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.
Spatial association of these attributes of seismic reflectors give an idea of the depositional environment.
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- Seismic Stratigraphy, Basin Analysis and Reservoir Characterisation by P.C.H. Veeken