Horst Zwingmann¹, P. Joe Hamilton¹, P. Tingate², N. Lemon², C. Cubitt²

(1) CSIRO Petroleum, Perth, Australia
(2) Adelaide University, Adelaide, Australia

ABSTRACT: The Origin and Timing of Illitic Clays in Reservoir Sandstones of the Cooper Basin, South Australia

Potassium-Argon (K-Ar) ages have been determined on illitic clay separates from tight gas sandstones of Permian age from the onshore South Australian Cooper Basin. Illite is one of the commonest clay mineral components in these sandstones and is a major cause of their low permeability. It displays a wide range of morphologies, modes of occurrence and origins. As a matrix cement it occurs both with platy and pore bridging morphologies which are highly deleterious to reservoir quality. Volcanic clasts from several centimetres to a few millimetres in diameter contain illite as a major alteration product occurring as tightly packed arrangements of platy morphology It is the only K-bearing phase in these clasts. Remaining occurrences of illitic clay appear to be of a detrital origin.

A total of 24 K-Ar illite dates for a range of grainsizes of 0.5 - 2-6 microns were determined and these range from 390+/-5 to 92+/-1 Ma. This wide spread in ages is interpretetable in the context of petrographic and XRD characterisation of the clay separates and burial history modelling of the sampled wells. In this way distinct ages are discerned that variously reflect, in order of decreasing age, the age of detrital illitic material, timing of volcanic clast alteration, timing of illite neoformation and timing of illite polytypic transition in a well that had experienced a high maximum burial temperature (>250degreesC).

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado