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Abstract The oil-bearing, fractured granite basement rocks are a very important and complicated hydrocarbon reservoir in offshore Vietnam. However, the poor S/N ratio and imaging of these steeply dipping fractures by conventional Kirchhoff migration are quite difficult to interpret for exploration. The recent development of Controlled Beam Migration (CBM) has generated a much clearer fracture image inside the basement, but the velocity inside the basement remains an issue. The imaging velocity field needs to be much slower than the well sonic velocity to produce a decent fracture image. With the help of the CBM stack sweeping technique, we have developed a new model building flow which takes into account the complex TTI/HTI anisotropy due to the micro-fracture system inside the basement. In this flow, the azimuthally dependant velocities for fractures are first picked through CBM stack sweeping; they are then compared with the well velocity trend to estimate the TTI/HTI anisotropy parameters. The azimuth angles are further updated by CBM stack sweeping with different azimuth angles. The resultant seismic velocities much better fit the sonic velocity logs. Also, the migrated section has much less migration swings from multiples, and the imaging of the major fault/fracture system is enhanced. Azimuth angles of the TTI (velocity) model and the improved fracture imaging provide the much needed information for the "From Seismic interpretation to Tectonic Reconstruction" methodology, which has been proven to be effective in exploration in the Cuu Long Basin for the last five years (Ngoc et al., 2010). The existence of TTI/HTI anisotropy inside the fractured basement is demonstrated through this case study. We have presented a new velocity modelling flow which can estimate the azimuth information for fracture systems and improve the imaging quality inside the basement. |
