• Class and Course

    Practical Depth Conversion with Petrel

    Depth conversion (domain conversion) of seismic time interpretations and related data within Petrel is a critical skill set for interpreters. However, there is no single methodology that is optimal for all cases, since the available seismic and geologic control varies in quantity and quality within each project. To impart an effective approach to depth conversion, the first part of this course prioritizes understanding the nature of velocity fields and practical approaches to velocity representation. Next, appropriate (vertical) time-to-depth conversion methods suitable for time migration are presented in case history and exercise form. Single-layer and more sophisticated multi-layer approaches are reviewed along with structural-uncertainty analysis.

    Depth conversion must also embrace the process of database validation. The database QC issues reviewed include: incorrect well locations and deviations, improper assessment of seismic polarity and phase, mis-correlated seismic horizons, and inconsistent formation tops. The issues above compromise the interpretation, introduce distortions in the implied velocity field, and can result in false structures. Database validation is addressed via the formation of synthetic seismograms to confirm horizon correlations and the use of intuitive QCs such as seismic horizon versus well-top cross plots to detect inconsistencies.

    Prestack depth migration is now commonplace, and there is always the need to calibrate the depth volumes with well tops. The same basic QCs and methods used for vertical time-to-depth conversion are used to validate the fidelity of the formation tops and the seismic depth surfaces used for calibration. This is particularly important during anisotropic depth migration where inconsistencies between well control and the seismic interpretation impact the estimation of anisotropic parameters, resulting in a compromised depth-imaging project.

    This course emphasizes the formation of velocity models appropriate for the available data. This is in context to creating Petrel Velocity Models suitable for initializing reservoir characterization employing depth-calibrated seismic inversions and other attribute cubes precisely integrated with the well information. Structural uncertainty analysis is covered using various approaches to provide a critical metric for depth estimation accuracy.

    The course agenda can be shortened if required.

    Day 1

    Module 1:  Overview of Depth Conversion

    Learning Objectives and Importance:    

    • Discuss goals for vertical time-to-depth conversion


    • Accuracies for relative structure, well prognoses, volumetric estimates and reservoir models
    • Database validation QCs
    • Indicators for prestack depth migration (PSDM)


    Module 2:  Sources of Velocity

    Learning Objectives and Importance:    

    • Review sources of velocity information


    • Sonic logs, check shots and VSPs
    • Seismic (refraction and reflection)
    • PSDM Tomography
    • Full Waveform Inversion


    Module 3:  Defining Velocity Types

    Learning Objectives and Importance:    

    • Review definitions and characteristics of velocities for Petrel representation


    • Types of velocities and Petrel Templates
    • Conversion of velocity types (e.g. RMS to Interval)
    • Import and conversion of velocity text files
    • Creating Petrel seismic velocity cubes from text files

    Module 4:  Functional Representation of Velocities

    Learning Objectives and Importance:    

    • Define velocities fields using vertical functions


    • Velocity representation via Time-Depth functions
    • Velocity as a function of time, V0+KT, and basic database QCs
    • Velocity as a function of depth, V0+K(Z-Z0), from well-tie TDRs
    • Petrel Velocity Models with time and depth functions

    Day 2

    Module 5:  Gridded Representation of Velocities

    Learning Objectives and Importance:    

    • Define velocities fields using grids


    • Spatial velocity variations (lateral gradients)
    • Creating an edited PSTM velocity model in Petrel

    Module 6:  Vertical Time-to-Depth Conversion (Basic)

    Learning Objectives and Importance:    

    • Implement well-top calibrated depth conversion using T/Z functions and/or Petrel Velocity Models


    • Simple, intuitive depth conversion (no velocity model)
      • T/Z Function Lookup Table plus Well Adjustment
    • QC methods that define Time Depth Relationships (TDRs)
    • Basic Petrel Velocity Models
      • V0+KT
      • Seismic velocities
      • V0+K(Z-Z0) from well TDRs
    • Time to Depth QCs

    Day 3

    Module 7:  Well and Seismic Data Integration

    Learning Objectives and Importance:    

    • Understand methods for linking well and seismic information in Petrel


    • Sonic and density log editing
    • Check shot loading and QC
    • Establishing seismic data polarity and phase (the wavelet)
    • Creating synthetic seismogram well-tie correlations
      • Image volumes (“wiggle trace”)
      • Inversion volumes (creating P Impedance logs and synthetics)
    • Issues with synthetic ties

    Module 8:  Database Validation

    Learning Objectives and Importance:   

    • Appreciate the need to review and correct the database prior to  incorporating well control


    • Confirm database settings
    • Using basic depth-conversion QCs to encounter data discrepancies
    • Conditioning Surfaces for input to Models

    Day 4

    Module 9:  Vertical Time-to-Depth Conversion (Advanced)

    Learning Objectives and Importance:   

    • Geologic and data-driven modeling considerations
    • Multi-layer Petrel Velocity Models
      • V0 + KT
      • V0 + K(Z-Z0)
      • PSTM Velocities
      • Pseudo Interval Velocities
      • Simple Grid Model with edited sonic logs
      • Hybrid Models (combinations of above)
    • “Simple” thrust models (forming complex models in Petrel)

    Module 10: Petrel Models and Uncertainty Analysis

    Learning Objectives and Importance:   

    • Implement domain conversion and uncertainty analysis with Petrel Velocity and 3D Models


    • Evaluating depth uncertainty
      • Mean and standard deviation workflow
      • Stochastic velocity modeling
      • Hidden-well workflow
    • Impact of structural uncertainty on volumetrics in 3D Models


    Day 5

    Module 11:  Pitfalls of Vertical Depth Conversion

    Learning Objectives and Importance:   

    • Understand accuracy of vertical time-to-depth methods and when they fail


    • QCs and validity of vertical depth conversion
    • Impact of geologic complexity
    • Alternatives to vertical depth conversion (depth migration, image rays, and map migration)


    Module 12:  Calibration of Depth Migration with Well Tops

    Learning Objectives and Importance:   

    • Learn approaches for stable integration of depth-domain seismic (PSDM) with well control


    • Working in the time domain
    • Creating the calibrated time/velocity model (the anisotropic Vz model)
    • Database QC
    • Conversion of time data (seismic/surfaces/faults) to well-top calibrated depth
    • Petrel Z-Z depth calibration (depth stretch only)
    • Depth Uncertainty Analysis

    Geoscientists involved in seismic interpretation and subsequent time-to-depth conversion or well-top calibration of depth-migration and inversion data.

    Attendees will gain an understanding of depth conversion methodologies, QCs for validating the methods employed, and tools for quantitative-uncertainty estimation. They will also learn to:

    • Review types of velocity data, and methods of representation and application in Petrel
    • Investigate the compatibility between well information and seismic data as a database QC
    • Evaluate the polarity and phase (wavelet) of seismic data
    • Create Wiggle and P-impedance synthetic ties to calibrate seismic volumes
    • Perform depth conversion of time-migration data with depth-uncertainty analysis
    • Implement depth calibration of depth-migration data with depth-uncertainty analysis

    Petrel Fundamentals and Petrel Geophysics courses.

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