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Geological Society, London, Special Publications; 1983; v. 12; p. 161-173;
DOI: 10.1144/GSL.SP.1983.012.01.15
© 1983 Geological Society of London

Geological and Geochemical Studies of Northwest European Continental Shelf

Aspects of Burial Diagenesis, Organic Maturation and Palaeothermal History of an Area in the South Viking Graben, North Sea

M. J. Pearson, D. Watkins & J-L Pittion

Department of Geology, The University, Marischal College, Aberdeen AB9 1AS, Scotland

D. Caston & J. S. Small

Total Oil Marine plc, Crawpeel Road, Altens, Aberdeen AB9 2AG, Scotland
Compagnie Francaise des Petroles, Laboratoires Exploration, 218-28 Avenue de Haut Leveque, 33605 Pessac CEDEX, Bordeaux, France

A detailed study has been made of changes in interlayered clay minerals, sediment mineralogy, vitrinite reflectance, kerogen composition, and chemistry of the soluble organic extract through Well 16/22-2 in the South Viking Graben. The well sampled sediments range in age from Oligocene to, at total depth, Middle Jurassic (Dogger).

Lithologies are typical of the South Viking Graben/Central Graben area with Upper Jurassic black pyritic shales, a thick Cretaceous chalk succession and prominent top Palaeocene tuff. Sandstones are common only in the Upper Jurassic Palaeocene and Eocene. Tuffs are sporadically present in the Middle Jurassic. Randomly interlayered illitesmectite is the dominant clay mineral through the Tertiary and Cretaceous and the change to an ordered structure which occurs just above the Jurassic/Cretaceous unconformity is interpreted as a diagenetic reaction. Estimates of illite-smectite composition indicate a reversal to smectite rich clays in the Lower Eocene-Top Palaeocene which is attributed to an insufficient supply of K+ from detrital feldspar and mica due to the dominance of basic volcanic weathering products.

The vitrinite reflectance profile for this well indicates progressive maturation of kerogen to marginal maturity in the Upper Jurassic. A large jump of 0.2% Ro occurs at the Upper/Middle Jurassic boundary below which kerogen maturity is at the peak oil-generating stage. This maturation break is supported by an increase in spore colour index. Chemical parameters are strongly influenced by a change in kerogen type from mixed algal and humic to predominantly humic at this stratigraphic level. Very high heat flow due to localized intrusive igneous activity during the late Middle Jurassic is thought to have caused the increased maturity of the Middle Jurassic sediments.