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AAPG Bulletin

Abstract

AAPG Bulletin, V. 103, No. 1 (January 2019), P. 215-240.

Copyright ©2019. The American Association of Petroleum Geologists. All rights reserved.

DOI: 10.1306/06071816525

Potassium-argon timing of episodic mica and illite crystallization in highly indurated Hassi Messaoud (Algeria) hydrocarbon-bearing sandstones

Norbert Clauer,1 Horst Zwingmann,2 Andrew Todd,3 and Amélie Aubert4

1Institut de Physique du Globe de Strasbourg, Université de Strasbourg, Université de Strasbourg–Centre National de la Recherche Scientifique (Uds–CNRS), Strasbourg, France; [email protected]
2Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto, Japan; [email protected]
3Commonwealth Scientific and Industrial Research Organization Earth Science and Resource Engineering, Bentley, Western Australia, Australia; [email protected]
4Laboratoire d’Hydrologie et de Géochimie de Strasbourg, Université de Strasbourg, UdS-CNRS, Strasbourg, France; [email protected]

ABSTRACT

Nanometer to micrometer mica and illite separates of indurated Cambrian and Ordovician oil-bearing sandstones from the Hassi Messaoud field (Algeria) were extracted, x-rayed, observed by scanning and transmission electron microscopy, and K-Ar dated. Electron microscope observations revealed typical euhedral shapes for the mica to illite particles of most size fractions; almost no odd-shaped detrital crystals were detected. The combined results document several generations of mineralogical and morphological identical mica to illite crystals that could not be differentiated by the traditional identification methods. Illite and mica genesis was multiphased with crystallization episodes at 340 ± 10 (ca. Middle Mississippian), 280 ± 10 Ma (ca. early Permian), and 170 ± 10 Ma (ca. Middle Jurassic). Younger than the stratigraphic age of the host rocks, which is incompatible with a detrital origin, the two older mica ages confirm that the hydrocarbon generation and emplacement had to start after the Variscan tectonothermal event and before exhumation of the meta-sediments. The younger K-Ar ages at 135 to 110 Ma (ca. Early Cretaceous) relate to further crystallization episodes, whereas those at circa 295, 265, and 210 Ma probably correspond to variable mixtures of the older and younger mica to illite end-members. Three average K-Ar values are statistically significant: the oldest at 340 ± 10 Ma corresponds to the start of the Variscan tectonic activity, and the intermediate at 280 ± 10 Ma sets its end, both episodes probably modifying the reservoir capacities of the potential hydrocarbon host rocks. The ages at 170 ± 10 Ma identify a further diagenetic activity characterized by illitization of dickite-type precursors in local reservoirs. These younger ages could correspond to the hydrocarbon charge into reservoirs, which stopped diagenetic illitization at a present-day depth of approximately 4000 m (∼13,000 ft).

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