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AAPG Bulletin


AAPG Bulletin, V. 97, No. 12 (December 2013), P. 21492171.

Copyright copy2013. The American Association of Petroleum Geologists. All rights reserved.


Episodic and simultaneous illitization in oil-bearing Brent Group and Fulmar Formation sandstones from the northern and southern North Sea based on illite K-Ar dating

Norbert Clauer,1 Nicole Liewig2

1Laboratoire d'Hydrologie et de Geochimie de Strasbourg (CNRS-UdS), 1 rue Blessig, 67084 Strasbourg, France; [email protected]
2Institut Pluridisciplinaire Hubert Curien, 23 rue Becquerel, 67087 Strasbourg, France; [email protected]


Size fractions (lt0.4 and 0.4–1.0 mum) of Brent Group sandstones from the northern North Sea contain mostly illite-smectite mixed layers with kaolinite, whereas the same size fractions of Fulmar Formation sandstones from the south-central North Sea consist of illite-smectite mixed layers with minor chlorite. Transmission electron microscope observations show elongated illite laths or agglomerates consisting of small laths when larger individual laths are lacking.

The K-Ar data of the fractions less than 0.4 mum of Brent Group samples plot on two arrays in a 40Ar/36Ar vs. 40K/36Ar diagram that have isochron characteristics with ages of 76.5 plusmn 4.2 and 40.0 plusmn 1.5 Ma, and initial 40Ar/36Ar ratios of 253 plusmn 16 and 301 plusmn 18, respectively. For the Fulmar Formation samples, the data points of the fractions less than 0.2 and less than 0.4 mum also fit two isochrons with ages of 76.6 plusmn 1.4 and 47.9 plusmn 0.5 Ma and initial 40Ar/36Ar ratios of 359 plusmn 52 and 304 plusmn 2, respectively. Some of the coarser 0.4–1.0-mum fractions also plot on the two isochrons, but most plot above indicating the presence of detrital components more than 0.4 mum. The almost identical ages obtained from illite-type crystals of sandstones with different deposition ages that are located about 600 km (373 mi) apart record two simultaneous illitization episodes. These events were not induced by local burial conditions, but are related to episodic pressure and/or temperature increases in the studied reservoirs, probably induced by hydrocarbon injection. This interpretation is indirectly supported by notably different K-Ar illite ages from cores of a nearby reservoir at hydrostatic pressure.

Illite is not as well crystallized as expected for potential crystallization temperatures above 160degC measured by fluid-inclusion determinations. In both the northern and south-central North Sea, the two illite generations remain unaffected after crystallization despite continued burial, suggesting notably higher crystallization temperatures than those estimated from geothermal gradients. No recent illite crystallization or alteration is recorded in the K-Ar ages, despite a dramatic regional acceleration of the subsidence in the southern North Sea.

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