Seismic, Sequence and Magnetic Stratigraphy

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Seismic, Sequence and Magnetic Stratigraphy
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Seismic Exploration
Figure 13.1
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Seismic methods

• Propagation of seismic (elastic) waves, reflect
  back, picked up by detectors
• Robert Mallet, 1848, measured velocity of seismic
  waves in the subsurface materials.
• Detection of rock structures, petroleum industry,
  salt domes, explosives.
• Refracted waves at discontinuity surfaces

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Reflection Seismic Method

• Waves reflected back directly from subsurface
  rock interfaces
• Shorter distance from explosion to the detectors
• Basic Principles
• Seismic waves travel at known velocities through
  rock materials
• Vary with type of rock, shale 3.6 km/s
  sandstone 4.2 km/s limestone 5.0 km/s
• Shotpoint origin of waves (explosives,
  vibrations, sound)
• Geophones - detectors

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Variable-Density Mode, waves of certain amplitude
shaded black, other light colored
Figure 13.2
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Marine Seismic Survey
Figure 13.3
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Seismic Stratigraphic Interpretation

• Interpreting stratigraphy and depositional facies
  from seismic data
• Factors that generate seismic reflections,
  critical to the entire concept
• Density-velocity changes at unconformities or
  bedding surfaces
• Reflection may not be the bedding surface of one
  bed by a set of beds

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Seismic reflection configuration and reflection
continuity
Figure 13.4
Primary depositional conditions Parallel,
divergent, prograding
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Undaform, Clinoform, Fondoform
Figure 13.5
Depositional Environments in relationship to wave
base.
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Reflection Parameters

• Amplitude indicates bed thickness, and spacing
• Fluid and gas in formation affect amplitude
• Bright spots dark bands
• Reflection frequency number of oscillations of
  seismic waves per second, hertz or kilohertz
• Frequency of seismic waves induced by energy
  source used to create the waves

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Reflection parameters 2

• Interval velocity average velocity of seismic
  wave between reflectors.
• Porosity, density, external pressure and pore
  pressure affect this.
• External form geometry of the stratigraphic
  body, seismic facies

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Seismic Stratigraphic Analysis

• Depositional processes interpreted from seismic
  profiles
• Based on analog from established stratigraphic
  and depositional models
• Used for lithofacies changes, relief, topography
  of unconformities, paleobathymetry (depth
  relationships), burial history

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Seismic Sequence Analysis

• Sequence any grouping or succession of strata
  (more restrictive) distinctive stratigraphic
  units
• Depositional sequence stratigraphic unit
  composed of a relatively conformable succession
  of genetically related strata and bounded at its
  top and base by unconformities or their
  correlative conformities

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Depositional sequences.
Figure 13.6
Sequence not defined by rock types, fossils, or
depositional processes.
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Internal Relationships

• Concordant parallel to the sequence boundary
• Discordant lack parallelism with sequence
  boundaries

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Relationships to strata
Figure 13.7
Non-depositional
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Seismic reflection patterns
Erosional truncation
Nondepositional hiatus
Toplap
Downlap
onlap
Figure 13.8
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Relationships that define Unconformable boundaries
Figure 13.9
Mapping unconformities key to seismic sequence
analysis
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Unconformities

• Outcrop 4 kinds
• Seismic stratigraphic 2 kinds of
  discontinuities good reflectors
• Erosional unconformity surface hiatus produced
  by subaerial or subaqueous erosional truncation
• Downlap surfaces marine surfaces representing a
  hiatus without evidence of erosion.

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Sequence Boundaries, Downlap, Reflection
terminations
Above discontinuities onlap, downlap
Below discontinuities truncation, toplap,
apparent truncation
Figure 13.10
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Sedimentary Sequences

• Vertical resolution 10 to 50 m
• What can be identified?
• Progradational delta-slope systems
• Carbonate shelf-margin systems
• Marine offlap-onlap systems

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Figure 13.11
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Common Seismic facies patterns
Seismic facies unit mappable, areally
definable, three-dimensional unit composed of
seismic reflections, differ from adjacent units
Figure 13.12
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Stratigraphic Bodies from seismic facies
Figure 13.13
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Lithologic and Environmental interpretation of
the seismic facies
Figure 13.14
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Sequence Stratigraphy

• Sedimentary successions can be divided into
  unconformity-bounded units Sequences
• They form during a single, major cycle of
  sea-level change
• Sequences can be split into smaller units,
  genetically linked, form during different stages
  of a single sea-level cycle, Table 13.2

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Sequence System

• Depositional System entire 3-D assemblage of
  lithofacies enclosed in sequence boundaries
• System tracts
• Parasequences
• Accommodation space

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Accommodation
Figure 13.15
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Parasequences and Parasequence sets
Figure 13.16
Range in thickness from 10 to 100 m
Progradational parasequences, retrogradational
parasequences, aggradational parasequences
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Highstand System Tract
Figure 13.17 A
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Falling-Stage System Tract
Figure 13.17 B
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Lowstand System Tract
Figure 13.17 C
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Transgressive System Tract
Figure 13.17 D
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Highstand System Tract
Figure 13.17 E
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Figure 13.17
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Why Sequence Stratigraphy

• To provide a high-resolution chronostratigrpahic
  (time-stratigraphic) framework for carrying out
  facies analysis
• Vertical facies analysis must be done within
  conformable packages of stratal units to
  accurately correlate coeval (equivalent age)
• Lateral facies relationships along a single
  depositional surface

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Global Sea-Level Analysis
Figure 13.18
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Eustatic Sea-Level Curves
Figure 13.19 A Hallam, 1984 B Vail, Mitchum,
Thompson, 1977
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Simplified Global sequence chart, Tertiary and
Quaternary
Figure 13.21
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Magnetostratigraphy

• Volcanic rocks and sediments younger than 5
  million years old
• Extended back to the Jurassic
• Basis
• Curie Point (500oC 600oC
• Minerals align to the magnetic field of the earth
  at this temperature
• Paleomagnetism

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Late Cenozoic Geomagnetic polarity time scale
Figure 13.24
Normal north (0o) Reversed south (180o)
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Marine Magnetic anomaly Biostratigraphic age of
sediment
Figure 13.25
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Terminology of magnetostratigraphy

• Chrons geochronologic time unit
• Polarity zone fundamental polarity unit for
  subdivision of stratigraphic sections
• Polarity superzone two or more polarity zones
• Polarity subzone subdivision of a polarity zone