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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 77 (1993)

Issue: 10. (October)

First Page: 1693

Last Page: 1709

Title: The Stratigraphy of Oxfordian-Kimmeridgian (Late Jurassic) Reservoir Sandstones in the Witch Ground Graben, United Kingdom North Sea

Author(s): S. D. Harker (2), K. A. Mantel (3), D. J. Morton (4), L. A. Riley (5)


Oil-bearing Upper Jurassic Oxfordian-Kimmeridgian sandstones of the Sgiath and Piper formations are of major economic importance in the Witch Ground Graben, United Kingdom North Sea. They form the reservoirs in 14 fields that originally contained 2 billion bbl of oil reserves, including Scott field, which in 1993 will be the largest producing United Kingdom North Sea oil field to come on stream in more than a decade.

The Sgiath and Piper formations represent Late Jurassic transgressive and regressive phases that began with paralic deposition and culminated in a wave-dominated delta system. These phases preceded the major graben rifting episode (late Kimmeridgian to early Ryazanian) and deposition of the Kimmeridge Clay Formation, the principal source rock of the Witch Ground Graben oil fields.

A threefold subdivision of the middle to upper Oxfordian Sgiath Formation is formally proposed, with Scott field well 15/21a-15 as the designated reference well. The basal Skene Member consists of thinly interbedded paralic carbonaceous shales, coals, and sandstones. This is overlain by transgressive marine shales of the Saltire Member. The uppermost Oxfordian Scott Member consists of shallow marine sandstones that prograded to the southwest (a precursor of the Kimmeridgian wave-dominated delta sequence of the Piper Formation, which prograded to the south and to the west). The contact of the Sgiath and Piper formations is a basinwide transgressive marine shale (I shale), which can act as an effective barrier to fluid communication between the Sgiath and Piper reservoir sandstones. The recognition of transgressive marine shales, or maximum flooding surfaces, is of great use in understanding regional geological history and in solving exploration and development problems.

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