The Judy Field pre-Cretaceous reservoirs produce from a series of tilted fault blocks that form a complex northwest-plunging horst in Block 30/7a, UK Central North Sea. The primary producing reservoir of this field is the Middle to Late Triassic Joanne Sandstone Member of the Skagerrak Formation with secondary contributions from the Middle Triassic Judy Member of the Skagerrak Formation and the Late Jurassic Fulmar Formation. Fluid properties and contacts vary throughout the field. Currently the field is producing from ten development wells.Based on core studies, the depositional environment of the Triassic Skagerrak Formation has been identified as ephemeral fluvial while the Jurassic Fulmar Formation is transgressive shallow marine. Sedimentological interpretation of core data from seven wells in the field indicates that the Joanne and Judy Sandstone Members comprise four major facies associations: fluviatile channel sandstones, sheetflood sandstones, argillaceous sandstones and lacustrine mudstones. The complex architecture of these four facies associations makes it difficult to accurately predict absolute reservoir productivity without recourse to long-term production tests. Although the reservoir complexity was appreciated from an early stage in the field development, the initial reservoir models proved to be somewhat simplistic and overestimated the length of the production plateau.Anew, longer offset 3D dataset was acquired in 1999 to replace the original pre-development 1986 3Dseismic volume. The 1999 data were designed to enhance the imaging of the pre-Cretaceous reservoirs and help define areas of additional potential, which could be developed through new high angle platform wells. Mapping of reflectors below the Base Cretaceous Unconformity with the newdata remains problematical due to poor signal to noise ratio and short period multiples. Despite these difficulties, a revised interpretation of the crestal portion of the Judy structure was completed. This new interpretation was coupled with the results from a detailed review of the geochemical, pressure and fault seal data, to create a petroleum charge model which accounted for the variations in fluid types and contacts throughout the field and identified three fault blocks (30/7a-P16, P17 and P18) in the crestal portion of the field for infill drilling. The fault blocks were successfully drilled in 2001 and 2002 despite significant drilling challenges posed by depleted reservoir zones above the target formation.

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