The west of Shetland area has scope for the stratigraphic entrapment of hydrocarbons within the Paleocene Vaila Formation. 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, 48 exploration wells (Fig.1) 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. An analysis of the 48 Paleocene exploration wells which specifically targeted stratigraphic traps has helped develop a model for explaining the success rate of about 1 in 5. Two key questions are, by using hindsight, how many wells can be said to have actually tested valid traps, and are there ways to improve exploration success for stratigraphic traps in the future? Stratigraphic and combination traps west of Shetland have proved most successful in the Foinaven area and in the Flett Sub-basin (Fig. 1). The presence of a regional seal and a favourable sandstone-shale ratio are significant positive ingredients for a successful Paleocene play. Many remaining undrilled features rely on a stratigraphic trapping component and good top reservoir definition. Hence, higher quality and better-imaged 3D seismic data are seen as essential tools in the search for these subtle traps. Typically, the basin margins and the flanks of structures provide favourable areas west of Shetland to begin the search for potential pinch-out plays. Successful wells located close to or at the basin margins include seven discoveries in the Foinaven Sub-basin (Foinaven, South-east Foinaven, Schiehallion, Loyal, Alligin, Cuillin and Arkle) and a further three in the Flett Sub-basin (Laggan, Torridon and Laxford). The Flett Sub-basin discoveries all lie immediately west of the Flett Ridge. Most of the discoveries have a north-westerly structural dip and are sealed up-dip by an E-W or NE-SW fault in combination with stratigraphic pinch-out of the Vaila Sandstones. Of the 37 failed wells, 27 (73%) were positioned too far updip to trap hydrocarbons, and 10 (27%) were positioned downdip of any trapping potential (Fig. 2). Quite surprisingly, none of the failed wells are considered to have tested what constitutes a valid stratigraphic trap. 39 wells were positioned on an amplitude or AVO anomaly. Of these, nine encountered notable hydrocarbons. Following post-mortem studies, the majority of the 30 wells that failed to find hydrocarbons could be shown to represent poorly understood amplitude anomalies (various lithologies including igneous), AVO artefacts and spurious DHIs (which also include multiples). A large number of failed wells were positioned on interpreted AVO or high amplitude features believed to coincide with the termination or up dip limit/pinch-out edge of a sandstone interval. Furthermore, work carried out by most companies on these features implied that a hydrocarbon accumulation was present. For a number of failed cases, the cause of the AVO or high amplitude features was misinterpreted.
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