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Seismic, Sequence and Magnetic Stratigraphy

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Title: Seismic, Sequence and Magnetic Stratigraphy


1
Seismic, Sequence and Magnetic Stratigraphy
2
Seismic Exploration
Figure 13.1
3
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

4
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

5
Variable-Density Mode, waves of certain amplitude
shaded black, other light colored
Figure 13.2
6
Marine Seismic Survey
Figure 13.3
7
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

8
Seismic reflection configuration and reflection
continuity
Figure 13.4
Primary depositional conditions Parallel,
divergent, prograding
9
Undaform, Clinoform, Fondoform
Figure 13.5
Depositional Environments in relationship to wave
base.
10
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

11
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

12
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

13
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

14
Depositional sequences.
Figure 13.6
Sequence not defined by rock types, fossils, or
depositional processes.
15
Internal Relationships
  • Concordant parallel to the sequence boundary
  • Discordant lack parallelism with sequence
    boundaries

16
Relationships to strata
Figure 13.7
Non-depositional
17
Seismic reflection patterns
Erosional truncation
Nondepositional hiatus
Toplap
Downlap
onlap
Figure 13.8
18
Relationships that define Unconformable boundaries
Figure 13.9
Mapping unconformities key to seismic sequence
analysis
19
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.

20
Sequence Boundaries, Downlap, Reflection
terminations
Above discontinuities onlap, downlap
Below discontinuities truncation, toplap,
apparent truncation
Figure 13.10
21
Sedimentary Sequences
  • Vertical resolution 10 to 50 m
  • What can be identified?
  • Progradational delta-slope systems
  • Carbonate shelf-margin systems
  • Marine offlap-onlap systems

22
Figure 13.11
23
Common Seismic facies patterns
Seismic facies unit mappable, areally
definable, three-dimensional unit composed of
seismic reflections, differ from adjacent units
Figure 13.12
24
Stratigraphic Bodies from seismic facies
Figure 13.13
25
Lithologic and Environmental interpretation of
the seismic facies
Figure 13.14
26
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

27
Sequence System
  • Depositional System entire 3-D assemblage of
    lithofacies enclosed in sequence boundaries
  • System tracts
  • Parasequences
  • Accommodation space

28
Accommodation
Figure 13.15
29
Parasequences and Parasequence sets
Figure 13.16
Range in thickness from 10 to 100 m
Progradational parasequences, retrogradational
parasequences, aggradational parasequences
30
Highstand System Tract
Figure 13.17 A
31
Falling-Stage System Tract
Figure 13.17 B
32
Lowstand System Tract
Figure 13.17 C
33
Transgressive System Tract
Figure 13.17 D
34
Highstand System Tract
Figure 13.17 E
35
Figure 13.17
36
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

37
Global Sea-Level Analysis
Figure 13.18
38
Eustatic Sea-Level Curves
Figure 13.19 A Hallam, 1984 B Vail, Mitchum,
Thompson, 1977
39
Simplified Global sequence chart, Tertiary and
Quaternary
Figure 13.21
40
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

41
Late Cenozoic Geomagnetic polarity time scale
Figure 13.24
Normal north (0o) Reversed south (180o)
42
Marine Magnetic anomaly Biostratigraphic age of
sediment
Figure 13.25
43
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
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