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The following case history involves Jurassic reservoir sands in the Viking Graben of the North Sea and is based primarily on a study conducted while the author was with 17on Production Research (Wilson, 1977). It documents the relative importance of many of the major geologic controls on reservoir quality in clastic sandstones, and reviews methods used for prediction of porosity and permeability. The interpretations presented here are those of the author and do not necessarily reflect the opinions of Esso Exploration and Production, U.K. Ltd., or Shell Exploration, U.K. Ltd.

During the early stages of drilling in the Viking Graben, unexpectedly low permeability was encountered in the Middle Jurassic Brent Group in well 211/26-4 (Figure 21-1). In order to delineate factors causing this anomaly, a pilot study was first conducted on five wells. The pilot study consisted primarily of petrographic analysis along with limited amounts of scanning electron microscopy (SEM) and X-ray diffraction analysis. The following conclusions were reached in that study:

  1. Porosity reduction was controlled primarily by quartz overgrowth development.

  2. Anomalously low permeabilities were produced primarily by transformation of potassium feldspar and kaolinite/dickite to highsurface-area illitic clay.

Following completion of the pilot study, a detailed study was initiated involving 19 additional wells. Locations of all wells analyzed are shown in Figure 21-1. The wells are distributed stratigraphically as follows:

  • Brent Group (Middle Jurassic) - 17 wells

  • Magnus Sandstone (Upper Jurassic) - 2 wells

  • Statfjord Formation (Lower Jurassic) - 1 well

  • Sleipner Sandstone (MidldleJurassic) - 3 wells

  • Triassic Sandstone - 1 well


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