Seismic facies analysis

Most of the hydrocarbons (gas and oil) occur in sedimentary rocks that were generated in different depositional environments (e.g. river charmels, delta systems, submarine fans, carbonate mounds and reefs). Seismic waves penetrating into and reflected within sedimentary rock bodies yield a seismic image of their external shape and of their internal texture. Therefore, the analysis of the external shape of seismic bodies and its internal textures, which is called seismic facies analysis [2], helps to specify the depositional environment of the investigated sedimentary rocks. An analysis of the seismic facies is a must in seismic interpretation to determine the depositional environment and to locate potential reservoirs, especially in complex oilfields. 

For a seismic interpreter, seismic facies analysis is a monotonous and time consuming task because it still has to be done manually by scanning through hundreds to thousands of seismic cross sections. Hence, a process is highly required which makes this interpretation step automatic. 

The following of this chapter explains (1) the principals of seismic facies analysis, (2) what kind of principal external shapes and internal textures are desirable to extract, and, (3) what kind of strategy should be developed to achieve an automatic mapping of the specified features. The attention of this chapter is directed to carbonate mounds of the Barents Sea and their seismic facies. 

Fig. 12. Simplified scheme of seismic facies analysis. The thick lines indicate the boundaries of different shapes which represent seismic sequences. The different shapes show different textures.

A useful automatic seismic facies mapping tool has to combine information about shapes and textures within these shapes. Only a combination of shapes and textures enables a meaningful seismic facies analysis fulfilling the requirements of seismic stratigraphy. Mapping of seismic bodies has to be done on the basis of reflection terminations. As soon as a seismic data set is subdivided into different shapes, texture analysis within each of the shapes can be achieved. 

Seismic Texture Attributes. Subsection 1.2 introduced seismic sequence stratigraphy as a means of explaining the structure of the subsurface, helping determine the depositional environments and possible rock type distributions. Essentially, this process boils down to an analysis of seismic bodies defined by their internal textures and external shape, often referred to as seismic facies analysis. This type of analysis is a must in seismic interpretation to locate potential reservoirs, especially in complex oilfields. 

For further information, extensive overviews of the interplay between seismic facies analysis and seismic texture attributes are given in the chapter [8] of Carrillat and Vallfe. 

In our approach, we consider that reflections of seismic waves within sedimentary rock bodies produce an image of their external shape and internal configuration or texture. The study of these external shapes and internal textures is also referred to as seismic facies analysis. For further details on this, we refer to the chapter [37] of Schlaf and Randen. For instance a meandering channel produces an external shape that can be filled with different seismic... 

The general inversion scheme for reservoir characterisation and delineation from seismic multicomponent data involves the transformation of the converted shear waves (PS data) to PP time domain. This operation is performed in order to have the multi-component data with the same time reference for allowing direct comparison and analysis of both PP and PS data. The transformation of PS data to PP time requires a detailed analysis of the overburden, and interpretation of correlative reflection events on both data sets. The next step in the general inversion scheme involves 3D seismic facies analysis using seismic texture attributes as described earlier. 

The multi-component data have been classified into seven t3q>es of seismic facies. These facies include flat parallel continuous high amplitude, flat parallel continuous low amplitude, discontinuous low amplitude, discontinuous high amplitude, dipping continuous, transparent facies and chaotic patterns. Seismic facies analysis and direct comparison of the acoustic mode (PP) and the converted mode (PS) show that in parts of the Grane Field the converted... 

Recommendations. As discussed above, automatic seismic texture analysis is a very powerful tool in extensional tectonic domains and in tectonically undisturbed areas. There it quickly sorts the data set into a number of seismic facies. Compressional tectonic environments, however, are prone to alter the seismic texture and can lead, when applied uncritically, to wrong conclusions with respect to the reservoir quality. When automatic seismic texture analysis is applied to compressional regimes, the results should be interpreted carefully. To avoid wrong conclusions, the seismic data set under consideration should be interpreted by a geologist before automatic texture algorithms are applied to map out potential areas of error. 

Fig. 3. Current state of the art in 3D seismic stratigraphic/facies analysis.

These seismic facies geobodies, whether they are sedimentary, structural, diagenetic, or fluid related constitute an attempt to identify, isolate and extract geological/rock physical objects or properties in one single coherent modelling entity. The system level allows interactive visualization and analysis of these geobodies and their more efficient transfer to the reservoir model. 

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