USC Sequence Stratigraphy: Seismic Stratigraphic Interpretation

Seismic Sequence Analysis

This page is the first step of a seismic stratigraphy interpretation. Its objective is to define the genetic reflection packages that envelope seismic sequences and systems tracts. These bounding discontinuities are identified on the basis of reflection termination patterns and their continuity.

Boundaries are defined on a seismic line by identifying the termination of seismic reflectors at the discontinuity surfaces. Terminations occur:

Below a discontinuity and the definition of the upper sequence boundary. Examples of this include:

a. Toplap: termination of strata against an overlying surface, representing the result of non-deposition and/or minor erosion.

b. Truncation: this implies the deposition of strata and their subsequent tilting and removal along an unconformity surface. This termination is the most reliable top-discordant criterion of a sequence boundary. Such truncation can also be caused by termination against erosional surface, as for instance a channel.

Above a discontinuity and the definition of the lower sequence boundary:

a. Onlap: A base-discordant relationship in which initially horizontal strata progressively terminate against an initially inclined surface, or in which initially inclined strata terminate progressively updip against a surface of greater initial inclination.

b. Downlap: a relationship in which seismic reflections of inclined strata terminate downdip against an inclined or horizontal surface. Examples of downlap surfaces include a top basin floor fan surface, a top slope fan surface, and a maximum flooding surface.

Note: If onlap cannot be distinguished from downlap because of subsequence deformation, the term baselap is used.

Recommended procedures for performing seismic sequence analysis include:

Identifying the unconformities in the area of interest. Unconformities are recognized as surfaces onto which reflectors converge.
Mark these terminations with arrows.
Draw the unconformity surface between the onlapping and downlapping reflections above; and the truncating and toplapping reflections below.
Extend the unconformity surface over the complete section. If the boundary becomes conformable, trace its position across the section by visually correlating the reflections.
Continue identifying the unconformities on all the remaining seismic sections for the basin.
Make sure the interpretation ties correctly among all the lines.
Identify the type of unconformity:

Sequence boundary: this is characterized by regional onlap above and truncation below.
Downlap surface: this is characterized by regional downlap.

Recommended color codes:

Red: Reflection patterns and reflection terminations.

Green: Downlap surfaces

Blue: Transgressive surfaces

Other colors: Sequence boundaries

If using only black and white:

Thin solid lines: Reflection pattens

Thicker solid lines: Sequence boundaries

Dashed lines: Downlap surfaces

Dotted lines: Transgressive surfaces

References

Vail, P. R., 1987, Seismic stratigraphy interpretation procedure, in Bally, A.W. (ed.), Atlas of seismic stratigraphy: AAPG Studies in Geology No. 27, Vol. 1, p. 1-10.

Boggs, S. Jr., 2001, Principles of Sedimentology and stratigraphy, 3^rd Ed., Prentice-Hall, Inc., New Jersey, 726 p.