Basics of Seismic Interpretation

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  • 1Plan for today: Discussion Group #1 Pitfalls; discussion group #2 after spring break (no class next week)Basics of Seismic Interpretation (short lecture)Lab #2 part one, faults exercise (begin together, finish on your own just turn in the final map)Synthetics Tutorial TKS (Kingdom Suite)Lab #2 part two, synthetics tie

    What do we mean by interpretation?

    1) Geophysical interpretation

    - Tracing and mapping reflectors

    Requires understanding of velocity, geometric, and processing pitfalls

    True (but still approximate) cross-sections require time-to-depth conversion and migration

    Basics of Seismic Interpretation

  • 2- Identifying reflections

    - Structural mapping

    - Tying to surface outcrop or drilled subcrop

    - pattern recognition seismic facies and sequence strat

    - Interpreting the entire stratigraphic section/data volume

    - Inferring the geologic history

    * Consider scale: regional vs prospect mapping.Requires knowledge of what is geologically possible and what is geologically probable in a given area.

    Integration of entire geophysical/geological database

    Requires imagination

    2) Geologic interpretation

    In practice you cant 1) draw a good map

    until you have 2) identified the reflections

    but you cant properly identify the reflections until you have

    3) inferred the geologic history

    which you cant do until youve

    2) guessed at what the reflectors are

    and that is difficult unless youve already

    1) mapped them

    Interpretation is an iterative process

  • 31) have a model in mind2) interpret some data (be brave and use erasers!)3) draw some contours4) discover the model is wrong, and5) go back and redraw.

    So, be prepared to :

    Basic workflow

    1) Data validation.

    2) Preliminary interpretation.

    3) Choose the significant markers you want to map.

    4) Pick a section, typically by coloring a trough for each marker (mark faults).

    5) Tie to, and pick, the other sections (tie faults).

    6) Check line ties and loop closures.

    7) Post times to a map.

  • 41) Data validation

    Inspect the sections and test data (if available) for proper processing and for any lateral or vertical change of processing.

    2) Preliminary interpretation

    Inspect the sections for geological plausibility. Preliminary geological inspection.

    Begin to work out fault pattern, sketch on map.

    3) Choose the significant markers you want to map.

    -Choose a reflector that has good continuity and/or character, and that has some significance.

    -There may not be a good reflectorin which case you may want to trace a weak event instead, or choose a strong event as a proxy (phantom horizon). Very risky!

    -A fairly thorough interpretation will probably include at least three horizons

    -Regional mapping: want to be able to correlate across a wide area. If possible, choose major sequence boundaries.

    -Name the reflector/horizon.

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  • 63) Choose significant markers continued (subsurface ties).Ways to identify a reflection assuming we have depth control from a well penetration:

    * Stacking velocities* Sonic log (also 1-way time)

    - watch datum, cumulative error, validity check with seismic data

    * Synthetic seismogram- consider polarity and frequency spectrum, beware of forcing correlations

    * Velocity survey (check shots; 1-way time)

    Remember: a dipping reflection on unmigrated data will not tie directly with a well drilled right on the profile. Normally, you tie to off-line wells along strike or along plunge of a structure.

    Well ties: migration

  • 7Well ties contd

    Tie wells off-line

  • 8L 4 - Well Log Data Courtesy of ExxonMobil

    Lithology Logs Gamma Ray

    a scintillation detector (similar to a Geiger counter) that measures the natural radiation from a formation

    SP (spontaneous potential) a measurement vs depth of the

    potential difference between the voltage in the wellbore and an electrode on the surface

    For both logs:Deflections to the right = ShaleDeflections to the left = Sand

    Shal

    eSa

    nd

    Sd

    Gamma Ray

    ShaleBaseline

  • 9Resistivity Logs High deep resistivity means:

    Hydrocarbons Tight streaks (low porosity)

    Low deep resistivity means: Shale Wet sand

    Separation between resistivities means: The formation fluid is different from the

    drilling fluid The formation is permeable to the

    drilling fluid

    Deep, Medium, and Shallow refers to how far into the formation the resistivity is reading (4 ft, 2 ft, few in)

    ILD (deep)

    MSFL

    SFL

    Formation Fluiddifferent fromDrilling Fluid

    Formation Fluidsimilar to

    Drilling Fluid

    Porosity Logs Density porosity (solid black line)

    measure the bulk (average) density of the formation (rock & fluids)

    Neutron porosity (dashed red line) measures the hydrogen content

    Deflections to the left = more porousDeflections to the right = less porous

    Dashed red left of Solid black black = Shale Dashed red right of Solid black = Gas Sand Dashed red over Solid black = Wet Sand or

    Oil Sand

    Shal

    e

    Gas

    Oil or

    H2O

    Gas

    Neutron Porosity

    Density Porosity

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    L 4 - Well Log Data Courtesy of ExxonMobil

    Sonic (Velocity) Logs Sonic (DT)

    Acoustic energy emitted by a transmitter, travels through the formation/fluids, detected by multiple detectors

    Log displays the interval transit time (Dt) in msec/ft (actually an inverse velocity)

    T

    T

    R1

    R2

    R1

    R2

    Delta-T

    * Vertical seismic profiling (VSP)

    - combines best aspects of velocity survey and synthetic seismogram

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    4. Pick a section, typically by coloring a trough for each marker.

    -Where to start? Dipline, off-structure, deepest part of the basin.

    -What happens when reflectors split?

    - Type 1: no geologic hiatus

    sequence thickening or facies changes: go with the flow

    -Type 2: geological hiatus

    Overstep, unconformity, truncation or toplap: Stay high.

    Overlap, onlap, baselap, or downlap: Stay low.

    Be consistent throughout the mapping area for each horizon!

    Difficult to pick in this direction

    Easier to pick in this direction

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    5. Tie to, and pick the other sections.6. Check tie lines and loop closures.

    Use expanding loops rather than contracting loops.Causes for misties:

    Static misties (bulk shift between profiles)

    Survey errors

    Changed recording system between two different surveys

    Changed processing parameters

    Dynamic misties (variable at different times on a section)

    Different stacking velocities in dip and strike-line directions

    Noise

    Paper distortion. Or (digital) data loading problems.

    Mislabelling

    Trying to tie 2D-migrated dip and strike lines.

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    Interpreter errors

    -miscorrelation across a fault or across facies changes

    -picking the wrong cycle on diverging series

    -incorrect correlation through noisy areas

    OR, problems creating a realistic geological model from a limited dataset.

    7. Post times to a map.

    8. Contour the time values for each selected horizon.

    Use interpretive contouringapply your geologic understanding of structural and depositional style to develop a realistic interpretation of the data. See notes handed out with Lab 1 on contouring methods and advise.

    Maps require title blocks!

    Title blocks should include: company name, prospect area, geographical location, map scale, type of projection, dates drafted/revised.

    Maps with data and contours should also have: name of horizon mapped, interpreter, contour interval. May also need: special notations about the data, definition of any non-standard abbreviations.

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    Lab 2 fault mapping and contouring; synthetic ties

    TKS tutorial synthetics (SYNPak)

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