Solan, Strathmore and the back basin play, West of Shetland

Hydrocarbon migration from the Faeroe–Shetland Basin source kitchen into the Mesozoic back basins that flank its southern margin is prevented by basement highs such as the Rona Ridge. The back basins have long been considered non-prospective due to a perceived lack of source rock and insufficient burial to generate commercial hydrocarbons. The Triassic Strathmore discovery made in 1990, followed by the Upper Jurassic Solan discovery the following year, have demonstrated the prospectivity of the East Solan Basin and similar back basins along the same trend. The two discoveries typify the potential plays in this area, which consist of tilted Palaeozoic and pre-Upper Jurassic structural traps, and syn- to post-Upper Jurassic pinch-out plays against the surrounding structural highs. Strathmore, which straddles UK Blocks 204/30a and 205/26a, consists of dipping Triassic sandstones truncated beneath a broad structural nose mapped at Base Late Jurassic level. The truncated Triassic, which is over 10000 ft thick down-dip, consists entirely of sandstone, but only the lowest, quartz-rich, 550 ft thick Otter Bank Sandstone approaches reservoir quality. The overlying Foula Sandstone has a similar grain size and depositional setting, but its immature detrital mineralogy has resulted in early compactional porosity loss, making the unit an effective top seal to the Otter Bank Sandstone reservoir. Overlying the eastern portion of the Strathmore accumulation in Block 205/26a is the oil-bearing basin floor Solan Sandstone, which sits within the Kimmeridge Clay Formation (KCF), thickening and dipping northeastward into the East Solan Basin. To the southwest the Solan Sandstone forms a stratigraphic trap, onlapping and pinching-out against an intrabasinal high created by the Judd Transfer Zone, which separates the East Solan Basin from the South and West Solan basins. The oil in both the Triassic and Late Jurassic accumulations was generated in the East Solan Basin from the KCF. Both accumulations have similar oil–water contacts, but may be partially isolated in terms of aquifer support. In addition, both share a heterogeneous oil column, which becomes heavier and richer in asphaltene with depth, possibly reflecting two hydrocarbon charges. At present, the 23 × 10 6 BBL oil reserve size (78 × 10 6 BBL STOIIP) of the Solan accumulation is deemed insufficient to justify stand-alone development. Strathmore contains over 200 × 10 6 BBL STOIIP, but the low reserves, estimated at approximately 36 × 10 6 BBL oil, are a function of low matrix permeabilities in all but a few reservoir layers. Only if fracture production can be established will the field be capable of commercial development. Exploration for potential Solan analogues requires the presence of sufficient Late Jurassic section to be confident that the thickening is due to Solan Sandstone presence. Identifying Strathmore analogues is less straightforward. The reservoir is regionally extensive, but is difficult to define seismically without biostratigraphical control (rare in the Triassic) or nearby penetrations of the Otter Bank Sandstone, which are few and far between. Despite the East Solan Basin discoveries proving the play concept, the volumes of hydrocarbons generated by the back basins still remain one of the limiting factors.