oa Unmasking the Crest, Imaging Below Shallow Gas Using Prestack Q Depth Migration, Irong Barat Field, Malaysia

Seismic imaging of many oil and gas fields in Peninsular Malaysia is degraded by amplitude and frequency attenuation (Q) associated with shallow gas. In addition to the amplitude attenuation, seismic travel time delays caused by abnormally slow velocities associated with these shallow gas accumulations cause time structure distortions in the seismic image. These amplitude, frequency, and seismic traveltime distortions often obscure the seismic image sufficiently that seismic reservoir characterization is impossible. In some areas the seismic image is completely distorted and lost, in these situations conventional marine seismic P wave imaging is not an effective tool. However in other areas the seismic image is degraded, but seismic energy is still transmitted through the shallow gas attenuator. In these situations seismic processing technology can attempt to compensate for the image degradation. Using a Prestack Depth migration engine, we have developed a processing technique to recover the amplitude and frequency loss below these shallow gas zones. This technology uses an integrated Q model, velocity model, and depth imaging system to correct and restore the amplitude, frequency and velocity distortions associated with the shallow gas attenuators.<br>Initial implementations of prestack Q depth migration have shown that a detailed velocity and Q model of the subsurface is not the only requirement for a successful project. Since Q modeling requires the data to be true amplitude, most conventional approaches for noise attenuation and acquisition footprint removal cannot be applied prior to imaging. This requires the development of special noise mitigation techniques; otherwise the increased noise level can overwhelm the final imaging results. In addition, incorporation of the well data offers clues to the rock physics issues associated with the Q and velocity distortion. ExxonMobil recently completed the first prestack Q depth migration project in Malaysia over the Irong Barat field. A recently signed Production Sharing agreement for this mature field gives ExxonMobil and partner PCSB production sharing rights to 2033. Improved seismic imaging is part of the strategy implemented to recover remaining oil reserves for this mature field. The 142 square kilometer Irong Barat 3D<br>survey was acquired in 1998. The survey combines conventional marine data with OBC undershoot of the platforms, which adds processing complexity. Image quality on the flanks of the structure is very good, however varying degrees of shallow gas associated distortion are present over the crest of the structure and therefore Irong Barat was considered a good test for the Prestack Q Depth Migration technology. This paper will describe the seismic processing lessons learnt and present the results of the project.