Potential Size of
Accumulation
Yesnaby Sandstone Reservoir
The Yesnaby Sandstone formation consists of over 200m of desert sandstone (dune bedding clearly shown in the sea stack of the Yesnaby Castle). They are full of bitumen indicative of being previously full of CRUDE OIL which has rapidly lost its volatiles on exposure. The Harra Ebb formation was tilted and eroded before the dune sands were deposited on the uneven unconformity surface . The sand thus varies in thickness over the area.
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The mature organic rich lacustrine Laminites of the Orcadian Basin
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Aeolian and fluvial sands of the Yesnaby Sandstones . Thickness is zero at the pinchout in the south thickening to over 200m towards the north. Porosity 13% to 25% and permeability 3mD to 2,000mD - this quality of reservoir could give high recovery factors.
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The upper fluvial sands are barren of bitumen and commonly have porosity less than15% and permeability less than 2mD (milli-Darcy) so may not have contained liquid hydrocarbons.
Seal:
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The top seal (cap rock) is muds and flagstones of the Stromness Flagstone Formation.
Trap:
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The Trap at Yesnaby (Combination Trap, Pinchout and Anticline) is formed by the reservoir sands dipping north away from the "Zero Edge" pinchout sealed by overlying Stromness Flagstones and underlying Harra Ebb Formation and the basement granite-gneiss.
Potential Size of Accumulation:
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Estimating the volume of the Potential Reservoir at Yesnaby requires the calculation of the Thickness and extent of the Sandstone. This is accomplished by carefully mapping the Geological Structure of the underlying Harra Ebb Formation and the overlying Lower Stromness Flagstones. Subtracting the one map from the other yields an
ISOPACH MAP of the reservoir Volume.
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With seal on the Borwick/Navershaw Fault the accumulation would cover a minimum 2,000 acres and contain about 850MMBOIP (204MMBO - million barrels recoverable).
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The potential for about 1 billion barrels of oil in place (in an excellent reservoir sand) could make this an attractive target if located at depth offshore.
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The Yesnaby Dune Sand or equivalent is not found at Graemsay or Stromness Basement outcrops or in the Warebeth Borehole therefore is discontinuous in distribution adding an extra risk factor to the "Exploration Play Concept" - it would be an attractive Secondary Objective to an Eday Sandstone Primary Target.