- The Athena field is situated in block 14/18b in the Outer Moray Firth area of the UKCS, lying approximately 18 kilometres west of the Claymore and Scapa fields and the associated production facilities. Ithaca currently holds a 22.5% interest in the block 14/18b. Two wells drilled and appraised the Athena discovery in the 1990s and tested oil.
- Ithaca Energy has drilled 3 appraisal wells on Athena, all of which were suspended at the time of drilling for later completion as production wells. The appraisal wells recovered 25 to 28 API crude with test rates of between 1,150 bopd and 2,330 bopd observed. All wells were been drilled from one central location to facilitate subsea development.
- The Athena field is located on a structural high point between the Jura and West Scapa depositional basins, which developed during rifting. Oil is trapped in deep basin sands which entered the basin off the Halibut Shelf ponding in stratigraphic lows. The reservoir in Athena consists of the Leek and Scapa Sandstones. To the north, Athena is a stratigraphically bounded while to the south the sand is truncated by the Athena bounding fault.
Detailed Subsurface
The Athena field is located on a structural high point between the Jura and West Scapa depositional basins, which developed during Mesozoic rifting. The Leek and Scapa sandstones of the Lower Valhall Formation were sourced from the Halibut Shelf to the south of the Athena field. Turbidite sands entered the basin at the junction through a transfer or fault zone between the NW/SE trending Jura Basin margin of the Halibut Shelf, and the SW/NE trending margin towards the West Scapa Basin. An area of low structural relief on the underlying BCU surface can be seen on the seismic interpretation. The sands were locally sourced from reworked Devonian alluvial clastic sediments that form the basement of the Halibut Shelf, possibly being eroded by fluvial systems on the exposed Halibut Horst, and stored in a shallow shelf environment. Turbidite sand lobes were deposited into the basin, initially directed to the accommodation space created in the Jura Basin, but eventually filling to the east and onlapping onto the basin margin towards the north. The Leek and Scapa Sandstones pinch out into the calcareous Valhall Marls, forming the stratigraphic trap for the Athena Field to the north. To the south, the Leek and Scapa Sandstones either onlap or are truncated against fault scarp conglomerates derived from the Halibut Shelf edge.
Deposition of conglomerate/breccia facies along the edge of the Halibut Shelf to the south of the Athena field is widely recognised in the area; several exploration wells encountered tight conglomerates (e.g 14/18a-8, 8z and 14/18b-17z) between Athena and the Scapa Field to the east. No conglomerates have been encountered in the Athena field main area. The location and nature of the boundary between the fault scarp conglomerates observed in 14/18b-17z and the Leek Sandstone reservoir of the Athena field area is not well defined.
Calcareous cementation of the reservoir is interpreted to increase towards the northern pinchout out edge of the field, and towards the edges of individual sand lobes. Increasing distance from the clastic source area (the Halibut Shelf), is interpreted to lead to a lower rate of clastic deposition and greater calcareous deposition of the Valhall Formation and a lower Net to Gross (NTG); this observation is constrained by existing wells in the core area. 14/18b-18, the most northern well in the field, encountered a relatively low NTG section but other reservoir properties (particularly permeability) are significantly better than well 14/18b-15a which lies further south. This evidence suggests that there is not a strong link between NTG and well productivity and therefore additional upside may exist along the northern flank of the field.