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Textural and Isotopic Variations in Diagenetic Kaolinite from the Magnus Oilfield Sandstones

Published online by Cambridge University Press:  09 July 2018

C. I. Macaulay ,
A. E. Fallick  and
R. S. Haszeldine
C. I. Macaulay
Affiliation:
Department of Geology and Applied Geology, University of Glasgow, Glasgow G12 8QQ
A. E. Fallick
Affiliation:
Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow G75 OQU, Scotland
R. S. Haszeldine
Affiliation:
Department of Geology and Applied Geology, University of Glasgow, Glasgow G12 8QQ
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Abstract

In Upper Jurassic sandstones of the Magnus Sandstone Member from the Magnus Oilfield in the North Sea, diagenetic kaolinite morphology and isotopic composition vary away from the Late Cimmerian unconformity. Near the unconformity, coarse-grained (15-30 μm) vermiform kaolinite morphology is most common, whereas downdip, 3 km away, fine-grained (∼10 μm) blocky morphology is prevalent. Kaolinite abundances increase downdip, away from the unconformity, and kaolinite near the unconformity has low δ18O (12·5-14·9%o smow) compared to that downdip (15·9−17·5%o). This reflects replacement of marine depositional pore-waters by meteoric water near the erosion surface. However, isotopic temperature estimates suggest that the majority of kaolinite formed at elevated burial temperatures of ~80°C, and not during subaerial exposure of the sandstone. The δD of kaolinite close to the unconformity is also lower than that downdip. Kaolinite morphology and isotopic composition record meteoric water ingress during Late Cimmerian subaerial erosion and retention of meteoric-derived water in the crest of the Magnus structure during burial diagenesis. Kaolinite formation during subaerial exposure is not of significance to the sandstone reservoir quality.

Type
Research Article
Information
Clay Minerals , Volume 28 , Issue 4 , December 1993 , pp. 625 - 639
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1993

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