Seismic Stratigraphy
Why Seismic Stratigraphy?
1) Imaging has improved to the point that the seismic line resemble geological cross sections, with all the potential for interpretation. Seismic lines are generally low resolution tools, but they have much better lateral coverage than outcrops
2) Seismic Data Allows characterization and quantification of lithology: Instead of grain size and composition use amplitude, frequency, continuity, and velocity. These features can be qualitatively described, and quantified easily when the data is in digital format.
3) Layer Geometry: Seismic data show gross geometry (within the limits of resolution over large areas. Vertical Exaggeration can be changed to reveal subtle angular relationships. Seismic reflectors may be commonly time lines.
4) 1-3 above comprise predictive tools that can allow you to interpret lithology, depositional environment, sea level changes, and even age.
Industry has been strongly support of Seismic Stratigraphy because of its predictive nature
Problems with roots in Stratigraphy. Descriptive and terminology bound. This lecture guides you through it at a simplified, but hopefully clear level. The material is not conceptually difficult, but just intricate and voluminous.
Nature of Reflector Packages
Feature of
Reflectors Geological
Interpretation Amplitude Impedance
contrasts, Layers spacing cause constructive and distructive
interference, Fluid content Polarity Sign of impedance
change (can be complicated by interference) Continuity Beddding or layer
continuity (keep resolution in mind) Frequency Layer thickness
(beware of interference) Interval
Velocity Lithology,
Porosity, Fluid Content Reflection
Coinfiguration Bedding patterns,
Depositionsl processes, Unconformities, Fluid
contacts
Time Significance of Seismic Reflectors
Seismic reflectors in many cases are time lines. They cut across major lithogic boundaries, especially those defined by outcrop sections or wells.
Why is this possible? Reflectors record individual beds (impedance contrasts). In seismic data the individual beds can be followed laterally in a way not possible in typically discontinuous outcrop sections
Graphic from Sherrif and Geldart, 1995, p. 403.
Geometry of Seismic Reflectors:
From Sequence Stratigraphy "the subdivision of sedimentary basin fills into genetic packages bounded by unconformities and their correlative conformities"
Seismic data, because of its large-regional scale, is ideal for defining these packages.
Seismic Sequences:
Seismic sequence: A group of genetically related strata bounded by unconformities or related conformities. This is the explicit definition.
- Often bound by angular truncations which are interpreted as unconformities.
- Expected to have a depositional affinity, closely tied to eustatic sealevel cycles.
Seismic Facies: Packages of reflectors with a set of seismic characteristics differing from adjacent units. (similar to definition of a formation-must be distinguishable from adjacent units and mappable on earth's surface)
Boundaries of Seismic Sequences
The nature of the contacts of the seismic reflectors with the boundaries of the sequences provide information on the growth of the unit, hence much detail is paid to these surfaces. See Graphic.
Terms: Top Boundary: Truncation, Toplap, Concordance
Bottom Boundary: Onlap, Downlap, Concordance.
Point Arena Basin example of downlap of Plio-Pleistocene over truncated Miocene
Santa Cruz Terrace Deposits Downlapping onto unconformity
Onlap and downlap are indicators of unconformities or very condensed sections, as is toplap.
The above figure shows a single sealevel rise and fall cycle.
Internal Reflection Configurations
Many varieties: Some that we will use:
Parallel, Divergent, Hummocy, Contorted, and others …..
Clinoforms: Inclined basinward prograding layers. Sometimes described as offlap. Indicating sediment supply overwhelming water level (usually sealevel in basin). Clinoforms have a lot of geometric variety. See examples from:
Peru, a temperate water
carbonate of Miocene age: Clinoforms merging into Toplap. 
and the Permian of West Texas
.
Why is this important: In a very straightforward way it tells you about depositional processes
Variation of sediment supply, subsidence and sea level rise. Variations in sea level and subsidence define the accommodation space&endash;or vertical limits for sediment accumulation. Variations in these parameters result in distinctly different sediment package geometries, or seismic sequences. Plus the depositional conditions at differing water depths and sediment supplies would result in differing sediment types. The predictive potential of this approach has been irresistible for the oil industry.
Sediment Supply verses Accommodation Space in a Basin
Other Geometric Aspects of Seismic Data:
See Overheads
System Tracts:
The seismic sequence can be divided into packages representing contemporaneously deposited strata: Classically: Low Stand, High Stand, and Transgressive.
Future of Seismic Stratigraphy: 3D and Seismic Geomorphology
Fluvial System: Offshore Java
Channel Complex, Nile Delta
Carbonate Buildups: Again a photorealistic slice from a 3D seismic volume off Boreno.
Term: HT00 Lecturer: Christopher Juhlin
Seismic Stratigraphy - Overview
Hours: 11
Goals of the course
Acquaint the student with the basics of seismic resolution
Understand the principles of seismic stratigraphy
Acquaint the student with the basics of geophysical wireline logging
Understand how these logs can be applied to sequence stratigraphy
Literature
Emery D. and Myers K. J.: Sequence Stratigraphy, Blackwell Science, 1996 (3 copies in library)
Nichols G.: Sedimentology and Stratigraphy, Blackwell Science, 1999
For additional relevant literature see: www.geofys.uu.se/~cj/seis_strat/literature.html
Reading assignment
Chapters 2 and 3 in Emery and Myers
Chapter 22 in Nichols
Notes
Notes for parts of the course are available online at
Basic principles of reflection seismics
www.geofys.uu.se/~cj/seis_strat/notes/interp/theory.html
Basic principles of seismic stratigraphy
www.geofys.uu.se/~cj/seis_strat/notes/interp/interp.html
Introduction to logging methods for sequence stratigraphy
www.geofys.uu.se/~cj/seis_strat/notes/logging/logging.html
Homework
There will be 3 homework assignments during the course
Course information
www.geofys.uu.se/undergraduate/seis_strat.html