Improving the Imaging of Pre-Messinian Reservoirs in the East Mediterranean Sea, Offshore Egypt, Using Converted Wave Attenuation, Full-Waveform Inversion and Reflection Tomography

Abstract

The offshore eastern Mediterranean region has received increased international interest in the last decade for its hydrocarbon potential in the pre-salt traps. The presence of a heterogeneous Messinian-age salt layer and complex pre-Messinian structures pose very difficult challenges in seismic imaging. In this paper, we provide a detailed workflow for seismic data preconditioning and imaging which resolves the subsurface challenges of the Mediterranean. Broadband acquisition was used to collect seismic data, which combines the responses of dual-sensor receivers to remove the effect of the receiver ghost. Adaptive source de-ghosting was then applied to address the source-side ghost. Data was processed using robust multiple attenuation and converted wave attenuation (CWA). A high-resolution velocity model building and imaging workflow was designed as follows: Diving waves full-waveform inversion (FWI) to capture detailed velocity for the complex overburden, followed by post-salt reflection tomography. Born modeling-based reflection FWI to update the velocity heterogeneities inside the salt body followed by reflection tomography for the deep section. Reverse time migration (RTM) to handle the waveform multi-pathing. De-ghosting corrected the wavelet phase and expanded the usable frequency bandwidth, resulting in a broadband dataset for imaging. Robust multiple attenuation and converted wave attenuation (CWA) techniques aided in revealing the true geological dips beneath the salt and facilitated picking accurate residual move-outs during the velocity model building. RTM in conjunction with the high-resolution velocity model significantly improved imaging of complex salt structures and pre-salt reservoirs. At well locations, our workflow resulted in a very good match between the available well data and surface seismic in terms of markers depths and velocity trends. This paper presents a novel approach for modelling the velocity heterogeneities inside the complex Messinian-age salt formation using the Born modeling-based reflection FWI. In addition, salt-related strong converted waves were successfully attenuated, whereas previously the presence of this energy misled interpreters and caused anomalous velocity updates in similar geological settings in the Mediterranean.