INTERPRETATION OF SEISMIC DATA
It is important to note that the interpretation of seismic data essentially begins at the survey design phase, when choices about offsets, line orientations, source characteristics etc are made, as against the notion that it is when the “final stack” is delivered by the processing team. This is because these choices can greatly influence the interpretability of the final data. Therefore the fact the interpreter must ascertain that the data before him has gone through proper acquisition and processing cannot be overemphasized; because even if you are the best interpreter you would have ended up interpreting the wrong data.
Also, to make an interpretation as robust as possible, the interpreter (or interpretation team) must integrate as many data types as possible, such as well log data, production data, pressure data and other types of geoscientific and engineering data. Multidisciplinary skills and approaches are required in order to maximize the return on the investment made in collecting and processing the data (Hart, 1997).
Therefore, the interpretation workflow as presented by Hart (2000) is presented below. This workflow can apply to both 2-D and 3-D seismic interpretation.
- Collect all pertinent Data & reports
- Scan records for polarity, static shifts, etc.
- Scan through sections (Line by Line). Overview of Data quality, structure, stratigraphy
- Tie Well & Seismic data. Use: paleo, Lithology, synthetics, VSPs, Tops, etc.
- Pick Horizons & Faults (Loop tying)
- Seismic Stratigraphy analyses
- Structural analyses
- Contouring/Mapping
Direct Hydrocarbon Indicators (DHI) in Seismic Data
Well logs can be used to identify the position of hydrocarbon-bearing reservoirs, and therefore can be tied to the seismic data for quicker and easier hydrocarbon indication (fig.16). But where there is no available well logs, especially in the exploration phase, Direct Hydrocarbon Indicators (DHI) can be employed to detect hydrocarbons on seismic data. These Direct Hydrocarbon Indicators (DHIs) include basically bright spot, flat spot and the dim spot.We know that the Direct Hydrocarbon Indicators (DHIs) respond to the seismic attributes like amplitude variations. Recent studies using the amplitude variation with offset (AVO) has shown that these bright amplitudes could be as a result of changes in Lithology, porosity or fluid change, since these are the rock properties that amplitude responds to. Therefore, it is one thing to have an amplitude, but another to know what is causing it.
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Seismic Stratigraphy /Interpretation
Seismic stratigraphy is a technique for interpreting stratigraphic information from seismic data.The resolution of the seismic reflection follows gross bedding and as such they approximate time lines. The key is that the contrast represented by seismic lines comes from bedding surface and not lateral variations (facies changes).
In addition to bed thickness constrains there are three other factors that limit final resolution of the seismic data.
1- the Earth acts as a filter that progressively attenuates the high-frequency components of the seismic data.2- Acoustic velocity increases with depth due to compaction and increased cementation. This increases the wavelength of the signal with detrimental effect on the resolution.
3- If there is high ambient noise on the raw data, the processing stream may include a high-cut filter which has the effect of removing the high frequency necessary for finer resolution.
The two main steps involved in seismic stratigraphy are seismic sequence analysis and seismic facies analysis. In the first phase, major stratigraphic packages are defined by the picking of unconformities, flooding surfaces or sequence boundaries. The seismic reflection terminations (downlap, onlap, toplap. Erosional truncation, etc) are used to identify the major sequence boundaries.
The next phase in seismic stratigraphy is seismic facies analysis,and it involves the determination of depositional environment of the rocks being examined. In this phase, the reflections in each sequence are described in terms of their frequency content, amplitude, continuity, and other shape descriptors (e.g. parallel continuous reflections, hummocky or chaotic discontinuous reflections).
Structural/Fault Interpretation
Structural interpretation involves the interpretation of faults and other structural framework that could be crucial in understanding the structure of a sedimentary basin as well as potential hydrocarbon traps. Normally, structural and stratigraphic interpretations can be carried out simultaneously. For example, to calculate the throw on a fault, you have to first identify the horizons on either side of the fault blocks. Also, to correlate a horizon from one side of a fault to another you need to understand the fault geometry (normal, reverse) (fig.).Fault / structural interpretation can focus on either the large-scale mapping of basin-bounding (i.e. km-scale) structures to small-scale mapping of faults that may affect the flow of hydrocarbons within a reservoir (e.g. metre to tens of metre scale faults).
Attribute analysis (like coherency attributes) of seismic data can help in significantly speeding-up the structural (fault) interpretation process. The coherency volume is generated from a conventional
3-D seismic amplitude volume. Also the coherency attributes can be very useful for detecting stratigraphic features such as channel systems (channel fairway analyses).
It is important to note that changes in seismic velocity with depth can affect the imaging of faults and accordingly may lead to erroneous interpretation of the fault geometry. In the time-section the fault appears to have a listric geometry (fig.). But in the depth-section, when velocity increase with depth is put into consideration, the fault is shown to have planar and not listric geometry. In any case, this is not to say that all listric faults are a result of limitations in time imaging!