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Isotope Techniques for Dating of Fluid Flow |
1 Centre de Géochimie de la Surface (CNRS-ULP), 67084 Strasbourg, France
2 CSIRO, Division of Petroleum Resources, North Ryde, NSW 2113, Australia
3 Institut für Geowissenschaften, Universität Mainz, D-55099 Mainz, Germany
4 Institut für Geowissenschaften, Universität Iena, D-07749 Iena, Germany
Authigenic illite is the most abundant clay mineral in Carboniferous and Rotliegend rocks of northern Germany. In spite of extreme present-day burial depths of 4580 to 5280 m, the Rotliegend sandstones are characterized by moderate permeabilities which are controlled, together with their reservoir qualities, by omnipresent illite. It is admitted that deep-crustal hydrothermal fluids, migrating along major fault systems, interacted with the Carboniferous Coal Measures and generated acidic fluids inducing primary leaching processes, mass transfers and concomitant pervasive clay authigenesis.
The investigated samples belong to wells located in a horst-graben structure covering an area of 20 by 20 km. This restricted area allowed investigation of a three-dimensional distribution of fluid flows and related illitization in distinct sandstone reservoirs. The K-Ar ages of the < 2 µm fractions decrease from 210-180 Ma (11 analyses with a mean at 198 Ma) for illite in the horst position to 190-155 Ma (24 analyses with a mean at 177 Ma) for illite in the graben area up to about 4 km away from faults, suggesting a relationship between timing of illite formation and structural position of the host rocks. The decrease in the illite K-Ar ages from the horst into the graben allowed calculation of a migration rate of 0.2 µm per year for the illitization front in the Rotliegend sandstones towards the centre of the graben, which might be of use for prediction of the illite distribution in these rocks.
