The Paleocene deepwater sandstone play West of Shetland

Abstract

Following the discovery of oil in Paleocene sandstones in the Foinaven and Schiehallion fields in 1992 and 1993, exploration in the Faeroe–Shetland Basin has been very active. Most of the basin has been covered with 3D seismic surveys and numerous wells have been drilled for the Paleocene deep water play. However, success in terms of commercial discoveries has been limited. Detailed analysis suggests that the Foinaven/Schiehallion geological setting is unique in terms of hydrocarbon charge history, reservoir quality and trapping style. Oil charge is attributed to the presence of a Mesozoic high (Westray Ridge), which both focused charge and retained oil prior to remigration into Paleocene traps. The T35–T36 sequence which includes the Andrew Tuff appears to provide an effective top seal across the basin and all significant discoveries have been made in the T31–T35 fairway directly underlying it. Although reservoir sandstones similar to those found in the Foinaven and Schiehallion area have been encountered across the entire Faeroe–Shetland Basin, their relatively shallow burial depth and their position at the base of slope at Foinaven and Schiehallion appears to be unique in the basin. Several gas discoveries have been made in the Flett Sub-basin, located to the northeast of the Foinaven/Schiehallion area. The Flett Sub-basin has all the ingredients of a successful Paleocene hydrocarbon play, such as pervasive charge, an effective regional top seal, and stacks of Lower Paleocene deep-water sandstones and shales. However, trap definition is difficult: most remaining structures rely on a stratigraphic trapping component. The definition of top reservoir sand on seismic data is not straightforward, as seismic imaging techniques, including direct hydrocarbon indicator (DHI) analysis, are often ambiguous. T31–T35 sandstones in the deep Flett Sub-basin tend to be tight, although a considerable number have been found to have good reservoir properties. Prediction of areas where reservoir quality is preserved will be a main challenge for continuing exploration in the deep part of the Faeroe–Shetland Basin, including the undesignated area adjacent to Quadrant 204.