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The Hebridean Basins and Adjacent Areas |
Department of Geology, The University, Manchester M13 9PL, UK
Sediments considered to be of Permian or Permo-Triassic age are exposed in many areas of Scotland and within several offshore basins. These sediments were deposited within fault-bounded, intermontane basins and represent a variety of continental facies typical of arid or semi-arid areas. A major study of the diagenetic evolution of the Scottish Permo-Triassic reveals that the basins were isochemical and experienced similar patterns of diagenesis, largely controlled by the climatic aridity of the depositional environment. Depositional pore-waters in the Permo-Triassic sediments of Scotland were well oxygenated and alkaline in nature, allowing the rapid alteration of the unstable mineral assemblage. Ionic complexes released as a result of this alteration were precipitated as a suite of eodiagenetic minerals with oxygen isotope analysis on the authigenic K-feldspar phase confirming an origin from meteoric waters in an arid climate. These eodiagenetic modifications resulted in a major loss of primary porosity. Further burial resulted in minor pressure solution while further depth-related changes have been identified in the authigenic clay mineral suite. Telodiagenetic precipitation of kaolinite and minor quartz was associated with a moderate development of secondary porosity. The hydrocarbon reservoir potential of the Scottish Permo-Triassic is low, primarily due to the early destruction of porosity.
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