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Abstract

All of the components of an exhumed Devonian petroleum system occur in Orkney. These include a good quality mature source rock to bitumen-bearing sandstone reservoirs, with several separate accumulations that could have held about 1.88 billion barrels of oil. The exhumed system presents an excellent analogue for deeply buried petroleum systems offshore. Whilst lighter oils are now absent, reported oil shows occur, commonly associated with faults cutting the Eday Group. On Orkney, Middle Devonian source rocks (750 m thick) were thick lacustrine laminites (fish beds) representing some 30% of the sequence. RockEval and vitrinite analyses show the organic matter is good quality Type I and II and within the early oil window. These source rocks underwent burial until Permian inversion. Several exhumed reservoirs occur on Orkney in aeolian and fluvial sandstones with porosities from 15 to 25%. These reservoirs have been ‘breached’, losing the light-end hydrocarbons, leaving pore space oil stain and bitumen residues. Thin fluvial and sheet-floods sands found within the lake cycles have bitumen residues and provided connectivity between the thicker reservoir units. All types of trap are found including a major, broad anticline running north–south on Mainland Orkney and an unconformity with fault traps and pinchouts.

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