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Exploration activity in the West of Shetland area offshore the UK has been carried out for more than three decades, but now the upstream industry – encouraged by UK government incentives – appears ready to tackle the challenge of finding and developing fields in one of the world's harshest offshore environments.
Exploration in the UK West of Shetland area has been taking place since 1974 in up to 1,800 m (5,906 ft) water depth. The first significant discovery to be made was the Clair field in 1977, which at the time was the largest discovered – but undeveloped – hydrocarbon resource on the UK Continental Shelf (UKCS).
After many appraisal wells, the development of the field was approved in 2001 and production started in 2005. The field is located on a structural high on the Rona ridge where oil is trapped in fractured Devonian and Carboniferous reservoir rocks.
Early drilling followed the Clair model and was generally targeting structural highs over the crests of the Rona, Westray, and Corona ridges at Jurassic reservoirs. However, in 1992, the discovery of the Foinaven field in deepwater Paleocene sands with a stratigraphic component to the trap invigorated a new approach to exploration in the area.
After 20 years and more than 100 exploration and appraisal wells, Foinaven represented the first economically viable reserves and proved the benefit of modern seismic techniques and AVO analysis. The discovery of the Schiehallion field one year later and the Loyal field the following year meant the combined recoverable reserves from these three fields were estimated at around 600 MMbbl of recoverable oil. After these discoveries, deep water has been regarded as the most promising source of huge reserves.
Interest picked up significantly again in 2004 with the appraisal of Laggan and the significant oil and gas discovery of Rosebank/Lochnagar in the Faroe-Shetland Channel near the UK/Faroes border. In 2009, there was an upsurge in the West of Shetland area, resulting in oil and gas discoveries like Tornado, Glenlivet, and Lancaster, the first planned exploration well on a basement target on the UKCS.
The West of Shetlands is still a relatively underexplored region, so to invigorate exploration the government has included a new class of deepwater field allowance in this year’s budget. Subsea development projects such as Laggan and Tormore and associated plans for infrastructure development further improve the commercial appeal of the area.
With regard to the prospectivity of the area, multiple reservoir targets exist. Pre-Base Cretaceous Unconformity and Cretaceous reservoirs will be sealed by Cretaceous mudstones, and the Paleocene reservoir is likely to be sealed by either volcaniclastic sediments or mudstones.
The principal oil-prone source rock in the Faroe/West of Shetland basin is Late Jurassic organic-rich, basinal-marine mudstone (Kimmeridge clay formation). This has been proven to be the source rock of the hydrocarbon discoveries West of Shetland with a larger gas kitchen due to thicker Cretaceous-Tertiary overburden. Also, there is the possibility of an additional mid-Jurassic source rock. Basin modeling studies suggest that the hydrocarbons in place are likely to be gas/condensate, with migration likely to have started in the Cretaceous and continuing post-trap formation and enhancement through to the Miocene.
The main geophysical challenge in the area is the degrading of the subsurface imaging at pre-Tertiary level due to the attenuation and dispersion of the seismic signal.
This is due to the presence of Paleogene basaltic volcanism resulting from the development of the Iceland mantle plume before continental breakup. It is important to determine the seismic properties of the basalt flows as this affects how the seismic energy is attenuated and the way it affects the imaging of the deeper section. For instance, internal layering of the basalts may generate complex interbed multiples and interference patterns, and high-velocity basalts overlying low-velocity sediments create large impedance contrasts.
To improve reflectivity in the sub-basalt sediment, various seismic processing techniques have been tested by PGS focusing on frequency enhancement, velocity estimation, and multiple attenuation. These techniques have been used to great effect in the PGS large-scale reprocessing projects known as MegaSurveyPlus datasets where extensive data coverage is combined with processing techniques to extract more value from the data using improved technology.
The MegaSurveyPlus in the Faroe Shetland basin (FSB) covers more than 15,000 sq km (5,791 sq miles) and has imaged stratigraphic levels much deeper than previously seen. The main objective of the reprocessing effort has been to improve the multiple attenuation by using the latest data processing techniques such as 3-D surface-related multiple elimination and Backus deconvolution. Detailed attention to velocity analysis combined with the use of the latest 3-D Fourier regularization and migration techniques aim to produce an extensive seamless volume over a challenging part of the FSB area.
The final volumes show a significant improvement over the pre-existing datasets in this area. The reprocessed dataset provides an improved structural image that gives the interpreter a better understanding of the complex subsurface in this area. During the processing of the data, it has been ensured that the resulting dataset produces
• Improved signal to noise ratio;
• Improved de-multiple response revealing features and structures not previously observable;
• Enhanced imaging for reservoir interpretation;
• Clear characterization of intrusive aiding interpretation; and
• Optimized amplitude-vs.-offset response.
All of these improvements aim to lead to a greater confidence in interpretation.
Changing Future Of The Atlantic Margin
The FSB is similar in size to the UK Viking Graben but considerably less explored. Although some evidence suggests it contains more large undrilled structures, the uncertainty about basalt thickness and water depths makes it more challenging and expensive to explore.
However, new technologies and government incentives have helped keep exploration in the area at the forefront of UK activity with prospects such as North Uist and Handcross either drilled or scheduled to be drilled in the near future.
North Uist is in UK Block 213/25c approximately 125 km (78 miles) northwest of the Shetland Islands and approximately 30 km (19 miles) southeast of the UK-Faroes median line in 1,291 m (4,236 ft) water depth. It was completed by BP in May this year and plugged and abandoned with no comment on the well’s results from the operator or partners. The well was testing the Upper Triassic as the primary target and the edge of the Palaeocene Cardhu prospect at shallower levels.
Handcross, located in UK Blocks 204/18b, is expected to be drilled in 1Q 2013. According to operator Valiant Petroleum Plc, the Handcross prospect is a large stratigraphic trap, similar to the nearby Foinaven and Schiehallion fields, that benefits from a number of geophysical anomalies interpreted to reflect the presence of hydrocarbons in Paleocene sands. Gross prospective resources estimated internally could reach 180 MMboe (net 162 MMboe).
As the industry approaches 40 years since the first exploratory forays into the West of Shetland arena, activity and interest in the area is at its highest to date. With infrastructure being installed, the area is gaining a new viability, and with new discoveries being made, it is clear that the improved technology now available has given a greater confidence to exploration in this difficult area.