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In this paper we have compared the AVO analyses from four wells in the west of Shetland: Foinaven oil well (204/24A-2); Laggan gas well (206/1-2) and two prospect wells (204/17-1 and Assynt 204/18-1). We apply the spectral decomposition to the data from these four wells and provide further evidence that there are also differences in the spectral characteristics between the four wells. While Foinaven and Laggan Class III AVO anomalies are consistent theoretical predictions for frequency-dependent AVO behaviours. However, the results from the Assynt prospect 204/18-1 and another prospect 204/17-1 show rather complicated behaviour in the iso-frequency sections that cannot be fully explained. In summary, though the amplitude anomalies seen in the two prospects were originally interpreted as to be similar to the Foinaven, but our study indicated they are different in both the AVO behaviour and the spectral characteristics.

This paper incorporates the results of recent exploration wells to provide a re-evaluation and re-validation of earlier publications by the author.Following the discovery of the Foinaven and Schiehallion fields in the early 1990s, exploration success for Paleocene targets outside the Quadrant 204 area of the Faroe–Shetland Basin was initially rather limited. However, renewed interest in the last 10 years has seen 12 Paleocene exploration wells drilled, of which 9 have encountered notable hydrocarbons.Since 1972, 151 exploration wells have been drilled west of Shetland with 79 wells (52%) specifically positioned on Paleocene prospects which resulted in 23 discoveries. Analysis of the 56 failed Paleocene wells shows that around 80% were drilled on either a poor or invalid trap, with the remaining 20% failing mainly due to either lack of or poor reservoir or poor top or lateral seal. Intriguingly, only 4 exploration wells have been exclusively positioned on Paleocene 4-way dip (or structural closures) structures, with all of these encountering hydrocarbons.Fifty-six Paleocene prospects contained a stratigraphic component with 15 notable successes, all of which are within the Upper Paleocene Vaila sequence. Ten key discoveries are located in the Judd Sub-basin or adjacent Westray High (Foinaven, SE Foinaven, SW Foinaven, Schiehallion, Loyal, Alligin, Cuillin, Arkle, Amos and Tornado). A further four are located in the Flett Sub-basin (Laggan, Tormore, Torridon, Laxford) and one is in the Foula Sub-basin (Glenlivet).Forty-six of the wells were positioned on an amplitude or amplitude-variation-with-offset (AVO) anomaly, of which 16 encountered notable hydrocarbons. Following post-mortem studies, the majority of the remaining 30 failed wells could be shown to have drilled poorly defined amplitude anomalies (various lithologies including igneous), AVO artefacts or spurious direct hydrocarbon indicator (DHIs) (which include multiples). Wells 204/17-1 and 204/18-1 are good examples of poorly interpreted AVO responses, in which high amplitudes are mainly present on the nears (low offset data) and there is significantly decreased amplitude on the highs (far offset data). Moreover, post-drill AVO analyses categorically show that both wells were indeed drilled on Class I AVO anomalies.Not surprisingly, in recent years all of the wells positioned on sound AVO anomalies have been successful in encountering hydrocarbons. Moreover, these wells show both a clear increase in amplitude with offset and a conformance with structure. Undoubtedly, the recent triumphs firmly demonstrate there is potential for maintaining a high success rate in future Paleocene exploration west of Shetland. Equally, the evaluation and use of proven examples of successful traps such as Foinaven, Schiehallion, Laggan, Tormore, Glenlivet and other analogues can add value to future exploration programmes.

SUMMARY:The first section of the paper principally focuses on the analysis of four wells located west of Shetland, UK, to demonstrate that the appropriate use of AVO analysis forms a worthwhile and valuable tool for exploration. The wells analysed include two with hydrocarbons, the Foinaven oil well 204/24a-2 and the Laggan gas field discovery well 206/1-2, and two wells positioned on amplitude related prospects A second aspect to this paper centres on the application of spectral decomposition to the seismic data relating to these four wells, which provides further evidence that there are also apparent differences in the spectral characteristics between them. Both Foinaven and Laggan exhibit Class III AVO anomalies, which * Now at ExxonMobil Upstream Research Co., Houston, USA, enru.liu@exxonmobil.com), Tel. +1 713 431 7527 2 are consistent with the theoretical predictions for frequency-dependent AVO behaviours. However, the results from the 204/17-1 and 204/18-1 wells show rather complicated behaviour in the iso-frequency sections that cannot be fully explained. In summary, though the amplitude anomalies seen in the two prospect wells were originally interpreted to be similar to that in Foinaven, this study demonstrates they are different in both their AVO behaviour and spectral characteristics.

The West of Shetland area has scope for the stratigraphic entrapment of hydrocarbons at various Jurassic to Palaeogene stratigraphic levels. Mapping and identification of such traps requires a fundamental understanding of the regional geology, the study of analogues and source kitchens, and a thorough approach to trap validation. Since 1982, 47 exploration wells have been positioned on Paleocene prospects with a significant stratigraphic component, but few have found hydrocarbons — many failing as a result of poor trap definition and overconfidence in the predictive use of amplitude anomalies. Hydrocarbon sourcing of many of the failed prospects was also poorly constrained. Few amplitude-related stratigraphic features could be tied with confidence to a viable source kitchen. The presence of a regional seal is a prerequisite ingredient for a successful Paleocene play. Many remaining undrilled, subtle prospects rely on a stratigraphic trapping component, and high-quality 3D seismic data are seen as an essential search tool. Examples of undrilled prospects are presented from the Paleocene of the northern Faroe-Shetland Basin and the Mesozoic of the East Solan Basin and Corona Ridge.