Depth Imaging beneath a Seismic Obscured Area (SOA) and Analyzing Its Character - Eldfisk Field Experience

To overcome the challenge of imaging beneath a previously seismic obscured area (SOA) ConocoPhillips(CoP) and its partners used depth imaging utilizing OBC data with converted waves on the Eldfisk field. The imaging challenge is caused by overburden effects predominantly in the Miocene interval. Laterally compact low velocity streaks combined with compartmentalized and overpressured thick shales distort the wave field traveling through it. In 2006/2007 CoP and its co-ventures reprocessed the 2001 OBC data with emphasis on improved multiple attenuation, converted wave preprocessing and depth imaging to uplift the reservoir image within the SOA area. This resulted in a significant reduction of the SOA area (conservative ~30%) for the pressure data and a much improved converted wave image of the reservoir compared to previous results. The velocity field used in depth imaging was consistent with the expectations of a sharp lateral transition and exhibited both extremely low velocities (1450m/s at around 1.8 km depth) and sharp lateral transition (up to 30% reduction for 1000m lateral distance). The workflows and techniques were novel with respect to the role detailed interpretation played in the constraint tomography and how synthetic modeling helped to understand the imaging problem at hand.