Seismic data gives only reflection image of subsurface generated by sound waves. Seismic stratigraphy techniques help us for stratigraphic interpretation of seismic reflectors. It is important because geological concepts of stratigraphy can be applied on seismic data and hence, seismic stratigraphy can be used as a predictive tool for petroleum system elements like reservoir, seal and source rock. The basic assumption behind seismic stratigraphy is that individual reflector can be considered as timelines i.e. it is representing a very short time interval of similar sedimentation conditions. This assumption signifies that seismic reflector can have the different depositional environment and therefore it has information of various lithofacies units. However, for seismic stratigraphic analysis, only sedimentary reflections should be used.
Seismic data contains many non-geological reflectors. These can be artefacts like diffractions, multiples etc or non-sedimentary reflections like fault planes, fluid contact etc. These non-geological elements need to be recognised before any seismic stratigraphic analysis.
The basic assumption is that Seismic reflection represents bedding plane. So, Its characteristics should change with conformable changes in depositional regime. These changes can be energy level, depositional environment, sedimentation rates, source, diagenesis and pore contents.
There are several features of seismic data that can help us to interpret depositional regimes.
- Reflection continuity: It shows the continuity of layers. It is related to energy levels and sedimentary processes in the depositional environment.
- Reflection amplitude: It shows lithology contrast, bedding spacing and fluid content.
- Reflection configuration: It shows the geometry of bedding pattern. Important to interpret palaeogeography.
- Reflection frequency: It gives bed thickness and sometimes fluid information like gas.
- Interval velocity derived from seismic: It is important for gross lithologies, porosity distribution and fluid contact.
Interval seismic velocity also gives additional information on gross lithology, porosity and fluids. Spatial association of these attributes of seismic reflectors give an idea of the depositional environment.
Unconformities are surfaces of erosion and/or non-deposition. It signifies time-gaps in the geological record. Unconformities can generate reflections because they separate stratigraphic beds of different lithologies and hence, different physical properties.
Often, there is angular contact between beds of two units across unconformity. This angular relationship shows tectonic deformation before the deposition of the younger sediments were deposited. Erosion truncates the underlying strata or time units. If beds of both units are parallel then it would be difficult to recognize unconformity from seismic. In this case, other techniques like bio-stratigraphy, isotopic analysis may be helpful.
Several types of reflection terminations can be seen on seismic at these unconformable interfaces:
Older sediments are eroded. Underlying sediments may be deformed. This can indicate a time of hiatus before other overlaying strata was deposited.
If erosion is affecting a prograding geometry. The underlying unit must show a depositionally inclined layering.
It is the case when interface and the overlain or underlain stratum are deformed in the same manner.
It is the case when the younger sediments are progressively overstepping each other.
It is the case when younger strata's foresets abutt on the unconformity. The inclined foresetting gives indication of sediment supply direction.
The structural configurations of the beds of both sides of the unconformity and the internal reflection patterns displayed by overlain an underlain units gives tectonic and environmental significance of the interface. It can give hints such as
- It was subaerial, submarine, fluvial or glacial in origin
- Time gap is erosional or non-depositional. Non-depositional may indicates sediment bypass.
- Topographic relief is planar, irregular
- Unconformity is local or regional
- It may indicate relative sealevel behaviour, sediment supply etc.
Above mentioned reflection terminations need to be mapped out on seismic data. After mapping, it is possible to outline the unconformities. These boundaries separate genetically related depositional units and hence, subdivide seismic sections into various Depositional Sequences. A variety of seismic facies units may be present in a single depositional sequence. Next step in seismic stratigraphy to map out these seismic facies units.
Seismic facies units
These seismic facies units are interpreted on the basis of reflection configuration, continuity, amplitude, frequency and interval velocity.
Sequence Straigraphy is the study of rock units within a chronostratigrphic framework bounded by erosional surfaces, nondeposition, or conformities.
The concept of sequence stratigraphy was first developed by Peter Vail in the 1960s. While he was a senior scientist at Exxon, he come up with three core concepts that define sequence stratigraphy. 1) Seismic reflections are generated by physical bedding surfaces, 2) Seismic reflections create patterns or sequences that can be used to interpret depositional environment and lithology, and 3) Unconformities that form sequence boundaries are the same age when located in multiple basins are located. Vail also believe that eustacy is the main control on unconformities, sedimentary systems, and system tracts within a stratigraphic sequence.
Sequences, Parasequences, and Parasquence Sets
Sequences are bounded by unconformities and their correlative conformity. A sequence is formed in response to sea level change, subsidence, and sediment supply. They can be broken down into system tracts (the next section). Within in a sequence is a parasequence and parasquence set. Parasequences are defined by marine flooding planes, and are usually aggradational. Parasequence sets are genetically related parasequences which form distinct stacking patterns that are bounded, usually by flooding surfaces.
Both squences and parasequences are defined by their physical relationship of the strata. Sequences and parasequences allow geologists to predict stratal relationships and infer geologic age.
System tracts are associated with seismic stratigraphy and eustacy. A system tract is an indicator of the deposition sequences that would be present within a sea level cycle. They fall into three categories.
1. Low System Tract (LST): This is created when sea level is low, but sea level is starting to rise. There is high sedimentation rates coupled with little accomedation space which causes layer stacking. Within a LST there will be progradational parasequences moving out from the shelf.
2. Transgressive System Tract (TST): This is created when sea level is rising. Here, there accomedation space is greater than the sedimentation rate, which causes a retogradational set of parasequences. At the very top of the TST is the maximum flooding surface (MFS). This is where the furthest amount of flooding occurred on the surface before sea level begins to drop.
3. Template:High System TractHighstand System Tract (HST): This is created when sea level starts to drop. Accomedation space is once again less than sedimentation rates first causing aggradational stacked parasquences then to progradational stack. At the top of the HST is where a squence boundary is formed. This boundary can be affected by erosion, especially if there is a long period of low sea level.
Migration Smiles: These are caused during migration. The velocity is inaccurate and causes upward curved artifacts within the data. The data is over-migrated, because too high of a velocity was chosen.
Multiples: This is caused by waves traveling between two strong reflectors before reaching the receiver. They usually show up as artifact since they do not follow the linear assumption that migration does. There are two types:
1. Short Period: These are harder to find and remove since they have the same velocity as the original waves do. Sometimes deconvolution can be useful in removing the multiple.
2. Long Period: These multiples are easier to locate because the velocities are different. This allows for removal during the stacking process.
Ocean Bottom Multiple: This occurs during deep water seismic imaging. There is a bouncing effect of a wave between the interface of the water and the sea floor, and is reflected in the substructure.
Recommended Seismic Stratigraphy Workflow
This is a recommended seven step workflow created by Peter Vail on seismic stratigraphy. For more in-depth information look at external source 5; Seismic Stratigraphy Interpretation Procedure.
- Seismic Sequence Analysis: This will define the genetic packages related to the seismic sequences and system tracts by finding discontinuities.
- Well-Log Sequence Analysis: Use well log information to try to determine lithofacies to then estimate where the sequences and system tracts are located. They are double checked by trying to correlate the seismic data with global cycle charts.
- Synthetic Well - to - Seismic Ties: This step is to link seismic data to the well data depth. This allows for geologists to plot individual wavelets from each interface.
- Seismic Facies Analysis: This step determines as many variations within the individual seismic sequence and system tracts as possible in hopes of determining lithofacies or fluid changes. This should also located where discontinuities occur.
- Interpretation of Depositional Environment and Lithofacies: Use the seismic data as well as the known geologic information of the area to determine environment of deposition and lithofacies.
- Forward Seismic Modeling: This step serves multiple purposes. One is to interpret stratigraphy and fluid composition near or at the seismic resolution. Two, is to create a geologic cross section using seismic information. This shows stratal surfaces and impedence contrasts. Three, is to create a basin simulation model.
- Final Interpretation: Using all the information gathered in the past six steps is to make an interpretation that makes sense.
- Seismic Stratigraphy, Basin Analysis and Reservoir Characterisation by P.C.H. Veeken
2. Depositional Squences by University of Georgia Stratigraphy Lab; http://www.seddepseq.co.uk/SEQ_STRAT/SeqStrat_UG/tracts.htm
3. Geophysical Exploration Technology: Applications in Lithological and Stratigraphic Reservoirs. Chapter 2 - Sequence Stratigraphy Research Technology by: Li Ming and Zhao Yimin. Pages 21-62; https://www.sciencedirect.com/science/article/pii/B9780124104365000022
4. An Overview of the Fundamentals of Sequencce Stratigraphy and Key Definitions from The Society of Economics Paleontologists and Mineralogists. By: J.C. Van Wagoner, H.W Posamentier, R.M. Mithchum, J.F. Sarg, T.S. Loutit, and J. Hardenbol.
5. Seismic Stratigraphy Interpretation Procedure by P.R. Vail. http://archives.datapages.com/data/specpubs/oversiz2/data/a188/a188/0001/0000/0001.htm