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Exploration Outside of the Mute Zone; the Geophysics of a Billion Barrel Discovery in the Mature North Sea

Lie, Jan E.*1; Danielsen, Vidar 1
(1) Lundin Norway, Lysaker, Norway.

The area around southern part of the Utsira High in the Norwegian sector of the North Sea has been a repeated target for exploration ever since the first well was drilled in 1967. Four substantial discoveries have been made on and around this part of the Utsira high with Lundin Norway as operator the last few years. The combined resource potential of the Luno, Tellus, Avaldsnes and Apollo discoveries are reported to be in the order of 800-1600 MMboe.

This area has proven to be very challenging to image by the use of conventional streamer seismic as the Upper Jurassic sand stone- and weathered Granitic Basement Reservoir is situated directly below a thin high velocity Chalk layer. To diagnose the seismic imaging problem an OBC data-set was acquired over the Luno Discovery.

Conventional coherency based seismic velocity estimations below the Chalk was difficult due to the lack of coherent reflectors. Tomographic inversion of the refracted energy gives valuable information about the sub-chalk seismic velocities and allowed us to distinguish between weather granitic reservoir and un-weathered basement. Lessons learnt from the OBC dataset were transferred to the larger-scale Geostreamer acquisition and interpretation. The dual-sensor GeoStreamer allows for elimination of the effect of the receiver Previous HitghostNext Hit (free surface Previous HitreflectionNext Hit).

Marine seismic acquisition is going through a major step change with regards to imaging quality due to the improved ability to handle the receiver and source Previous HitghostTop. Lundin Norway recognized this ‘deghosting’ step change very early, and has taken advantage of these developments. We have acquired benchmark test-profiles with the competing ‘de-ghosting’ acquisition techniques. Direct comparisons between OBC, conventional streamer, GeoStreamer (PGS) and Broadseis (CGGVeritas) data will be shown across the Utsira High.

The deep towed ‘de-ghosting’ streamers enhance the low frequency bandwith which has resulted in a better seismic image with improved standout of the lithological units especially in areas with low signal to noise ratio.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California