In 2004/5 NAM acquired a proprietary long-cable survey with 6 km streamers in the Dutch offshore, the first 3D survey to be shot in the Southern Permian Basin with these streamer lengths. The long-cable acquisition geometry was deemed necessary because step change improvements in imaging was required to support additional exploration drilling. The quality of the seismic image is crucial to optimally leverage Shell’s DHI enhancement technique called CTD stacking. This technique was specifically designed for the Rotliegend stratigraphy. For the discovered fields, CTD stacks of the vintage data delivered a high success rate (70%) in estimating the encountered gas water contact (GWC). For the remaining prospect portfolio, however, the approach provided less conclusive results. Improved data quality was considered essential to achieve a similar success rate. The combination of long-cable 3D data and CTD stacking technology had significant impact on our business: • We were better able to image deep Rotliegend fault blocks • We obtained better structural definition underneath salt ridges • We achieved more consistency in the DHIs and were therefore better able to rank the portfolio


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