Fault Shadow Correction by Advanced Tomographic Velocity Model Building
Fault shadow has been a challenging issue for seismic imaging. The presence of faults above target could induce fault shadow; manifesting themselves as pull-up and sagging onto target horizons. In case the faults are part of the prospect structure, the shadow zone could also hinder a correct structural interpretation. False structures could be created on seismic in the footwall of fault as a result of velocity error across fault plane. The velocity contrast across the fault plane could not be restored from RMS velocities with Dix-based equation. The lack of correct velocity contrast in the depth imaging process causes non-hyperbolic move-out and kinematic distortion that deteriorates residual curvature analysis. Conventional reflection tomography inversion for fault shadow seems inadequate. Eliminating fault shadow can de-risk hydrocarbon exploration and enhance appraisal activities. Various techniques and workflows have been proposed to tackle the issue. Fault Constrained Tomography (FCT) incorporates interpreted fault planes into the reflection tomographic inversion. Diving wave based Full Waveform Inversion (FWI) can provide high-resolution velocity field than reflection tomography, but at limited depth. FWI based on reflected signal can reach greater depth, but has not yet been proven effective in production. FWI may not be a practical solution for routine production due to cost and other constraints. In this paper we propose a novel method called Coherency Guided Dip Constrained Tomography, where an additional term besides residual move-out (RMO) was added into the cost function involving the dip of kinematically migrated coherent events. A “coherency cube” generated from a depth-migrated volume portrays faults and other discontinuities. The coherency cube can guide the reflection tomography inversion to locate p-wave velocity perturbation in the right place. This results in a velocity model with proper velocity contrast across the fault planes. When depth migrated, the fault shadow can be resolved. This approach has been proven successful in a recent seismic survey in Gulf of Moattama, offshore Myanmar. The extensive polygonal networks of normal faults that extending to great depth makes other alternative approaches (FCT and FWI) impractical. As a result of the method we proposing in this paper, proper seismic image has restored non-geological undulations on target horizons at approximately 4100m below MSL. The common imaging gather (CIG) quality is improved with non-hyperbolic move-out curvature minimized. Target horizon reflection continuity also improved. Comparing to the existing techniques, the proposed method overcomes the shortcomings of various other methods and can prove to be a practical and effective alternative to tackle fault shadow issues for velocity model building and pre-stack depth imaging.
AAPG Datapages/Search and Discovery Article #90336 ©2018 AAPG Asia Pacific Region, The 4th AAPG/EAGE/MGS Myanmar Oil and Gas Conference, Myanmar: A Global Oil and Gas Hotspot: Unleashing the Petroleum Systems Potential, Yangon, Myanmar, November 13-15, 2018