FLUORESCENT GROWTH BANDS IN IRRADIATED-BITUMEN NODULES - EVIDENCE OF EPISODIC HYDROCARBON MIGRATION

Minute rims of solid bitumen (similar to 40-50 mu m thick) surround de trital radioactive grains in the Permian-Triassic sandstones and Arran oo Member of the Kockatea Shale from the northern Perth basin, Austral ia. The bitumen formed as Th- and U-bearing minerals (monazite, xenoti me, zircon, thorite) irradiated and immobilized fluid hydrocarbons com ing within range of alpha-particle emissions. Using transmitted light and scanning electron microscopy the rims appear compositionally homog eneous, but under blue/violet epifluorescent illumination the bitumen displays complex concentric and contorted banding. These fluorescent t extures 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 th rough a process of swelling and expansion outward from the mineral cor e during subsequent oil influxes, producing graded fluorescent growth bands. Oil droplets and lamellae also were adsorbed onto the outer por tion of the nodules. Such bitumen nodules are a new and potentially im portant source of data for understanding the movement of hydrocarbons in sedimentary basins, specifically for identifying hydrocarbon pathwa ys, the number of discrete hydrocarbon pulses, and the relative timing of oil migration.