The Reservoir quality aeolian dune sands at Yesnaby are full of bitumen indicative of being previously full of CRUDE OIL which has rapidly lost its volatiles on exposure. This area had the potential for about 1 billion barrels of oil in place (in an excellent reservoir sand).
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Much of thepopular visitor site at Yesnaby is dominated by the Sandstone exposures showing spectacular Sand Dune Bedding at the Castle of Yesnaby and the nearby cliffs.
The southwest corner of the "Millstone Quarry" shows the Dune Sands passing up into fluvial (river) sands with several trace fossil burrows.
The Unconformity surface and quartzite pebble layers are observed on the north side of the quarry.
The Harra Ebb formation is 60-100m thick and consists of breccias, sands muds, and conglomerates deposited on an uneven surface of an eroded basement gneiss hill.
Considerable erosion led to an unconformity surface with elevated topography over the ancient granite hill and the development of Solution Hollows over much of the surface.
Sand dunes of the Yesnaby Sandstone Formation piled up against this topography pinching out over the crest of the hill. See the Cross Section.
The Yesnaby Dune Sands would have formed an uneven pile of sand the top of which was later reworked by rivers and streams to form a flatter surface.
Tectonic movements tilted this surface ( 10 deg) prior to the establishment of the the lacustrine environment of "Lake Orcadie".
This first advance of the lake produced a few metres of very sandy sediments over the Yesnaby area before normal lake conditions prevailed.
Differential compaction over the granite hill maintained the anticline form of this Structure.
The ISOPACH MAP (contours of equal thickness) was constructed by subtracting the Harra Ebb structure from the Base Stromness Flags structure. The appropriate application of the Trapesoid and Pyramid rules - depending on slope - gave a reservoir volume of 786,000 acre feet. Not much imagination is required to see that this could extend to the north west for another 600,000 acre feet. and across the fault for another 1,000,000 acre feet.
The upside for this type of exploration target if found offshore would be in excess of 2.5 Billion Barrels of Oil in Place.