Three-Dimensional Seismic Applications in Interpretation of Dunlin Field, U.K. North Sea

Abstract

Summary In 1977 and 1978, a seismic survey was shot over the Dunlin field and processed using the technique of three-dimensional (3D) migration. Earlier seismic control together with appraisal drilling had proved that development of the field would be commercially viable, but the reservoir configuration was poorly defined. The 3D survey achieved its objectives of increasing confidence in the structural interpretation and providing a firmer basis for field development planning. planning. Introduction Dunlin field was discovered in 1973 by the drilling of exploration Well 211/23-1, which was drilled jointly by the Shell/Esso and Conoco/Gulf/British National Oil Corp. (formerly National Coal Board) groups to test a structure lying within Blocks 211/23 (Shell/Esso) and 211/24 (Conoco/Gulf/BNOC). Tests in this well showed the reservoir to have high potential for profitable development if reserves were potential for profitable development if reserves were proved to be large enough and if it could be proved to be large enough and if it could be developed efficiently. Further appraisal drilling revealed unforseen structural complications, and seismic surveys conducted before 1974 provided only a general outline of the field.Despite the lack of detailed reservoir delineation, the well results were sufficiently encouraging to justify unitizing the field as mapped and beginning plans for development of the field from a centrally plans for development of the field from a centrally located platform. The five unit partners, with Shell appointed as operator, agreed to the development of the field with the order for construction of a 48-slot concrete platform with planned throughput capacity of 150,000 B/D. Before platform installation in 1977, a seismic survey was shot over the center of the field and processed using 3D migration, a technique successfully applied to marine seismic surveying only in the last few years. The survey was extended in 1978 to cover the entire field.The objectives of the 3D survey were to increase confidence in the geological model developed for the field and provide better data for mapping the reservoir, thus optimizing field development.This paper will describe the general characteristics of the Dunlin field, explain the problems encountered in mapping the reservoir, and briefly discuss the 3D seismic survey and the results of its interpretation. Dunlin Field Dunlin is one of several related hydrocarbon traps located in the U.K. portion of the Viking Graben in the northern North Sea (Fig. 1). It lies in 480 ft of water about 100 miles northeast of the Shetland Islands near the U.K./Norway boundary. In the Viking Graben, there are nine major fields on which production platforms presently are installed, plus production platforms presently are installed, plus several other proved structures that are awaiting development plans.Several major fault trends traverse the area, and all the producing fields are related to these faults (Fig. 1). In general, the structures are tilted fault blocks that have been truncated by erosion on the updip edges. Sands within these tilted blocks and beneath the erosional unconformity provide the reservoir, and impermeable overlying shales provide both the source of the hydrocarbons and the seal for the trap.The major producing interval in these fields is the Middle Jurassic Brent group, composed of interbedded sands and shales of deltaic origin. JPT P. 407