Azimuthal analysis of reflectivity for fracture characterization: Rock physics modeling and field example

Abstract

Aligned vertical fractures can produce anisotropy in the shallow crust that can be observed in seismic data, such as amplitude variation with angle of incidence and azimuth (AVAZ). AVAZ can be a useful technique for determining the main fracture orientation, fracture density and sometimes the type of fluid in the fractures. The azimuthal variations in the P‐wave reflections have been observed in field data by different authors, such as Lynn et al. (1996), Grimm and Lynn (1997), Mallick et al. (1998), Teng (1998), Perez et al. (1999), Shen et al. (2002). One important advantage of this method is that it can provide localized information at the interface of interest, unlike the travel time methods, which give global estimates. Also, if the reservoir is very thin, travel time techniques are not very useful for fracture characterization This paper presents the analysis of the PP reflectivity (Rpp) from a 3D seismic data set acquired over a gas reservoir, represented by a fractured carbonate in Eastern Texas. The final goal is to determine the main orientations of the fractures and their spatial densities. We also present the results of our site‐specific rock physics fracture modeling, based on the well‐log data. The AVAZ modeling shows that the polarity of the azimuthal variation of the reflectivity changes with the fluid and fracture compressibility. Therefore, rock physics modeling is a key link between the observed seismic amplitude variation with azimuth and the actual fracture parameters, such as fracture strike and density. Based on the results of the rock physics modeling, we derive the mean fracture orientations from the seismic data using a bootstrap method. By using this technique we can quantify the uncertainty in fracture orientation due to the possible measurement errors. We find an excellent agreement between the fracture orientation derived from the azimuthal variation of the seismic amplitudes at far offsets and the fracture orientation derived from the FMI logs from the nearby wells. There is also a very good agreement between the mean fracture strikes from the azimuthal variation in the seismic amplitudes and the present regional stress field. The mean fracture orientation is parallel to the maximum horizontal stress in the region.