Using azimuthal P-wave AVO to determine the principal fracture orientations in the Austin Chalk, Gonzales County, Texas

The Austin Chalk is a naturally fractured reservoir with predominantly vertical aligned fractures. In this study, we use a practical method to determine the principal fracture orientations in the Austin Chalk in Gonzales County, Texas. The database consists of nine 2-D P-wave seismic lines of different azimuths. The raw seismic data was processed through a sequence that preserved the relative changes of amplitudes with offset. The AVO gradient at every CDP in a seismic line was calculated. Then, the median AVO gradient of all the CDPs in a seismic line was chosen to represent AVO of the whole line. This procedure was repeated for all seismic lines. Well log data indicates that the Austin Chalk and its underlying formation, the Eagleford Shale, are fairly homogeneous in the study area. Therefore, any variation among the median AVO gradients of lines with different azimuths is interpreted as an effect of the fractures. Different combinations of the nine median AVO gradients and their corresponding line azimuths were used to find the azimuth of fracture symmetry axis. The resultant solutions clustered about two, nearly perpendicular, azimuths N58°E and S31°E. The ambiguity of the solution is inherent to this method and extra information is needed to resolve which of these two azimuths is the fracture orientation. Other non-seismic data indicates the presence of a predominant NE-trending fracture set in the Austin Chalk in the study area. Therefore, the N58°E direction is the fracture azimuth in the study area. This result agrees with the results of other studies in and around the study area using different methods.