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Radiogenic and stable isotope evidence for age and origin of authigenic illites in the Rotliegend, southern North Sea

Published online by Cambridge University Press:  09 July 2018

K. Ziegler ,
B. W. Sellwood  and
A. E. Fallick
K. Ziegler
Affiliation:
Postgraduate Research Institute for Sedimentology, The University, Whiteknights, PO Box 227, Reading RG6 2AB, Berkshire, UK
B. W. Sellwood
Affiliation:
Postgraduate Research Institute for Sedimentology, The University, Whiteknights, PO Box 227, Reading RG6 2AB, Berkshire, UK
A. E. Fallick
Affiliation:
Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow G75 0QU, UK
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Abstract

Aeolian sandstones of the Lower Permian Leman Formation (Rotliegend Group) provide the best gas reservoir in the southern North Sea, but permeability is greatly reduced by the presence of authigenic fibrous illites. New radiogenic (K/Ar) and stable (oxygen and hydrogen) isotope data are presented for fibrous illite cements (<0.1 µm), so that the absolute timing and controlling diagenetic factors for their formation can be more fully evaluated. Thus, the expected quality of gas reservoirs in the southern North Sea might be better predicted. Samples have been analysed from five wells in areas with contrasting structural evolution: the Sole Pit Basin, and the Indefatigable Shelf. The K/Ar ages of between 160 and 190 Ma have been obtained from the Indefatigable Shelf illites, and between 120 and 160 Ma for those from the Sole Pit Basin, reflecting different times of basin inversion. These K/Ar ages are interpreted by reference to burial/thermal models for each well. The temperature of illite precipitation falls between 88 and 140°C. Calculated pore-fluid compositions derived from oxygen and hydrogen isotopic analyses give values of ∼ + 1 to +9‰ (SMOW) δ18O and +1 to −50‰ (SMOW) δD. The illite δD values have probably been affected by isotopic exchange and fractionation with the surrounding gaseous hydrocarbon. The δ18O values reflect the degree to which evaporative concentration had affected Zechstein marine waters which subsequently invaded the Leman Sandstone. Comparisons between δ18O and δD values in clays and in formation water for the Leman Field suggest that oxygen isotope exchange might have taken place, and that the initial K+ and radiogenic 40Ar contents within illites may have been modified.

Type
Research Article
Information
Clay Minerals , Volume 29 , Issue 4 , October 1994 , pp. 555 - 565
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1994

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