Seismic & Sequence Stratigraphy

A field of study in which basin-filling sedimentary deposits, called sequences, are interpreted in a framework of eustasy, sedimentation and subsidence through time in order to correlate strata and predict the stratigraphy of relatively unknown areas. Sequences tend to show cyclicity of changes in relative sea level and widespread unconformities, processes of sedimentation and sources of sediments, climate and tectonic activity over time. Sequence stratigraphic study promotes thorough understanding of the evolution of basins, but also allows for interpretations of potential source rocks and reservoir rocks in both frontier areas (having seismic data but little well data) and in more mature hydrocarbon provinces. Prediction of reservoir continuity is currently a key question in mature hydrocarbon provinces where sequence stratigraphy is being applied.

The field originated during the 1960s with the study of the stratigraphy of the continental USA, where numerous unconformities could be correlated widely, and led to the proposal that major unconformities might mark synchronous global-scale events. Through sequence stratigraphy, widely-separated sediments that occur between correlatable unconformities could be compared with each other. Studies of outcrops and seismic lines bore out these concepts, which initially were called "Seismic Stratigraphy" and first published widely in 1977. Further study of seismic lines led to the interpretation of the geometry or architecture of seismic events as representing particular styles of sedimentation and depositional environments, and the integration of such interpretations with well log and core data. Because of the simultaneous, competitive nature of the research, numerous oil companies and academic groups use the terminology of sequence stratigraphy differently, and new terms are added continually.

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.

Illustration of Seismic Stratigraphy

NAJEEB'S NORTH AMERICAN STRATIGRAPHIC CODE

Najeeb's Home Page

Your Comments are more then welcome