Impact of high-resolution FWI in the Western Black Sea: Revealing overburden and reservoir complexity

Abstract

The Western Black Sea (WBS) is now an area of active petroleum exploration, and several seismic surveys have been acquired to support the ongoing activity. Seismic imaging in the WBS is challenging due to the presence of complex overburden, such as canyons, shallow gas channels, and complex shallow stratigraphy. Such overburden complexities create amplitude dimming and wipeouts in the images and cause structural distortions of the reservoir units. Conventional velocity model building and seismic migration methods are not optimal for addressing such challenges. Full-wavefield inversion (FWI) technology was applied to overcome the complex seismic-imaging challenges in the WBS and to support the exploration and development program. To date, a suite of FWI products has been used to identify shallow hazards, derisk prospects, optimize well placement, perform accurate resource estimation, and support development planning. Unlike traditional seismic migration, generated FWI products are not simply depictions of the seismic reflectivity but rather are high-resolution volumes of subsurface properties, such as velocity and impedance that are directly interpretable. In contrast, FWI in most published examples is applied only on low-frequency data (typically less than 10 Hz) to enhance the velocity models needed for migration. Brute-force generation of high-resolution subsurface property volumes using FWI can be computationally prohibitive because of the high cost of the required finite-difference simulations. Many of these practical obstacles have been overcome through the use of advanced wave propagation and optimization algorithms and faster computer architectures. In the WBS, detailed subsurface property models generated with FWI have provided an accurate description of the overburden complexity and helped mitigate imaging distortions. The resulting imaging uplift has enabled improved structural definition, stratigraphic characterization of the reservoir, and quantitative interpretation. Predrill predictions using FWI volumes have shown excellent agreement with well results.