oa Compensating for Gas Wipeout Effect Through Velocity Tomographic Inversion: A Case History from Malay Basin

Tomographic inversion has been widely employed to solve for the occurrences of gas accumulation problem in seismic imaging. In Malay Basin and offshore Malaysia, there are many imaging problem related to the so-called ‘gas wipeout” effects. The occurrence of the gas wipeout zone has creates difficulties in structural tracking and estimation of the hydrocarbon resource potential for reservoirs. We present here a case history of addresses the issue of gas wipeout effect through velocity tomographic inversion and its application prior to Pre-Stack Depth Imaging processes in the South of Malay Basin. Full azimuth hybrid tomography was used to derive the sediment velocity depth model and identify velocity changes across fault, structure and gas wipeout zone. Multiple passes of gas-flood modelling and imaging allowed accurate delineation of the anticlinal structure filled with gas. The key element of this method is the use of hybrid velocity model with control from the geological structures and well velocities.<br>The workflow of the hybrid 3D tomography involves automatically performs the focusing analysis on the depth-imaged gathers and updates velocity models through a global inversion process. Focusing parameters were measured and projected along the computed ray paths from the imaged subsurface reflectors back to the surface. The new velocity updates are derived from this inversion process will be used to improve the focusing of the seismic gather and generating new traveltimes for each of the reflectors in the next iteration of Pre-Stack Depth Imaging with the updated model. After 3 or 4 iterations of the tomographic velocity updates, the velocity depth model was ready and sufficient to image the structures and gas effect in the area. The final velocity model derived from the PSDM process is a geological product that can be used to identify lithology changes, overpressure zones and importantly, changes due to the fluid content of the reservoirs. The result of the study shows that the final velocity model has given significant improvement in the imaging and interpretation of the structures within the gas wipeout area. The issue has been compensated in producing a structural image, producing image-gathers, propagating the wavefield throughout the domain and determining the velocity via migration algorithms and tomographic inversion.