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

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Authors:Ziegler, K.; Sellwood, B. W.; Fallick, A. E.
Author Affiliations:Primary:
University of Reading, Postgraduate Research Institute for Sedimentology, Reading, United Kingdom
Other:
Scottish Universities Research and Reactor Centre, 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.555-565; 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. 32 refs.; illus., incl. sketch map
Summary: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 ppm (SMOW) δ18O and +1 to -50 ppm (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.
Subjects:Absolute age; Authigenic minerals; Clastic rocks; Clay mineralogy; D/H; Dates; Deuterium; Hydrogen; Illite; Isotope ratios; Isotopes; K/Ar; Leman Sandstone Formation; Lower Permian; Natural gas; O-18/O-16; Oxygen; Paleozoic; Permian; Petroleum; Reservoir rocks; Rotliegendes; Sandstone; Sedimentary rocks; Sheet silicates; Silicates; Stable isotopes; Atlantic Ocean; North Atlantic; North Sea; Southern North Sea
Coordinates:N530000 N540000 E0030000 E0010000
Abstract Numbers:95M/3627
Record ID:1995025462
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Abstract, Copyright, Mineralogical Society of Great Britain and Ireland
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