Balanced crosssections and computeraided modeling

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Balanced cross-sections and computer-aided
modeling
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Balanced cross-sections

• A balanced cross-section (or interpretation) can
  be restored to the original (horizontal) form
  without voids or gaps.
• This means that the cross-section may be correct.
• A cross-section that cannot be restored must be
  incorrect.
• Assumptions
• Volume is neither created nor destroyed during
  the process.
• In 2D cross-sections, mass does not move in or
  out of the section.
• Usually done in depth, not time.
• Works best in sedimentary strata in upper crust
  (low temperature and brittle deformation).
• Most common in thrust terrains (poor seismic
  images and less syndepositional growth)
• Can be applied in extensional as well with care
• Does not work well in metamorphic or igneous
  terrains
• 3D balancing possible but usually requires
  computational help (useful in salt or
  strike-slip).

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An example from the Zagros - Both are balanced
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Classical balancing techniques

• Volume balancing.
• In 2D, reduces to balancing the area of a
  formation.
• Usually assume that bed thickness is constant
  over area of the cross-section.
• May need to include compaction effects.
• Line balancing
• If large-scale internal flow deformation does not
  occur within the formation, then the bed length
  will remain constant during deformation.
• If a sedimentary bed is 2km long before
  deformation, it should be 2km long after
  deformation (but may be broken up).
• Thick shales are usually the exception.

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Line balancing

• Begin with pinning points where no deformation
  has occurred.
• Measure bed lengths between these two points.
• If the lengths do not match Houston, we have a
  problem.

Which cross-section can be proven incorrect? (in
two ways)
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Volume balancing example depth to detachment (in
a region with deformable layer/decollemont)

• Pick two pinning points.
• Estimate amount of shortening between pinning
  points.
• Measure amount of structural relief.
• Given the known shortening, calculate the
  original thickness of the deformable layer.

Current length (L)
relief
Fold (area)
Amount of horizontal shortening (known)
Depth to detachment (fold area)/(shortening)
detachment
Original length (L0)
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Difficulties
Great! We can check to see if a cross-section is
correct. But.how do we make a good one to start
with? the blank space problem With just
surface data and maybe some wells or bad seismic,
how do we start?
????
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Make some assumptions! (e.g. Dahlstrom and Suppe)

• Thrust faults always step up.
• Thrust faults step up sharply and are not curved.
• Thrust faults in a given area step up at the same
  angle for example, about 35-40 in basement
  (e.g. granite) thrusts.
• Folds can be drawn as kink folds.
• Dips steeper than the normal angle of thrusting
  mean that several overlapping thrusts
  (imbrications) exist.

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• Use of kink methods
• Areas of constant dip separated by an axial
  surface.
• Bisect the angle between the bed dips.
• Use a scale of one-to-one
• Do rocks really do this?
• Sometimes.but it is an approximation also.
• In Suppe style, thrusts are assumed to have a set
  cutoff angle.
• Higher dips are the result of overlapping
  thrusts.

From Katterhorn, 1994
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Computer-aided
Ranges from simple (line balancing) to complex
(finite-element models). Conducted in
depth. Usually takes a lot of time to construct
3D models. Can be used for input for depth
migration, ray tracing, etc.
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Compressional tectonics and seismic expression
From Chatelier, AAPG, Search and Discovery
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Tectonic settings
Convergent boundaries Accretionary prism
(Barbados) Occur at subduction
zones Sediments scraped off descending
plate Fold and thrust belts Thin-skin
(Canadian Rockies) Most deformation occurs in
sediments above a regional decollemont May
involved a foreland basin Thick-skin
(Wyoming) Basement involved Higher angle
faults Inversion Re-activated older
faults Normal faults become compressional Transf
orm with compression (Los Angeles) Toe of
growth faults (Gulf of Mexico)
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Seismic data

• Often poorly imaged, especially on land
• can be difficult and expensive acquisition due
  to mountains with poor geometry.
• large elevation changes and large statics
  corrections
• complicated velocity structure
• requires migration
• Use all other available data (surface, gravity,
  etc)
• Use knowledge of characteristic geometries and
  lithologies
• possible decollemonts (shales, salts)
• competent sections (carbonates)
• look for repeated sections
• anticlines and folds must have something
  underneath
• Constrain possible solutions with balanced
  cross-sections and restorations
• Think in 3D and be imaginative!

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Accretionary prisms
thrusts
decollemont

• Nankai trough (subduction zone off southeast
  Japan)
• Sediments scraped off descending plate
• High fluid pressures
• Thrust faults
• Note decollemont

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3D seismic (from Shipley) Amplitude variations
along decollemont possible variations in fluids
and fault strength?
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Thin-skin thrusts

• Deformation occurs above a regional decollemont
• usually in sediments
• layers above decollemont may appear undisturbed
• ramp and flat geometry
• flat-lying thrust along weak layers (shales,
  salt)
• ramps up at defined angle in stronger layers
• often forms imbrications and duplexes
• Eventually, tectonic shortening involves basement
  somewhere
• Canadian Rockies are classic example

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• A variety of structures
• usually hard to recognize on seismic
• useful for filling in blank spots to balance
  sections

From Boyer and Elliott (1982)
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Thick-skin

• Basement involved
• Higher-angle

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Sort of a dumb animation
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And now think in 3D
Sheep mountain, Wyoming - An anticline above a
basement thrust
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Thrust faults and 3D

• Vary along strike
• Smoothly with gradual loss of slip
• Abruptly with tear faults
• Variations create structures in the layers above

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Inversion

• Structures which have undergone a reversal of
  regional stress with reactivation of faults are
  referred to as inversion
• Usually, extensional to compressional
• Occur in salt tectonics as well
• Change in correlation across faults
• Changes in formation thickness opposite to
  expected (thicker at structural highs instead of
  depocenters)

An inverted extensional fault - from Gibbs, AAPG
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Balanced cross-sections

• Restorable sections
• No missing pieces
• Thrust faults follow set geometries
• Faults break at certain angles
• Controls dip of overlying layers
• Higher dips indicate imbrication

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Sometimes the structures arent as neat
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Software can help

• 3D restoration software
• Import from interpretation
• Convert to depth
• Restore along faults

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Structural inversion in more complex structures
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And looking at rocks can help
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Somewhere below Santa Barbara- from Chatelier,
AAPG SD