The organic geochemistry of "Kolm", a unique analogue for the understanding of molecular changes after significant uranium irradiation

The highly uraniferous and organic-rich material which occurs as small lenticular nodules in the Upper Cambrian Alum Shale Formation of middle Sweden is known as “Kolm”. The uranium contents in Kolm samples range from 2000 ppm to 6000 ppm (>100 times higher than typical shale), making the organic matter one of the most irradiated in the geological world with an exposure time of >500 Myr. Here, we show that uranium is mostly enriched on the surface of various minerals, instead of being bound to the organic matter. Although Kolm formation predates the evolution of advanced plants and the organic matter is marginally mature, thermovaporisation and pyrolysis products are exclusively composed of gaseous and low-molecular aromatic compounds. The kerogen structure is extremely heterogeneous, and rare bitumen can be extracted although the TOC content of the Kolm sample is 62 wt%. These features are attributed to the irradiation effects of uranium which resulted in radiolytic bond breaking, aromatisation, and polymerisation of the organic matter. The organic-uranium interactions in the investigated Kolm sample shed new light on unexpected phenomena while petroleum exploration.