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ABSTRACT: Reservoir imaging in a carbonate environment: P and S-waves

Urosevic, Milovan, Miroslav Brajanovski, and John A. McDonald , Curtin University of Technology, Perth, Australia

Seismic imaging of carbonate reservoirs has always been a difficult task. These reservoirs are typically found in a structural area where secondary porosity is likely to be developed. Low P-wave reflectivity sequence are yet another characteristic of such environments. We show an example where the imaging problem was particularly difficult because both the reservoir and the sealing rocks are of predominantly carbonate composition. The difference between seal and reservoir is a function of the degree of fracturing; the sealing carbonates being compact rock unit, whilst the reservoir carbonates are fractured.

This study presents the results of extensive tests performed in such an environment using two multi-offset three-component VSP data sets. Initially, the overall elastic properties of the carbonates down to the target depth were estimated. These results combined with geophysical log information were used in isotropic and anisotropic pre-stack depth imaging (PSDM). The P-, P-to-S and S-waves sections were analysed and compared. Superior images of the reservoir rock were produced with S-waves when using raw single shot common-receiver gather. A good quality structural image was obtained with P-waves only when all the shot gathers from both VSP surveys were used in PSDM. Joint P- and S-wave interpretation of the results is presented.
