PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Thirty-Third Annual Convention & Exhibition, May 2009 INTERPRETATIVE PSDM WORKFLOW FOR SOLVING COMPLEX IMAGERY BELOW FAULTS AND CARBONATES REEFS IN SOUTH MAHAKAM, KUTAI BASIN: AN INTERPRETER'S POINT OF VIEW

Being located structurally in a transition zone between the Paternoster high platform and the Kutai basin, the study area is strongly compartmentalized. Lateral velocity variations across faults with very significant throws induce severe fault shadow effects below fault planes. Pinnacle carbonates overlaid by shale produce pull up anomalies above and seismic image degradation below. In this context, depth imaging is crucial for optimizing field development and reducing uncertainty. An interpretative workflow of Pre-stack depth migration (PSDM) has been performed on the South Mahakam seismic dataset. The objectives are to improve fault imagery and seismic imaging below carbonates. The main difference with time imaging lies in the early integration of interpretation in the velocity model building. Therefore interaction between the interpreter and the depth imager is indeed critical to build a correct depth-velocity model that will noticeably improve the seismic image. Velocity model building has been iteratively done using seismic interpretation surfaces as main inputs and also by incorporating well data. An automated tomography approach is used to update the velocity model, which is strongly constrained by the interpretation input. However, some strong lateral and vertical velocity contrasts (carbonate reefs, pinch outs, faults) have to be manually picked in order to help tomography to converge towards an adequate velocity model. The final velocity model has to meet 3 criteria: 1. flatness of gathers, 2. geological meaning, and 3. honour well data.