Seismic penetration problem in the Wadi field, Sirte basin, Libya

Abstract

The Wadi Field is located within the north central part of the Sirte basin, Libya. Over the past few years, Sirte Oil Company of Libya has acquired several seismic surveys in this area. The latest survey was an experimental line shot in 1995, with five heavy vibrators sweeping over the bandwidth 8-40 Hz and 720 recording channels. The objective was to obtain interpretable information beneath the top Upper Cretaceous horizon (top Kalash) which is generally the lowest horizon visible on the processed seismic sections. The primary exploration target is the Nubian formation within the Lower Cretaceous. The acquired dataset was processed by COG in 1996, and subsequently reprocessed by me as part of this project. Some improvement in the static corrections is evident in the reprocessed data, but neither processing sequence was successful in delineating the oil-bearing structure within the Lower Cretaceous. The rest of the project was an investigation into the reasons for the lack of signal penetration beneath the top Kalash horizon. Synthetic seismograms were generated using sonic log data from well D7-NC149 to understand the effects of the primary reflectivity, transmissivity, and multiples on the primary reflection events in general and on signal returns from the target horizon in particular. The behaviour of the reflectivity at the top Kalash and the top Nubian horizons and the transmission losses within the geological sequence overlying these horizons were examined in both time and frequency domains. Spectral analysis was conducted for 64-sample windows around the two-way travel times of both horizons in order to understand the effect of each factor separately.