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Department of Geology, University of Southampton, Southampton SO9 5NH, UK
The Devonian Orcadian Basin contains thick sequences of organic-rich lacustrine sediments of interest as hydrocarbon source rocks. Throughout the basin the range of organic maturation is large, spore colours vary from yellow-orange to black, and vitrinite reflectance for Type III kerogens ranges from 0.7 to 10.5%. Maximum palaeotemperatures are estimated to range from those typical of moderate burial diagenesis (100°C) to those characteristic of greenschist facies metamorphism (400°C). The highest maturities are related to contact metamorphism by Devonian plutons, and it is postulated that a large pluton underlies much of Caithness. High and low maturity regions are often juxtaposed across major faults which were active during Variscan inversion. Regionally, the lowest maturity is about 0.8% Ro, implying uplift and erosion of 2–3 km of cover, basin wide. This cover was probably a sequence of Late Devonian and Carboniferous rocks removed largely during Variscan uplift.
Geothermal gradients were high but variable during Devonian extension. This, together with the contact metamorphic affects of the Devonian plutons, resulted in early maturation/metamorphism of large areas of the basin. Most of the onshore sequence entered the oil window during Devonian and Carboniferous times. Significant generation of hydrocarbons from Orcadian lacustrine facies offshore during subsequent Mesozoic burial will have occurred only in those areas which escaped maturation during the Palaeozoic.
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