The Schiehallion development is one of the largest on the United Kingdom Continental Shelf in the last ten years, with mean recoverable reserves of 425 × 10 6 BBL. It is the second development West of Shetland, following the Foinaven Field. The Schiehallion development comprises two fields; Schiehallion and Loyal, which are located approximately 200 km west of the Shetland Islands and a similar distance northwest of the Orkneys in water depths of 300–500 m. The Schiehallion Field was discovered in late 1993 by Well 204/20-1 and side-track -1Z. Oil is trapped in submarine slope reservoir sandstone of Paleocene age. The discovery well penetrated an oil–water contact at 2064 m TVDSS. The field extent was mapped on 3D seismic data through the first half of 1994. This was followed by the drilling of five further vertical appraisal wells in 1994–95, three operated by BP/Britoil and two by Amerada Hess. In 1995, a horizontal well, 204/20-5, was drilled and subsequently tested over an extended period. The Loyal Field, to the north of Schiehallion, was discovered in late 1994 by Well 204/20-3 which encountered an oil column in sandstone of Paleocene age. The well was successfully side-tracked into a second, adjacent oil-hearing sandstone. An oil–water contact was penetrated at 2397 m TVDSS. A second well, 204/20-6A, drilled in 1995, confirmed the presence of oil at the crest of Loyal in a second separate segment with an oil–water contact inferred at 2160-m TVDSS. The mapping of net pay thickness directly from 3D seismic underpins the reservoir description of the Schiehallion and Loyal fields. Vertical appraisal wells were targeted to calibrate the net pay model and reduce reservoir uncertainties. This enabled a rapid appraisal of the key elements of the static reservoir description. Seismic attribute mapping and detailed seismic facies mapping, combined with sedimentological analysis of the cores and borehole imaging electric logs, defined the channelized architecture of the reservoirs. Horizontal Well 204/20–5 and subsequent extended well test (EWT) were designed to answer questions on the dynamic performance of the reservoir, particularly the connectivity and productivity of the sandstone reservoirs. This well was completed in the summer of 1995 after penetrating a gross reservoir length of c . 1700 m. The subsequent EWT achieved a stabilized, constrained flow rate of c . 18000 BQPD and demonstrated the productivity and good reservoir connectivity of the field. The well was suspended as a future producer. The development of the two fields was sanctioned in 1996. Development drilling began later in 1996 and oil production commenced on schedule in July 1998. The fields will be exploited using a floating production, storage and off-loading (FPSO) vessel which is jointly owned by the field partners (BP Exploration (Operator), Shell UK Exploration and Production, Amerada Hess Ltd, Murphy Petroleum Ltd, OMV (UK) Ltd and Statoil (UK) Ltd).