Abstract
Devonian rocks from the Orcadian Basin show an extreme range of organic maturity. Mean vitrinite reflectance ranges from 0-6 to 10.5~ and spore colour from yellow- orange to black. Illite/smectite (I/S) is abundant in the clay fraction of lacustrine mudrocks and shows variation in expandability from 30 to < 5~. A correlation is observed between expandability and vitrinite reflectance, but the specific relationship is different in the Moray Firth and the Caithness-Orkney areas. Illite 'crystallinity' measurements in the Caithness- Orkney area are also correlated with vitrinite reflectance. Geological evidence favours an I/S origin from precursor smectite, and mineral and chemical analyses of lacustrine shales show no evidence for regional differences in composition that might affect iUitization rates. The different relationships within each region are believed to result from the different relative response of organic maturity indicators and clay minerals to temperature and reaction time. The lower I/S expandabilities in the Caithness-Orkney region at a given vitrinite reflectance are attributed to relatively longer burial times.
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