The Effect of Oil Emplacement on Diagenetic Clay Mineralogy: the Upper Jurassic Magnus Sandstone Member, North Sea

  1. Richard H. Worden2 and
  2. Sadoon Morad3
  1. R. H. Worden2 and
  2. S. A. Barclay1

Published Online: 17 MAR 2009

DOI: 10.1002/9781444304336.ch20

Clay Mineral Cements in Sandstones

Clay Mineral Cements in Sandstones

How to Cite

Worden, R. H. and Barclay, S. A. (1999) The Effect of Oil Emplacement on Diagenetic Clay Mineralogy: the Upper Jurassic Magnus Sandstone Member, North Sea, in Clay Mineral Cements in Sandstones (eds R. H. Worden and S. Morad), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304336.ch20

Editor Information

  1. 2

    Department of Earth Sciences, University of Liverpool, Brownlow Street, Liverpool L69 3GP, UK

  2. 3

    Department of Earth Sciences, Uppsala University, Villa vägen 16, S-752 36 Uppsala, Sweden

Author Information

  1. 1

    Department of Geology and Geophysics, University of Edinburgh, EH9 3JW, UK

  2. 2

    Department of Earth Sciences, University of Liverpool, Brownlow Street, Liverpool L69 3GP, UK

Publication History

  1. Published Online: 17 MAR 2009
  2. Published Print: 7 OCT 1999

ISBN Information

Print ISBN: 9781405105873

Online ISBN: 9781444304336

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Keywords:

  • effect of oil emplacement on diagenetic clay mineralogy, Magnus sandstone;
  • geochemistry of Magnus sandstone;
  • clay mineral distribution and timing of kaolin relative to oil emplacement and diagenetic phenomenon;
  • clay-mineral diagenesis in MSM;
  • depositional facies - no significant control on kaolin and illite distribution;
  • illite and kaolin distribution significance - petrophysics;
  • controls on diagenetic system

Summary

The Upper Jurassic Magnus turbidite sandstone has higher illite/(illite + kaolin) and K2O/(K2O + Al2O3) ratios in the oil zone than in the water zone. Both clay minerals grew during burial diagenesis at similar times as quartz and ankerite cementation and oil emplacement. Illite may have grown at the same time as, or slightly later than, kaolin. Both clay minerals probably grew as a result of the dissolution of aluminosilicate minerals (e.g. K-feldspar). Bulk chemical (X-ray fluorescence) data show that the oil zone has a greater amount of potassium than the water zone. Illite thus grew where the aqueous potassium could not escape from the sandstone, whereas kaolin grew where aqueous potassium was able to escape the host sandstone. The oil zone would have had many orders of magnitude slower transport of potassium than the water zone (by either advection or diffusion mechanisms). The clay-mineral distribution pattern is thus likely to result from the inability of the oil-zone sandstone to lose potassium relative to the water-zone sandstone. The pattern is not the result of primary facies or depositional environment variations because all samples come from the same facies and turbidite sands tend to have minimal lateral variation. Nor is the pattern the result of early diagenetic processes beneath the Cimmerian Unconformity because kaolin is found in greatest abundance furthest away from the unconformity. The prevalence of kaolin cannot be the result of minor differences in reservoir temperature between the crest (oil zone) and flank (water zone) of the structure because illite, and not kaolin, would be the favoured mineral at the greater depth and thus higher temperature of the water zone. The data presented here imply that diagenesis is modified (in terms of physicochemical mechanism and geochemistry) rather than stopped by oil emplacement. The data also imply that oil-zone mudstones may be slightly less illitized than water-zone mudstones, although appropriate data to test this hypothesis have not yet been collected.