Application of Walkaway VSP for Improving Seismic Imaging of Fractured-cavernous Carbonate Reservoirs

Ordovician carbonate rocks are widely distributed in Tarim basin, western China, and they are the important yet very difficult targets in the oil and gas exploration and development. The carbonate reservoirs are ultra-deeply buried and their storage spaces are mainly secondary dissolved caves, holes and fractures of various sizes. The main challenges for seismic imaging are that the target formations are usually in very low signal-to-noise ratio due to the seismic attenuation, and the reservoir media are strong heterogeneous (caused by caves and holes) and having varying degrees of anisotropy (caused by fractures). Conventional processing techniques fail to image and identify the fractured-cavernous reservoirs accurately. In order to better image and identify the reservoirs, an 8-azimuth walkaway VSP (with a maximum offset of 6600m from the wellhead and covers the 510m-6470m depth interval with receiver spacing 10m) was carried out to estimate the true amplitude recovery (TAR) factor, Q factor and VTI anisotropic parameters, and then proper energy compensation approaches and VTI anisotropic migration algorithms are introduced in the seismic processing flow. The results show remarkable improvement in seismic imaging, and the depth errors of the geological targets are reduced considerably.