Success in exploring for reliable, robust Paleocene traps west of Shetland

Abstract

Abstract 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.