Birger Rasmussen²

Minute rims of solid bitumen (~40-50 µm thick) surround detrital radioactive grains in the Permian-Triassic sandstones and Arranoo Member of the Kockatea Shale from the northern Perth basin, Australia. The bitumen formed as Th- and 
U-bearing minerals (monazite, xenotime, zircon, thorite) irradiated and immobilized fluid hydrocarbons coming within range of alpha-particle emissions. Using transmitted light and scanning electron microscopy the rims appear compositionally homogeneous, but under blue/violet epifluorescent illumination the bitumen displays complex concentric and contorted banding. These fluorescent textures indicate that multiple influxes of hydrocarbons passed through the reservoir sandstones. Following radiation-induced immobilization of hydrocarbons from the first oil influx, the bitumen nodules grew through a process of swelling and expansion outward from the mineral core during subsequent oil