Abstract

Minute rims of solid bitumen ( approximately 40-50 micrometers 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 influxes, producing graded fluorescent growth bands. Oil droplets and lamellae also were adsorbed onto the outer portion of the nodules. Such bitumen nodules are a new and potentially important source of data for understanding the movement of hydrocarbons in sedimentary basins, specifically for identifying hydrocarbon pathways, the number of discrete hydrocarbon pulses, and the relative timing of oil migration.

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