Borehole seismic profiles in the Ekofisk Field

Abstract

In October 1983 a major borehole seismic survey was carried out in the Ekofisk oil field in the Norwegian sector of the North Sea on behalf of the Phillips Petroleum Licence 018 group of companies. A vertical seismic profile with the source vertically above the geophone in the highly deviated wellbore and three multilevel walk‐away borehole profiles were acquired in an area showing very poor surface seismic returns owing to gas charging in the overlying sediments. The processing of the data through to a series of conventional common‐midpoint sections has permitted detailed interpretation of the top of the Ekofisk formation and the top of the Tor formation apart from well control. Both formations are producers separated by a tight zone. The Tor formation is the primary horizon to waterflood. Information as to its lateral continuity is important in the location of proposed waterflood injector wells. Prior to the survey, the field was considered effectively unfaulted. An apparent graben lying subparallel to the borehole was detected by the surveys. Reflections from below the reservoir formations are evident. A byproduct of the survey is strong evidence for the existence of lateral velocity gradients or apparent transverse velocity isotropy associated with the overlying gas‐charged sediments. Subsequent to the oral presentation of this paper, the graben was drilled and found to have a 41 m fault throw, in its western flank.