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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
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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
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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.
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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
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7Well ties contd
Tie wells off-line
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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
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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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