Pre-stack Depth Imaging In Southern Caspian; A Step-wise Approach For Complex Imaging

The prospective Southern Caspian Basin is an active area for hydrocarbon exploration and development. The large anticlinal structures typical of the area have varying degrees of dip and an overburden with generally smoothly varying velocity gradients. Target depths are between 3-7 km and optimum imaging and positioning of the structures in depth is necessary for reserve estimation and well prognosis. Cost effective and timely depth migration is required for imaging. We have taken a two-step approach to 3D depth imaging in this structurally complex basin. In the first step, we depth migrated the common offset data with a single V(z) velocity function. The residual errors computed from the migrated common image gathers were used as input to 3D tomography to build a spatially varying velocity model in depth. These residual corrections were also used to obtain an optimized stack of the V(z) migrated gathers. The optimized stack was later demigrated with the same V(z) velocity function and subsequently re-migrated poststack using the spatially varying V(x,y,z) velocity model derived from 3D tomography. At the second step, using the newly constructed V(x,y,z) model, we migrated the same offset data using a prestack Kirchhoff algorithm. This integrated methodology yields a cost effective, stepwise approach to provide optimum depth imaging in this complex region where a priori information on subsurface features is limited. Results show that the prestack Kirchhoff migration based on spatially varying velocity has better focusing in complex areas; however, the overall difference between images from the two methods is minimal.