Stratigraphic Analysis
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- Printing Instructions:
- 11a - "Interpreting Line C"
- one document, 5 pages, letter size, B&W
- one document, 1 page, seismic line, about 30x18 inches (can have one copy for 3 students)
- 11a - "Interpreting Line C"
- Supplies:
- Color pencils, eraser
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Slide 1
- Introduction slide – photos of key people in the development of seismic/sequence stratigraphy
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Slide 2
- Similar to our structural analysis, we will make observations from the seismic data
- Have a set of depositional models in mind
- And combine the observation & depositional models to make stratigraphic predictions
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Slide 3
- A caution about seismic images
- Seismic data has a resolution that is not as fine as most stratigraphers are used to working
- Units are often 10s to 100s of meters
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Slide 5
- The strengths of seismic data are:
- Good areal coverage
- Able to image major depositional units
- Able to identify potential source, reservoir, and seal units
- Provides a stratigraphic framework within which other data can be understood
- Well data
- Basin fill history
- HC systems
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Slide 6
- The weaknesses of seismic data are:
- Limited vertical and lateral resolution: can’t resolve “small” features
- Stratigraphic interpretation is limited by the quality of the seismic data/imaging
- Seismic responses are non-unique – e.g., low amplitude could be a massive sand or a thick shale
- In new areas, we often have to ‘jump’ correlate from adjacent outcrops or basins
- Post-depositional erosion and/or structuring can hamper stratigraphic correlations and paleo-depositional reconstructions
- Typically we can’t “see” hydrocarbons
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Slide 7
- When correlating stratigraphy – the rock record, there are two basic methods, both have pluses and minuses:
- By rock type i.e., Lithostratigraphy or
- By age-equivalence i.e., Chronostratigraphy
- The next couple of slides illustrate this point
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Slide 8
- When using a lithostratigraphic methodology:
- Units are defined based on lithology
- Rock units vary in space and time
- Boundaries are subjective, and not physical since lateral facies changes are gradational
- The diagrammatic cross section shows 4 litho-units
- a fluvial or non-marine member
- two nearshore sands, distinguishing different physical properties (e.g. grain size, sorting, color)
- an offshore shale
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Slide 9
- When using a lithostratigraphic methodology:
- Units defined based on time-equivalent stratal surfaces, natural stratigraphic subdivisions
- Chronozones vary in space but not time
- Correspond to physical boundaries, which can generate reflections
- The diagrammatic cross section shows 3 time-units
- one major episode or pulse of deposition color-coded yellow
- a second episode shown in orange
- a third erisode indicated by white
- How do we break the rock record into time units?
- there may be index fossils
- there may be beds that mark time – e.g., a volcanic ash layer
- we can use a method we call sequence stratigraphic analysis – which is beyond the scope of this course
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Slide 10
- The question for us is, what does the seismic data reveal, Lithostratigraphic Units, or Chronostratigraphic Units?
- We can do a simple thought experiment
- For the diagrammatic section we used to illustrate lithostratigraphy and chronostratigraphy, how might the seismic respond for each case?
- If seismic reflections reveal lithostratigraphy, then we would expect a reflection in a prograding system to march out & up
- But, if seismic reflections reveal chronostratigraphy, then we would expect a reflection in a prograding system to march out & down
- The next slide shows the answer
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Slide 11
- This is one of 50+ examples of seismic data from a prograding (deltaic) setting
- Red lines have been added to mark some of the (black) peaks
- What do we see? The reflections march out & DOWN
- This strongly suggests that seismic reflections parallel stratal surfaces and therefore have time or chronostratigraphic significance.
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Slide 12
- A basic tenet of seismic stratigraphy is that seismic reflections parallel time lines
- Since stratal units above the scale of beds mark units of time, we conclude that seismic reflections are time-stratigraphic
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Slide 13
- Why Is that???
- Reflections are generated where there is a change in acoustic properties (z = ρ * v)
- Where would sharp changes in impedance occur?
- horizontally as lithofacies change?
- vertically across stratal boundaries?
- Consider this West Texas outcrop 1200 ft thick
- This section consists of primarily offshore shales with some nearshore silts and sands
- The sandy layers appear as white ledges – more resistant than other layers
- Let’s say I walked one of these sandy ledges for 5 miles and took a sample every 300 ft
- The first sample might have 80% sand, then 78% sand, then 77%, then 74%, then 75%, then 73%, then 71%, then 74%, then 72%, etc.
- The point is that sand % changes slowly, with an overall trend to lower sand as I move towards the basin (right in this case)
- Now, one of you can repel down the cliff and sample the outcrop every 10 ft – I’m too old for that
- Your first sample has 5% sand, then 18%, then 12%, then 47%, then 23%, then 82%, then 22%, then 7%, etc.
- Vertically there is more lithologic variation over a shorter spacing as we sample different beds, bedsets, and parasequences
- You would also notice stratal surfaces across which there are abrupt lithologic breaks
- There are very gradational lateral changes in physical properties
- But there can be abrupt vertical changes in physical properties especially at parasequence boundaries
- The abrupt impedance contrasts that generate seismic reflections are the abrupt vertical changes in physical properties at larger-scale stratal unit boundaries
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Slide 14
- Our next key question is: How can we define stratal units, especially on seismic lines?
- Consider for a moment how you would do this with a photo of an outcrop, such as this one of the North wall of the Grand Canyon
- Wouldn’t you look for evidence of a significant break in deposition?
- Can you see evidence for a major break in deposition on this photo?
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Slide 15
- Here I’ve added some interpretation of the stratal layering
- In the bottom half, strata are dipping down towards the right (east)
- These are pre-Cambrian aged rocks
- On the top of the wall, the layers are nearly horizontal
- About midway down there is a change in the dip of the strata – marked by the magenta line
- This is a classic angular unconformity
- We can deduce the following geologic history:
- The pre-Cambrian rocks were deposited horizontally near sea level
- There was a tectonic event that tilted the area down to the east
- When sedimentation resumed, the Paleozoic rocks were deposited horizontally near sea level
- More recently, the whole area is uplifted without tilting since the nershore Paleozoic rocks are now 7000 to 8000 ft above sea level
- The angular discordance between the Pre-Cambrian rocks and the Paleozoic rocks marks an unconformity that separates an older stratigraphic package from a younger stratigraphic package
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Slide 16
- If we accept Vail & Mitchum’s premise that seismic reflections parallel stratal units / time lines
- Then we can infer stratal dips by looking at the dips of seismic reflections
- In this example, there are high amplitude, continuous reflections dipping down to the right
- Some of the reflections are marked with green lines, with arrow heads near where they terminate
- There is a series of arrow heads along the red (trough) reflection marked in yellow
- Above the yellow line, the reflections have different dips, lower amplitudes and less continuity
- We would interpret the yellow line as an unconformity, separating an older episode of deposition from a younger episode of deposition
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Slide 17
- Vail & Mitchum gave us a set of terms to use when marking seismic unconformities
- There are two types of terminations we might observe at the base of a depositional package, a sequence
- If the reflections that terminate are at a lower dip than the surface against which they terminate, then we call these onlap terminations
- If the reflections that terminate are at a higher dip than the surface against which they terminate, then we call these downlap terminations
- NOTE: These definitions are based on observed geometric relationships, not on inferred depositional processes
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Slide 18
- There are two types of terminations we might observe at the top of a depositional package, a sequence
- If the reflections terminate along an irregular, erosional surface, then we call these erosional terminations
- If the reflections come up an approach a common level in a “lazy S manner,” then we call these toplap terminations
- Toplap is a syn-depositional pattern in a deltaic depositional setting
- Going from the basin towards the land, units thin below seismic resolution, hence the reflection terminations
- The common level they approach would be storm base
- NOTE: These definitions are based on observed geometric relationships AND some inference about depositional processes
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Slide 19
- This figure was first published in AAPG Memoir 26 (1977) and has since appeared in 50+ publications
- It shows how seismic terminations (onlaps, downlaps, toplaps, erosion) are used to identify seismic sequence boundaries
- The interval between two successive seismic sequence boundaries is called a seismic sequence
- A seismic sequence is a depositional sequence identified on a seismic section
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Slide 20
- Armed with these concepts and definitions, you are now ready to do some interpretation
- You will work this classic seismic line from offshore West Africa
- There are lots of onlaps, downlaps, toplaps, and erosional truncations that you can use to mark seismic sequence boundaries
- There are also some faults, now that you are experienced at identifying faults on seismic data
- Pull this line out, and I’ll help you get started