Stable isotopes in illite; the case for meteoric water flushing within the Upper Jurassic Fulmar Formation sandstones, UK North Sea

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Authors:Wilkinson, M.; Fallick, A. E.; Keaney, G. M. J.; Haszeldine, R. S.; McHardy, W. J.
Author Affiliations:Primary:
Glasgow University, Department of Geology and Applied Geology, Glasgow, United Kingdom
Scottish Universities Research and Reactor Centre, United Kingdom
Macaulay Land Use Research Institute, United Kingdom
Volume Title:Issue devoted to papers read at a conference on Diagenesis, overpressure and reservoir quality
Volume Authors:Bain, D. C., editor; Hall, P. L.; Shaw, H. F.; Spears, D. A.
Source:Clay Minerals, 29(4), p.567-574; Diagenesis, overpessure and reservoir quality, Cambridge, United Kingdom, March 26, 1993, edited by D. C. Bain, P. L. Hall, H. F. Shaw and D. A. Spears. Publisher: Mineralogical Society, London, United Kingdom. ISSN: 0009-8558
Publication Date:1994
Note:In English. 21 refs.; illus., incl. 1 table, sketch map
Summary:The extent to which the diagenesis of sedimentary sequences within the UK North Sea is influenced by the passage of meteoric water is subject of current debate. Petrographic (SEM, TEM, thinsection) observation and stable isotope ratios from authigenic illite separates are used to deduce the pore-fluid evolution of the Fulmar Formation within well 29/10-2 from the Central Graben, with emphasis on the past importance of meteoric water flushing. Petrographic observation shows the illite to have two modes of occurrence (pore-filling and as pseudomorphs after feldspar). These can be equated with two authigenic crystal morphologies (plates and laths) observed by TEM within illite separates prepared for isotopic analysis. The laths are equated with late stage 'hairy' pore-filling illite, the plates with illite pseudomorphs after feldspar which are difficult to date petrographically. The δD (-55 to -87 ppm SMOW) and δ18O (12.4-14.6 ppm SMOW) analyses can be interpreted in terms of a conventional hydrogen-oxygen cross-plot to indicate illite precipitation from pore-waters of meteoric origin at 50-60°C. However, these predicted precipitation temperatures are irreconcilable with the observed petrographic sequence. The hydrogen isotope compositions may have undergone resetting during burial. Consequently, only the δ18O data may be confidently interpreted in terms of the conditions of illite precipitation. This implies illite growth at temperatures above 80°C from waters with δ18O between -2.5 and +6 ppm SMOW, leaving the case for meteoric water flushing of the sandstones unproven.
Subjects:Case studies; Clastic rocks; Clay mineralogy; D/H; Fulmar Formation; Hydrogen; Illite; Isotope ratios; Isotopes; Jurassic; Mesozoic; Meteoric water; O-18/O-16; Oxygen; Pore water; Sandstone; Sedimentary rocks; Sheet silicates; Silicates; Stable isotopes; Upper Jurassic; Atlantic Ocean; Central Graben; North Atlantic; North Sea
Coordinates:N560000 N570000 W0020000 W0040000
Abstract Numbers:95M/3628
Record ID:1995025463
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Abstract, Copyright, Mineralogical Society of Great Britain and Ireland
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