Black opaque hydrocarbon rims about 0.05mm thick are preserved around detrital monazite grains in Permian-Triassic arenites of the northern Perth Basin, Western Australia. Chromatographic analysis indicates derivation from oil chemically like that elsewhere in the sequence, and the rims seem to be remnants of oil, which was largely flushed away. The hydrocarbon rims may have adhered to the monazite grains because of irradiation;they are not found on other grains. Monazite-rich heavy mineral bands are probably responsible for abnormally high radioactivity recorded locally by gamma-ray logs in wells penetrating the sandstones.

A preliminary search has revealed similar rims around monazite in other Western Australian arenites ranging in age from Silurian to Cretaceous. Thin-section examination of heavy mineral concentrations may therefore be a useful, simple technique to indicate the former presence of hydrocarbons in dry arenites.

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