 25

Reflection Transmission - Geometric Spreading – Attenuation. 25

Exercise 39: Peak to Side Lobe Ratio. 26

Exercise 40: Spectrum Bandwidth: 29

Exercise 41: Attenuation. 30

Attenuation of P-Waves. 30

Attenuation of S-Waves. 30

Exercise 42: Attenuation Due to Reservoir Thickness. 30

Exercise 43: Calculate α and Q for a Gas Field. 32

Exercise 44: Thin Beds Thickness Estimation. 33

Exercise 45: Instantaneous Attributes. 35

Exercise 46: Attribute Matching and Selection. 37

Exercise 47: Design Processing Parameters: 50

Elevation Change. 50

Seismic Data Problems: 50

Statics and Fold. 50

60 Hz Pickup. 50

Target Reservoir: 50

Ground Roll 50

Previous Acquisition Parameters: 51

Previous Processing Sequence on Old Data: 52

Modify Processing Flow for AVO and Attributes. 53