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Depth Conversion Methods & Pitfalls

  • This is a course for practicing seismic interpreters tasked with creating depth maps and estimating uncertainty for volumetrics and well planning. Time is split equally between teaching and exercises which are designed to illuminate concepts and cement understanding through hands-on application. The course incidentally provides attendees with insights into seismic imaging, tips on mapping & interpretation, and several efficiency enhancing spreadsheets to take away.

  • This is a five-day classroom course where a variety of exercises using Excel and easy-to-use mapping software are interspersed with lecture material including case-studies, recommended workflows, and tips on dealing with problems. A final comprehensive depth conversion exercise consolidates understanding of the workflow.

  • Overview

    Depth conversion is approached as part of the geological-geophysical interpretation process, which starts during seismic time interpretation and ends with maps for STOIIP/GIIP calculations and well planning. Tips and techniques in data QC, practical velocity modelling methods, and recognition of geological and geophysical pitfalls are emphasised. The techniques are equally appropriate for those with sophisticated tools who must understand the ‘black box’, and by those who have to make depth maps using only mapping software and a spreadsheet. Attendees will learn how to optimally use their velocity data, see how greater control on the velocity model can be achieved by incorporating geological understanding, and appreciate the importance of their choice of method for tying maps to wells. A wide range of experience will be catered for in the course which is biased towards worked examples. Attendees will leave with the skills necessary to independently undertake depth conversions, assess sensitivity and quantify depth uncertainty.

    Course content and learning outcomes

    Introduction

    • Understand the relevance of depth conversion in the era of Pre Stack Depth Migration
    • Appreciate the impact of overburden velocity on reservoir depth structure

    Geological factors influencing velocity

    • Learn the intrinsic properties of rocks which influence their velocity and anisotropy
    • Understand how diagenesis, overpressure and fluid fill modify these properties

    Sources of velocity data and their problems

    • Know the different sources of data providing a measure of velocity
    • Understand the limitations of these data and know checks to help ensure data integrity
    • Be aware of seismic velocity origins and velocity terminology

    Synthetic seismograms and time-depth relationships

    • Understand how interpretation systems use time-depth data and how this influences calculated velocities
    • Learn how to optimally calibrate logs and tie synthetic seismograms for use in depth conversion – to stretch/squeeze or not?

    Analysing velocity data

    • Appreciate the importance of looking at all of the data, including logs and seismic velocities
    • Learn data analysis techniques to determine the best approach to velocity modelling and depth conversion
    • Understand the importance of scale in displaying quantitative data, and learn tips to do it right

    Velocity model building methods

    • Understand when one layer models (e.g. t-d polynomials, Vavg maps) can and can’t be used
    • Appreciate the benefits of layering even with simple velocity models
    • Know how to implement common velocity models, including linear velocity functions (‘Vok’)
    • Learn techniques to calibrate and condition seismic velocity
    • Reveal the hidden limitations of less standard velocity models (isochron-isochore etc)

    The process of depth conversion

    • Be aware of generic workflows for depth conversion
    • Take away velocity analysis check-lists and good depth conversion model recording protocols

    Tying to wells

    • Learn strategies for tying maps to wells, and when not to use restricted radius tying
    • Understand why tying in depth, velocity or time domains is different, and how to choose
    • Appreciate the implications and value of mapping Vo, learn how to do it, and when not to
    • Know when it is best not to tie

    Dealing with geophysical pitfalls

    • Understand the basics of time migration and statics
    • Be aware of the reasons why the seismic image is not a vertical scaled version of the real earth
    • Understand the impact of channels and other shallow anomalies on seismic reflection times and how they limit the accuracy of seismic velocities
    • Be aware that imaging limitations can cause problems with layered depth conversion models

    Depth domain seismic

    • Be aware of the benefits and limitations of tomographic velocity inversion
    • Understand how anisotropy is used in PSDM
    • Learn to evaluate the quality of PSDM velocity models and so believability of PSDM depths
    • Adapt good depth conversion practice to tie depth domain seismic horizon to wells

    Quality control: tips and techniques

    • Learn techniques to QC the velocity model through ‘time conversion’
    • Be aware of quick and effective technique for QC’ing others’ work

    Quantifying uncertainty

    • Understand the sources of depth error in the time and velocity domains and the concept of systematic and random errors
    • Learn techniques to analyse errors and to quantify them

    Dealing with geological pitfalls

    • Understand that, even when properly implemented, there are times when linear velocity modelling methods will not work (in the presence of uplift, hydrocarbons, deep sea floor channels etc)
    • Learn solutions or work-arounds for when the recommended techniques need modification

  • Learning outcomes:

    1. Understand the geological factors influencing velocity and be familiar with sources of velocity data
    2. Interrogate velocity data to determine best approach to depth conversion
    3. Construct geologically reasonable velocity models and depth convert using well data derived velocities or calibrated seismic velocities
    4. Create depth maps using appropriate well tie techniques
    5. Adapt velocity function techniques (Vok) when geological factors (water depth, uplift etc) lead them to produce incorrect results
    6. Recognise when seismic imaging limitations mean that vertical stretch depth conversion is inappropriate and seismic velocities are compromised
    7. QC PSDM velocities and determine whether PSDM interpretation in depth is valid
    8. Determine the sensitivity of depth structure to velocity models and well tying
    9. Quantify depth uncertainty and understand its origins

  • Who should attend?

    Geologists, Geophysicists, and Technical Support staff engaged in producing depth maps from seismic interpretations, as well as technical management and other subsurface professionals involved in evaluating and using depth mapping results.

    The experience of attendees on the course has ranged from less than 1 to more than 30 years. Junior geoscientists gain awareness and practical experience while more senior geoscientists gain clarity and in-depth knowledge of advanced techniques from the course.  

    Experience required?

    A basic understanding of geology, structural interpretation and the seismic method is required. Experience in mapping and seismic interpretation will be a significant advantage.

  • The course is delivered by Alan Atkinson. Alan has over thirty years of exploration, field development, and production geoscience experience with operators in West Africa, South Asia and the North Sea.

    Specialising in the application of high end geophysics to structural and stratigraphic seismic interpretation, his areas of expertise are reservoir characterisation (using pre-stack inversion, AVO analysis) and velocity modelling (pre-stack depth migration and depth conversion).

    He started his career working the North Sea with Phillips Petroleum and Amerada Hess in London, and subsequently moved to CNR International, working Angola and Côte d’Ivoire. He then moved to Cairn Energy, living and working in India, and becoming Chief Geophysicist during their successful Rajasthan campaign. Here, he supervised several significant seismic acquisition and processing programmes, and managed a geophysical studies team undertaking, amongst other projects, AVO modelling work and pioneering 4D studies on India’s largest onshore fields.

    He is now a partner in Rockflow Resources Ltd, a specialist technical and management consultancy serving the international petroleum industry, primarily advising on reserve evaluations for a variety of clients worldwide and providing services as an Expert Witness in oil and gas disputes. He is also an honorary lecturer at Imperial College London, contributing to the Petroleum Geoscience MSc.

    Alan devised the ‘Depth Conversion Methods & Pitfalls’ course in 2009 and ‘Depth Conversion Methods & Petrel Workflows’ two years later. The courses have been delivered over 50 times to audiences across the world.

  • Oil companies, service companies who are interested in running the course in-house should contact Alan Atkinson, the course creator and presenter. Individuals may also contact Rockflow to enquire about possibilities for participating in a course.

    The Depth Conversion methods & Pitfalls course is available through the RPS Group training organisation. Members of their Nautilus Training Alliance (NTA) should contact RPS directly in order to book places, or to arrange in-house training.

     

    Oil companies, service companies who are not members of RPS’ NTA but who are interested in running the course should contact Alan Atkinson, the course creator and presenter, at Rockflow to enquire about in-house courses. Individuals may also contact Rockflow to enquire about possibilities for participating in a course.

  • “Provides a comprehensive overview of depth conversion methodology, clearly delivered and progressively consolidated throughout progress of the course”

    Origin Geophysicist (25 years experience)

    “Where Alan’s course stands out is his emphasis on making the depth conversion defendable; namely choosing the right way to represent your geology whilst not building in unnecessary complications”

    Conoco Phillips Interpreter

    “I have been highly impressed by Alan’s course. Not only is Alan a superb teacher but he has put in in an enormous mountain of effort in building the spreadsheets which are keys to understanding in this subject area”

    Tullow Lead Geoscientist (14 years experience)

    “A really great course. I’ve learned a lot of new stuff that is very practical and applicable in my job”

    Wintershall Geologist (1 year experience)

    “Alan Atkinson has that rare skill to turn complex concepts into (almost) obvious matters”

    GDF Suez Geophysicist (6 years experience)

    “Depth conversion is a tough area to produce a course for…Alan has produced a course which strikes an excellent balance between the theoretical and practical”

    BG Interpreter (17 years experience)

    “I think this may be the best Nautilus classroom course I have had. The instructor manual and exercises were great”

    Marathon Oil Senior Geophysicist (26 years experience)

  • Course Information (PDF)

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