• Seismic stratigraphic studies of a basinfill is to delineate the Depositional

Sequences (Mitchum et al. 1977).

ü“The Depositional Sequences are lithologic units bounded by unconformities”

• The basic method for the delineation of Depositional Sequence boundaries, is

called ‘reflection termination mapping’ technique (Vail et al. 1977).
• The seismic section is subdivided into several depositional units based on
reflection terminations that define unconformities.
• Reflections are ended and the geometry of the terminations defines the
position of the unconformity (Figure 1).
Figure 1: The seismic section is subdivided into several depositional units based on reflection terminations that define
unconformities. A depositional sequence is a lithologically related unit bounded by unconformities at its top and base.
Different packages are lying on top of each other and the observed geometries can be used for a relative age dating. The
degree of tectonic deformation is also changing. The older layers (DS1 and DS2) are the most steeply dipping. Reflections
in layer DS2 are clearly terminating against the base of DS 4. The erosive boundary is an unconformity and this interface
represents an important time break.
1. Basic seismic observations
Four major groups of systematic reflections are distinguished on seismic
sections (Figure 2):
• Sedimentary reflections: representing bedding planes.
• Unconformities: or discontinuities in the geological record.
• Artefacts: like diffractions, multiples etc.
• Non-sedimentary reflections: like fault planes, fluid contacts etc.
Figure 2: Examples of different types of seismic reflections. Not only rock interfaces but also fluid
contacts can have sufficient acoustic impedance contrast to generate a seismic reflection (Veeken 2007).
 1.1 Sedimentary reflections
One of the basic concepts behind seismic stratigraphy is the observation that
individual sedimentary reflections are some sort of timeline.
• More precisely: They represent a rather short time interval with pretty
similar sedimentation conditions.
• The timeline concept :means that each reflector coincides with a short time
period of very similar depositional conditions in a geological sense.
• This timeline idea equally implies that seismic reflections tend to cross
laterally through different depositional environments and therefore they
can incorporate various lithofacies units.
• In Figure 3: it is seen that, if a specific seismic loop is followed from the left
to the right, it goes from a parallel sub-horizontal topset into a gently dipping
foreset and finally it is transitioned into a parallel sub-horizontal bottomset.
• This TWT time geometrical relationship can be translated in terms of
depositional environments: it goes from a sand-prone shelf (proximal = closer
to the source area of sediments), into a shale-prone slope (further from the
source and thus finer-grained) and ending up in the distal shale-prone deeper
basinal environment (distal = far from the sediment source).
• The word ‘prone’ means that the dominant (or gross) lithology is likely to be
of a certain type.
• The lithology is mainly related to the grainsize and reflects the energy of the
environment of deposition.
• The expression shelf is normally restricted to deposits upto a water depth of
200 meters.
• There is a distinction between the inner and outer shelves.
• The inner shelf comprises the coastal, transitional to marine environments,
while the outer shelf is purely open marine with a water depth of 50 to 200
meters.
• Beyond 200 meters the water depth is bathyal; it constitutes the domain of
the continental slope or rise and the abyssal plain (Figure 4).
• In the trenches the water depth can go up to 10 km, corresponding to areas
where the crustal plates are subducting into the mantle.
Figure 4: Zonation and environments of deposition for a continental margin setting illustrating basic nomenclature
used in oceanography.
The sedimentary reflections corresponds mainly bedding planes that
represent conformable changes in the depositional regime, i.e.:
• Lithology.
• Energy level.
• Sedimentation rates.
• Environment of deposition.
• Input source.
• Degree in diagenesis.
• Pore contents.
• As reflections tend to cross depositional environments, it is assumed that
also the pore contents (fluid fill and cement) plays a role in creating the
acoustic impedance contrast.
• Even changes in seawater temperature and salinity can cause seismic
reflections
• As seen earlier on, reflections are generated by interfaces that show
sufficient velocity-density contrast (= acoustic impedance).
• This can be a single lithological boundary or the combined response of
several closely-spaced interfaces.
There are several features that help to describe the reflection character of a
seismic loop:
• Reflection configuration: is related to the geometry of the bedding pattern resulting from specific depositional
processes, the original paleo-topography and fluid contacts.
• Reflection continuity: describes the continuity of the layers. It is directly related to sedimentary processes and
therefore also to the environment of deposition.
• • Reflection amplitude: providing information on the reflection strength, lithological contrast, bedding spacing
and fluid contents.
• • Reflection frequency: gives an estimation of the bed thickness and possibly also the fluid contents.