Top Reservoir Mapping Problems in the United Kingdom Sector of the North Sea
Authors: Moore Jack W. 
Published: December 2007

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Abstract
In certain areas of the U.K. North Sea, it is not possible to develop a seismic structure map on top of the reservoir due to the absence of a reliable seismic reflector at this level. The high cost of drilling in North Sea waters precludes multiple well strat test programs to overcome this seismic deficiency and other techniques must be employed to evolve a top reservoir map. One such technique, which is used in the Leman and Indefatigable Gas Fields, consists of adding an isopach map to the seismic structure map contoured on the reflector nearest the reservoir. Details of this approach and the uncertainties arising from the method are discussed.

Introduction
Reserve estimates, development planning, and economic calculations require an accurate and definitive estimate of reservoir geometry. Normally the major portion of this geometry is determined from a seismic map adjusted to the top of the reservoir and supplemented with the results of appraisal and development drilling programs. The size of the appraisal drilling program usually is a function of the quality of the seismic data with larger appraisal programs being required to supplement poor seismic resolution. The optimum solution to reservoir delineation is the development of a reliable map on top of the reservoir unit using only seismic data. Such a map would minimise the appraisal programmes and the pre-development programmes and the pre-development expenditures.
Direct mapping of the top of the reservoir in portions of the gas sector of the southern U.K. North Sea is not possible. In the Leman Gas Field, the lowest reliable seismic reflector is found approximately 1000 feet above the Rotliegend (Permian) sandstone reservoir. The situation is similar in the Indefatigable Gas Field where the lowest seismic reflector is found about 200 feet above the Rotliegend reservoir. A structure map on top of the Rotliegend is developed in both fields by adding a geological isopach map of the interval between the lowest seismic reflector and the top of the Rotliegend sandstone to the seismic structure map on the lowest reliable reflector. It is apparent that the resulting Rotliegend structure map then becomes dependent on the configuration of the geological isopach which is developed using only limited subsurface data from wells.