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Abstract

The 34/10 Delta structure is located in Norwegian North Sea Block 34/10, about 200 km west-northwest of Bergen, and about 20 km east-southeast of Statfjord field.

Initial exploration was based on the interpretation of a 1-km two-dimensional (2-D) migrated seismic grid. In 1979, at a relatively early stage in the exploration phase, it was decided that better quality data were needed to map the structurally complex, highly faulted area adequately, and to guide further delineation drilling. A three-dimensional (3-D) seismic survey was therefore shot, covering the structure.

It is obvious from a comparison of individual lines that the 3-D data is of significantly higher quality than the 2-D data. Furthermore, the extremely dense grid of lines makes it possible to develop a more accurate and complete structural and stratigraphic interpretation.

Specifically, the 3-D data made it possible to map the dip and the subcrop of the subunconformity strata more accurately. The early geologic model had the Jurassic reservoir sands dipping steeply westward over the entire structure, which implied extensive erosion of the main reservoir sands toward the east. With the 3-D seismic data, however, it was possible to recognize and map much gentler dips in the eastern part of the structure. As a result, the mapped subcrop of the reservoir sands was placed much farther eastward, extending the eastern limit of the field considerably.

Based on this 3-D interpretation, four successful oil wells have been drilled. These are located in parts of the field that could not previously be mapped accurately on the basis of the 2-D seismic data because of their poor quality. This eastward extension has increased the estimate of reserves such that it was possible to declare the field commercial in late 1980.

This experience with 3-D seismic data in the North Sea indicates the usefulness and effectiveness of the tool when used early in the exploration phase.

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