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Late Jurassic Sedimentation and Tectonics, Main Area Claymore Reservoir, North Sea

By
S. D. Harker
S. D. Harker
Occidental Petroleum (Caledonia) Limited, 1 Claymore Drive, Bridge of Don, Aberdeen AB2 8GB
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C. E. Maher
C. E. Maher
Occidental Petroleum (Caledonia) Limited, 1 Claymore Drive, Bridge of Don, Aberdeen AB2 8GB
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Published:
January 01, 1988

Abstract

Late Jurassic sands constitute the major reservoir of the Claymore oil field, located in U.K. North Sea Block 14/19, 110 miles northeast of Aberdeen. These sands, known as the Main Area Claymore Reservoir (MACR), were deposited during the development of the Witch Ground Graben. This graben is a northwest-trending arm which developed off the central North Sea graben system. The base of the MACR is represented by the Sgiath Formation, a widespread pre-rift unit of Oxfordian paralic sandstones, mudstones, and coals (up to 121 ft thick). This unit marks the onset of Late Jurassic marine transgression over an eroded Triassic surface. Deepening of the Jurassic seaway, following deposition of the Sgiath, resulted in 160± ft of delta-fringe to marine shelf silty mudstones and bioturbated sandstones of the overlying Late Oxfordian-Kimmeridgian Piper Formation. The uppermost unit of the MACR is up to 1630 ft thick and is composed of fine-grained syn-rift turbidite sandstones of the Kimmeridgian to Mid Volgian Claymore Sandstone Member, Kimmeridge Clay Formation. This unit has a net sand ratio in excess of 0.9, though it is characterized by high gamma radiation levels (up to 120 API units), with up to 4% TOC in some of the sandstones. Two informal reservoir subdivisions are recognized: the Low Gamma-Ray Sands (LGRS) and the overlying High Gamma-Ray Sands (HGRS). These are distinguished by their different reservoir characteristics as well as an increase in average gamma radiation levels in the sandstones from 50 to 70 API units. The LGRS constitute the better reservoir unit with thicker bedding (1.5 ft vs 0.5 ft) and higher permeabilities (20-1300 md vs 0.2-400 md). This difference results from a waning of depositional energy levels in response to decreasing tectonic activity and/or a rise in sea level after deposition of the LGRS. The Claymore Sandstone Member is overlain by Mid Volgian to Ryazanian organic-rich siltstones and mudstones of the upper Kimmeridge Clay Formation. These argillites were deposited in a low-energy stratified water column prior to a major episode of Late Cimmerian uplift and erosion that formed the truncated tilted fault block of the Claymore oil field.

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SEPM Core Workshop Notes

Giant Oil and Gas Fields: A Core Workshop

Anthony J. Lomando
Anthony J. Lomando
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Paul M. Harris
Paul M. Harris
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SEPM Society for Sedimentary Geology
Volume
12
ISBN electronic:
9781565761001
Publication date:
January 01, 1988

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