Plan for today: Discussion Group #1 – Pitfalls; discussion 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 Tutorial – TKS (Kingdom Suite) Suite) Lab #2 – part two, two, synthetics synthetics tie tie
Basics of Seismic Interpretation What do we mean by interpretation? 1) Geophysical interpretation
- Tracing Tracing and mapping reflector reflectorss Requires understanding of velocity, velocity, geometric, and processing pitfalls True (but still approximate) cross-sections require time-todepth conversion and migration
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2) Geologic interpretation
- Identifying Identifying reflectio reflections ns - Structural Structural mapping mapping - Tying to surface outcrop or drilled drilled subcrop - “pattern “pattern recogniti recognition” on” – seismic seismic facies facies and sequence sequence strat - Interpreting the entire stratigraphic stratigraphic section/data volume volume - Inferring Inferring the geologic geologic history history * Consider scale: regional regional vs prospect prospect mapping. Requires knowledge of what is geologically possible and what is geologically geologically probable probable in a given area. Integration of entire geophysical/geological database Requires imagination
Interpretation is an iterative process In practice you can’t… until you have… but you can’t properly identify the reflections until you have… which you can’t do until you’ve… and that is difficult unless you’ve already…
1) draw a good map 2) identified the reflections
3) inferred the geologic history
2) guessed at what the reflectors are
1) mapped them
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So, be prepared to : 1) 2) 3) 4) 5)
have a model in mind inte interp rpre rett som somee dat dataa (be (be brav bravee and and use use era erase sers rs!) !) draw some contours discover the model is wrong, and go back and redraw.
Basic workflow 1)
Data val validation. on.
2)
Prel Prelim imin inar ary y inte interp rpre reta tati tion. on.
3)
Choose the significant markers you want to map.
4) 5)
Pick Pick a secti section, on, typi typical cally ly by by colori coloring ng a trou trough gh for for each each marker (mark faults). Tie to, to, and pick, pick, the the other other secti sections ons (tie (tie faul faults) ts)..
6) Check Check line line ties ties and loop loop clos closure ures. s. 7) Post Post tim times to a map map..
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1) Data Data vali valida dati tion on
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 reflector…in 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 wide area. If possible, choose major ‘sequence boundaries.’ -Name the reflector/horizon.
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3) 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 cumulative error, validity validity check with with seismic data * Synthetic seismogram - consider polarity polarity and frequency frequency spectrum, beware of forcing correlations * Velocity survey (check shots; 1-way time) Remember: a dipping reflection 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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Well ties cont’d
Tie wells off-line
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Lithology Logs •
Gamma Ray
Gamma Ray
– a scintillation detector (similar to a Geiger counter) that measures the natural radiation from a formation
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SP (spontaneous potential) measurement vs depth of the – a measurement potential difference between the voltage in the wellbore and an electrode on the surface
Shale Baseline
e l a h S
d S
For both logs: Deflections to the right = Shale Deflections to the left = Sand L 4 - We Well ll Log Log Data Data
d n a S
Courtesy of ExxonMobil
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Resistivity Logs • High deep resistivity means: – Hydrocarbons
ILD (deep) MSFL SFL
– Tight streaks (low porosity)
• Low deep resistivity means: – Shale – Wet sand
• Separation between resistivities resistivities means: – The formation fluid is different from the drilling fluid
Formation Fluid different from 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)
Formation Fluid similar to Drilling Fluid
Porosity Logs Density Porosity Neutron Porosity
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Density porosity (solid black line) – measure the bulk (average) density of the formation (rock & fluids)
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Neutron porosity (dashed red line) – measures the hydrogen content
e l a h S
Deflections to the left = more porous Deflections 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
Gas
Gas Oil or H2O
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Sonic (Velocity) Logs •
Sonic (DT)
Delta-T
– 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
R1
R2 R1 R2
T L 4 - We Well ll Log Log Data Data
Courtesy of ExxonMobil
* Vertical Vertical seismic profiling (VSP) - combines combines best aspects aspects of velocity survey survey and synthetic synthetic seismogram seismogram
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4. Pick a section, typically 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 geologic hiatus sequence thickening or facies changes: ‘go with the flow’
-Type -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. 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 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 d iverging series -incorrect correlation through noisy areas OR, problems creating a realistic geological model from a limited dataset.
7. Post times times to to a map. 8. Contour the time time values for each selected horizon.
Use interpretive contouring—apply 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, interpreter, contour interval. interval. May also need: special notations about the data, definition of any n on-standard abbreviations.
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Lab 2 fault mapping and contouring; synthetic ties TKS tutorial tutorial – synthetics synthetics (SYNPak) (SYNPak)
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