This page is the first step of a seismic stratigraphy interpretation. Its objective is to define the genetic reflection packages by the surfaces 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. These terminations occur:
- Below a discontinuity and the definition of the upper sequence boundary. Examples of this include:
- Toplap: termination of strata against an overlying surface, representing the result of non-deposition and/or minor erosion.
- 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.
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:
- Above a discontinuity and the definition of the lower sequence boundary:
- 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.
- 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.
Recommended color codes:
If using only black and white:
- 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:
This hierarchy of surfaces
now provides a framework to the reflectors. Be as objective as possible when identifying the discontinuity surfaces
of the section. Where possible base your observations more on the geometric relationships of the reflectors than on an interpretation of their origin.
Your next step in the workflow is to interpret the origin of this framework. A suggested approach is to assume that the framework is result of the generation of repeat
s of accommodation
and sediment fill (the accommodation succession
of Neal and Abreu, 2009). These seismic units of the accommodation succession
s vary in magnitude and duration and can be interpreted to be the products of sedimentary packages that accumulated on a depositional profile. Succession
s can be seen to consist of component partial succession sets that sequentially prograde to aggrade, retrograde, and aggrade to prograde to degrade.
Neal and Abreu, 2009 propose that when interpreting seismic it is assumed that deposition has responded to accommodation successions that range across time scales of 10–105 ka and have stacking patterns that are products of vectors of accommodation rate (δA)/sedimentation rate (δS). Thus: