High-resolution impedance estimation using refraction and reflection FWI constraints: the Fortuna region, offshore Equatorial Guinea

Authors: Ian F. Jones, Jeet Singh, Stuart Greenwood, Johnny Chigbo, Phil Cox and Colin Hawke
Journal name: First Break
Issue: Vol 36, No 11, November 2018 pp. 39 - 44
DOI: 10.3997/1365-2397.2018006
Language: English
Info: Article, PDF ( 7.06Mb )
Price: € 30

Summary:
Seismic imaging and reservoir characterization in the Fortuna region, offshore Equatorial Guinea, is beset with various geophysical challenges related to the presence of extensive, but small-scale low-velocity gas pockets, which give rise to significant and cumulative image distortion at target level. This distortion had not been resolved in a vintage 2013 broadband pre-stack depth migration project, as the velocity model was not sufficiently well resolved, but was subsequently addressed successfully in a project conducted using high-resolution non-parametric tomography with improved broadband deghosted data. The primary objective of that subsequent project was to improve the understanding of the internal structure of the Viscata and Fortuna reservoirs, and this objective was met via clearer internal imaging of these reservoir intervals and the overlying gas-charged sediments. The follow-on work considered here deals with the use of full waveform inversion to further delineate small-scale velocity anomalies associated with the highly compartmentalized reservoir units, and also to use the full waveform inversion velocity model as a constraint during acoustic impedance inversion. We compare the results of impedance inversion using both a conventional approach (with the well-log velocities to build the background trend), with a new approach using a trend derived from high-resolution waveform inversion velocities.