No abstract

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.

The Jade Fieldi sl ocated inBlock 30/2ci nt he UK CentralNorthS ea andwasdiscovered byw ell 30/2c-3in1 996. Itw ast henappraised byw ell 30/2c-4 in1 997,withfi rst production in2 002. The decision to develop the fieldafteronly oneappraisalwell led to adegree ofuncertainty inthe fieldreservesandestimated production profiles. Consequently,the development programmewasdesigned to include anelement off urther appraisal. The development planincluded simultaneous drillingandproduction operationssoreservoirpressure depletion,dueto production,wasrecognized asasignificant projectchallenge.Detailed contingencyplans were formulated to ensurethatwell objectiveswereachieved, despitethe potentially problematic reservoirconditions. The fieldproducesgasandc ondensatef rom the Triassic Joannea ndJ udySandstonesandi sclassified asa high-pressure, high-temperaturefield, withawetgas-in-place estimatefor the JoanneSandstoneofaround1000 BCF andfor the JudySandstoneof150BCF.Thispaperoutlinesthe petroleum geologyanddevelopment history ofthe fieldandthe current understandingofthesecomplexreservoirs.

There is considerable diversity in petroleum type within the Judy and Joanne Fields of the Central Graben. Superficially, the three major reservoir systems can be considered to contain the following fluid types: gas-condensates in the Pre-Cretaceous, undersaturated black oils in the Chalk and gas-condensates in the Palaeocene. Reality is, however, quite different.This paper presents the results of several geochemical studies undertaken in the area; oil/condensate analyses to identify differences in maturity and source input, a pressure data review, high resolution GC fingerprinting, strontium isotope analyses to investigate reservoir connectivity/compartmentalization, and 2D basin modelling to determine the timing and extent of petroleum expulsion and migration. These components have been synthesized to produce a composite petroleum charge model which adequately explains the differences observed. This, in turn, enables an assessment to be made of the likely impact, if any, on field development. Furthermore, the petroleum charge model can be applied to predict the charge risk, fluid type and likely petroleum-water contacts in untested parts of the field and in the immediate vicinity.The results of recent development wells are reviewed in the light of the charge model. Subsequent reservoir geochemistry studies have confirmed the validity of the model and highlight additional applications for reservoir management.