Solid bitumen envelopes around radioactive clastic grains in sandstones from the Perth and Bonaparte Basins of Western Australia display textures that resemble those induced by the irradiation of oil in the laboratory. The envelopes surround monazite grains in the Perth Basin and Bonaparte Basin, and locally surround zircon and rare xenotime in the Bonaparte Basin.Envelopes in the northern and central Perth Basin formed at different stages in the complex diagenetic histories of the sedimentary rocks, and examination of the envelopes has helped unravel the diagenetic sequences. In the Bonaparte Basin, some envelopes around monazite contain an inner, highly polymerised, non-fluorescent zone, and an outer, less polymerised, fluorescent zone. The inner zone is attributed to polymerisation of an early oil, and the outer zone to polymerisation of a later oil, an interpretation which accords with geochemical evidence indicating two episodes of oil migration. Where envelopes surround zircon, which is not as radioactive as monazite, they are not strongly polymerised, and fluoresce.This investigation shows that the combined techniques of optical microscopy, blue-violet fluorescent microscopy and SEM examination are useful in establishing the history of oil in sedimentary rocks. The presence of solid bitumen envelopes may reveal more than one generation of oil, or may demonstrate the former presence of oil in dry rock. No petrological examination of sandstone is complete until the radioactive clastic grains have been examined for signs of solid bitumen envelopes.
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- Polymerisation of Hydrocarbons by Radioactive Minerals in Sedimentary Rocks: Diagenetic and Economic Significance
J. E. Glover
C. B. Foster
- Springer Berlin Heidelberg