3D geologic modelling of channellized reservoirs: applications in seismic attribute facies classification

Abstract

Renjun Wen, president and CEO, Geomodeling Technology, presents a new methodology for modelling stratigraphic heterogeneity in channellized reservoirs. Geological models are usually used qualitatively in seismic interpretation. This paper illustrates that quantitative representations of detailed geological models can significantly enhance seismic attribute interpretation through facies classification. When applying seismic attribute classification to reservoir facies mapping, one often faces such typical questions as: ■ Which attributes should be used as input to classification? ■ How many classes should be used in the unsupervised classification method? ■ How many levels of hierarchy should be selected in the hierarchical classification method? ■ Does the seismic facies correspond to the geological facies? ■ How can attribute-derived facies models be validated? There are no unique and easy answers to the above questions. In this study, we aim to create a more accurate representation of the reservoir by using 3D synthetic Earth models to guide seismic attribute classification. We consider a channellized reservoir for which seismic attribute analysis has proven to be very useful, but results can be difficult to interpret. The next section describes a 3D stratigraphic modelling approach for the channellized reservoir. The major channel components and parameterizations are illustrated with examples. This is followed by a summary of seismic attribute analysis and classification workflow applied to a synthetic seismic volume. Results of attribute classifications using a self-organized map (SOM) (Kohonen, 1989) and waveform correlation maps are compared in relation to different input attributes and classification parameters. The lessons learned from this synthetic example are summarized and the selection of attributes for facies classification is discussed. 3D stratigraphic models of channellized reservoirs There are several computer-based methods to build 3D reservoir model flow simulations, such as object-based methods or cell-based geostatistical methods (Dubrule and Damsleth, 2001). However, none of these methods are able to reproduce stratigraphic heterogeneity patterns at sub-seismic scale, which can be major controlling factors for fluid flow and spatial variations of acoustic properties. In this paper we report a new modelling method to generate 3D stratigraphic architectures of channellized reservoirs. The method is an extension of the bedding structure modelling method developed by Wen et al. (1998) and is being further developed in the SBED Joint Industrial led by Geomodeling Technology. The stratigraphic features within channellized reservoirs to be modelled in this study are below the resolution limit of conventional seismic data. The cell size is about 20 x 20 x 1 m3. At such a modelling scale, detailed geological features must be modelled, based on their formation process so that their 3D structures can be correctly represented in the geological model.