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K-Ar geochronology and the timing of detrital I/S clay illitization and authigenic illite precipitation in the Piper and Tartan Fields, Outer Moray Firth, UK North Sea

Published online by Cambridge University Press:  09 July 2018

S. D. Burley  and
M. Flisch
S. D. Burley
Affiliation:
Geologisches Institute, Universität Bern, Baltzerstrasse 1, CH-3012 Bern
M. Flisch
Affiliation:
Geologisches Institute, Universität Bern, Baltzerstrasse 1, CH-3012 Bern
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Abstract

Piper and Tartan Fields are Upper Jurassic reservoirs located in fault blocks on either side of the Witch Ground Graben. The reservoirs are divided into a series of structural units, each successively downthrown towards the graben, covering a total depth interval of 8500–15 000 ft. Mudrocks interbedded with and overlying the reservoir sequence show a progressive increase in illite content with depth from ordered I/S clays with around 40% smectite to almost pure illites with low expandibility. K-Ar analyses on 13 <0·5 µm illite separates from mudrocks covering this depth interval give ages that range from 153 to 68 Ma. Shallowest samples give the oldest ages which are considered to reflect clay neoformation in Jurassic weathering profiles. In the 40Ar/K Harper diagram this data set falls on a mixing line which reflects the amount of smectite present. Deepest samples give the youngest ages and show a major reduction of 40Ar* with increasing K content indicating formation of new illite. These data are considered to document that ‘illitization’ proceeds via a two-step reaction: initial destruction of smectite followed by neoformation of illite. According to decompacted burial curves, ‘illitization’ took place 60–50 Ma ago. Reservoir sandstones have undergone a complex diagenetic history, involving multiple illite authigenesis. K-Ar analyses of 21 <0·5 µm illite separates selected across the oil-water contact in each of the main structural units show progressively younger ages in the water zone and with increasing depth of burial. Oldest ages in the oil zone (144–120 Ma) reflect eogenetic illite authigenesis whilst progressively younger ages in the water zone result from successive mesogenetic illite generations. The youngest age of 29 Ma was obtained from an illite-rich zone at the present oil-water contact in the Tartan downthrown block. Assuming illite authigenesis in the Tartan downthrown block took place shortly after oil accumulation, ∼29–35 Ma can be considered as a maximum age for the present oil accumulation. A minimum age for oil accumulation is calculated as 22 Ma on the basis of contamination of the 29-Ma sample with up to 10% of 93-Ma authigenic illite.

Type
Research Article
Information
Clay Minerals , Volume 24 , Issue 2 , June 1989 , pp. 285 - 315
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1989

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