Abstract

Burmah well 12/27-1, drilled on a structural high in the Inner Moray Firth, penetrated over 3000 ft (914 m) of Lower Devonian, organic-rich sedimentary rocks. These have been analysed for source-rock potential, and contain a substantial proportion of oil-prone, thermally mature kerogen. A decompacted vitrinite reflectivity profile shows that the lower part of the Devonian section has a steeper thermal maturity gradient, which is interpreted as indicating a high Devonian geothermal gradient associated with basin extension. Solid bitumen reflectivities and organic petrography show that the most recent hydrocarbon generation occurred in the upper part of the Devonian interval during maximum Tertiary burial. Vitrinite reflectivity data from the Jurassic rocks in the Beatrice Field and onshore East Sutherland enable the position of the oil generation window to be predicted in relation to the depth of the top of the Devonian sequence. The main controls on Devonian source rock potential in the Inner Moray Firth are not the depth of Mesozoic and Tertiary burial but the distribution of source rocks, their degree of Permian truncation, and the pre-Permian thermal maturity level.

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