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Clair Field is located off the coast of the Shetland Islands. It sits on the Rona Ridge, as part of the Faroe-Shetland Basin.

Clair Field

Clair Field is geographically located 75 kilometers off the coast of Shetland in Scottish Territorial Waters. The field covers about 220 square kilometers or five license blocks. It is the largest undeveloped oil discovery on the United Kingdom Continental Shelf. Clair field was discovered with Well 206/8-1A drilled in 1977.

Geologically, "Clair Field is located in the southern part of the extensional Faroe–Shetland Basin, which is of Devonian to Cretaceous age. Clair is bounded to the NW by Rona Ridge Fault system and to the SE by the West Shetland Basin and Shetland Spine Fault."

Basin Development

The Faroe-Shetland basin was developed over a long period of time due to millions of years of rifting. Rifting of the Faroe-Shetland basin, as well as the Orcadian Basin, was first identified in the Devonian age. Subsequently, during the Carboniferous age, rifting resumed to form the Rona Ridge Fault system. "Rocks of Carboniferous age are locally preserved above the Devonian. During the Permian to Triassic period, thick sequences of continental deposits were formed in a series of basins, such as the Papa Basin." The Middle to Late Jurassic period was the primary period of rifting for the Faroe-Shetland basin, and is the time when the basin formed the structure it is in today. Finally, during the Late Cretaceous to the Paleocene age, rifting continued "that locally modified the existing basins and formed new depocentres such as the Foinaven Sub-basin". Atlantic rifting occurred mainly along NW or SE dipping faults.

Structural Geology

"The first reservoir fractures formed during the early tectonic history of the field, possibly during the collapse of the Caledonian Belt or during subsequent rifting. Normally, one or two dominant fracture sets are developed either parallel or perpendicular to nearby faults that are older than the unconformity. Favorably oriented pre-existing faults and fractures have been reactivated many times during Clair Field's tectonic history into the Mesozoic and Tertiary."

"Most hydrocarbons in the Faroe-Shetland Margin are stored within fractures in the Cretaceous basement made up of an up-faulted ridge of Precambrian metamorphic rock, or within fractures overlying Devonian to Carboniferous age sandstones." The hydrocarbons are sourced from "Devonian-age continental fluvial sandstones from the Lower Clair Group Reservoir."

The seismic section shows how listric faulting increased the thickness of the UCG and formed the ridge and core blocks of the basin.

Hydrocarbon Source/Reservoir: "Continental fluvial sandstones" from:

• Upper Jurassic Kimmeridge Clay Formation
• Upper/Lower Jurassic
• Middle Devonian

Trap: "A four-way closure extending over approximately 220 km"

Seal: Upper Cretaceous Mudstone

The Clair Field is composed of structurally prominent features, the Core, Graben, Horst and Ridge. "The most prominent structural feature within the Clair Field is the Clair Ridge Fault. It was clearly active during Devonian and Cretaceous ages given the thickness variations across it."

"The Clair Group contains more than a kilometer of biostratigraphically barren red bed deposits of Late Devonian to Early Carboniferous age. The sedimentology data suggest a fluvial depositional environment with medium to coarse grained sandstones with several fining upwards cycles and sporadic deposits of low energy fine grained mud and siltstones."

Play Types

Hydrocarbons in the Clair Field are sourced from two different play types in the Faroe-Shetland margin, fractured Lewisian basement or Clair Group sandstones. Most hydrocarbons are stored:

• "Within fractures in basement made up of an up-faulted ridge of Precambrian metamorphic rock"
• "Within fractures of overlying Devonian to Carboniferous age sandstones"

Well Planning in Clair Field

"Early appraisal wells in the Clair Ridge area failed to confirm economically recoverable hydrocarbons. Vertical wells are unlikely to penetrate many potentially open productive open fractures. Therefore, high angle appraisal wells in the 1990's penetrated vertical fractures and confirmed that oil was recoverable on the Clair Phase 1 Platform in 2005. There were 36 production wells drilled. Clair Phase 2 production well planning started in 2009."

References

1. Ogilvie, Steven, David Barr and Matthew Dorling. "Structural geology and well planning in the Clair Field." Geological Society, London, Special Publications (2015): 421. 10.1144/SP421.7. <http://sp.lyellcollection.org/content/early/2015/02/19/SP421.7>.
2. Baron, Martin, et al. "Evolution of hydrocarbon migration style in a fractured reservoir deduced from fluid inclusion data, Clair Field, west of Shetland, UK." Marine and Petroleum Geology 25.2 (2008): 153-172. ISSN: 0264-8172. <https://www.sciencedirect.com/science/article/abs/pii/S0264817207000682>.
3. Andrew Morton, Alex Milne; Heavy mineral stratigraphic analysis on the Clair Field, UK, west of Shetlands: a unique real-time solution for red-bed correlation while drilling. Petroleum Geoscience ; 18 (1): 115–128. doi: https://doi.org/10.1144/1354-079311-026
4. Utley, Thomas, et al. "The onshore structural geology of Foula, Shetland and implications for the Devonian-Carboniferous development of the offshore Clair Basin." October 2017. https://geospatial-research.com. PowerPoint. November 2018.
5. Patruno, Stefano. "A New Look at the Geology and Prospectivity of a North Sea Frontier Area with Modern Seismic - The East Shetland Platform." 79th EAGE Conference and Exhibition 2017 A Volume: Oral Presentation Session: Exploration Plays, Prospects and Prospects Evaluation II, Abstract Th C2 09. Paris, France, 2017. Conference Paper.
6. LLC, Revolvy,. "Faroe-Shetland Basin" on Revolvy.com https://www.revolvy.com/page/Faroe%252DShetland-Basin