INTRODUCTION

BP Petroleum Development (UK) Limited's Magnus Field is the most northerly field in the North Sea to have been developed so far. Magnus lies mainly in block 211/12A of the British sector of the North Sea, with a slight northerly extension into block 211/7A. The field is approximately 16 km in length with an almost north-south trend (Fig 1). The development plan called for one large, fixed central production platform. A total of 16 producing well are to be drilled from the platform; in addition, seven subsea completions, about the field, are to be tied back to the platform by buried flowline and control bundles. Lying in 186 m of water, the Magnus platform is a piled steel jacket and is the largest single-piece steel platform in the world. At deck level the platform is approximately 100m square. The four corners of the platform point approximately north, south, east and west.

Fig 1 Location of Magnus Field (available in full paper)

In 1983, before the commencement of production drilling from the platform, a review of the shallow geology across the field was performed by BP Dyce. The study entailed a study of all well and seismic data that was available for the field to a depth of 1000m. (All depths in this paper refer to metres below sea-level) This depth is the approximate depth of the boundary between the Nordland group, in the shallow section (Middle Miocene to Recent) and the Hordaland group (Eocene to Early Miocene), the two groups are separated by a Middle Miocene unconformity. The Nordland group generally occupies the geological column to a depth of at least 900 m (metres subsea) and occasionally exceeds 1000m. The group consists mainly of mudstones and clays with occasional silts. More important, however, is the presence of localized, poorly consolidated sand bodies in the sequence. It was the presence of the latter that the study was particularly interested in. In the early 1970s, during the development of BP's Forties Field, similar sand bodies were found to be gas bearing and to present a threat to drilling in shallow section. Indeed, the pioneering work into high resolution seismic, in the North Sea, was carried out by BP in the Forties area to map these sand units (Lucas, 1974). The techniques put forward at this time have become the standard techniques, for rig and platform site surveys and have changed little. With the commencement of drilling plans for which the presence of shallow gas sands was important.

From the 15 well logs available for the area, it was apparent that there were two particular sand bodies that were correlatable across the field. The upper sand unit lay at approximately 450–540 m and varied from 10 to 60 m in thickness. This bed was also seen to be locally lignitic. The lower of the two sand units was typically 20–30 m in thickness and occurred in the 750–820 m interval. Other sand bodies were present in the logs; however, they did not seem to be correlatable and were through to be fairly localized.

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