Implications of linearly correlated oxygen and hydrogen isotopic compositions for kaolinite and illite in the Magnus Sandstone, North Sea

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Authors:Fallick, Anthony E.; Macaulay, C. I.; Haszeldine, R. S.
Author Affiliations:Primary:
Scottish Universities Research and Reactor Centre, Isotope Geology Unit, Glasgow, United Kingdom
Other:
Glasgow University, United Kingdom
Volume Title:Clays and Clay Minerals
Source:Clays and Clay Minerals, 41(2), p.184-190. Publisher: Clay Minerals Society, Clarkson, NY, United States. ISSN: 0009-8604
Publication Date:1993
Note:In English. 34 refs.; illus. incl. 1 table, sketch map
Summary:Authigenic kaolinite and illite are important diagenetic minerals in the Magnus Sandstone, a giant oil reservoir in the northern North Sea. These clay minerals, separated from three wells, show considerable ranges in their oxygen isotopic composition (δ18OSMOW = +9 to +16 ppm) and hydrogen isotopic composition (δDSMOW = -55 to -105 ppm). The variations in δ18O and δD are positively linearly correlated with a high degree of statistical significance for both kaolinite and illite: Formation of the clays in a pore fluid of uniform isotopic composition over a range of temperatures appears unlikely. It is suggested that the observed relationships between clay mineral δ18O and δD are perhaps best explained by a model of precipitation at more or less constant temperature from pore fluids which varied isotopically across the oilfield. The isotopic composition of the formation waters would then lie along the line: δDw = 6.2 δ18Ow - 50. This is most plausibly interpreted as a mixing line with suggested minimal endmembers at (δ18O, δD) values of (+4, -24) and (-4, -76). The first of these represents reasonable isotopic values for Magnus Sandstone formation waters. Although δ18O of the second is compatible with an evolved Cretaceous meteoric water, its δD value is difficult to understand in the context of the model.
Subjects:Authigenic minerals; Clastic rocks; Clay mineralogy; Clay minerals; D/H; Deuterium; Diagenesis; Geologic thermometry; Hydrogen; Illite; Isotope ratios; Isotopes; Kaolinite; Meteoric water; O-18/O-16; Oxygen; Petroleum; Petroleum exploration; Reservoir rocks; Sandstone; Sedimentary rocks; Sheet silicates; Silicates; Stable isotopes; Atlantic Ocean; North Atlantic; North Sea; European Atlantic; Magnus Sandstone
Coordinates:N600000 N620000 E0020000 W0010000
Abstract Numbers:94M/1502
Record ID:1993023733
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Abstract, Copyright, Mineralogical Society of Great Britain and Ireland
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