Discovered in 1977, the Clair oil field lies 75 km west of the Shetlands, offshore UK (UKCS), in waters of up to 150 m in Block 206. Clair comprises of an elongate NE-SW trending ridge of Lewisian basement and an associated roll-over (or terrace) containing a thick sequence of Devonian-Carboniferous continental red beds (Coney et al, 1993). The first well drilled in 1977, Well 206/8-1A, tested oil at 1500 bbl/day from the red beds at the crest of the roll-over. Well 206/7-1 followed, producing oil at 960 bbl/day from the fractured basement on the ridge with the oil coming entirely through the fractures. Ten further wells drilled between 1977 and 1985 indicated oil in place measurable in billions of barrels. However, test results were disappointing. The success of the discovery wells (206/8-1A and 206/7-1) was never repeated and commercial test production rates were never achieved.

Two further appraisal wells were then drilled in 1991. The first, a horizontal well in the fractured basement, tested at 2100 bbl/day after acid wash stimulation. The second well tested the red beds on the flank of the roll-over and achieved sustained flow rates of 3000 bbl/day from two zones (Coney et al, 1993). An extensive fracture analysis was performed in the horizontal appraisal well. The objective of the well layout was to cross-cut the fracture zones located in the fractured basement which were believed to act as preferential drainage paths for the hydrocarbons situated in the overlying and adjacent red beds source rock (Falt et al, 1992).

There are still many uncertainties associated with the Clair field but the appraisal wells drilled in 1991 demonstrated that significant and potentially commercial flow rates are achievable. Clair was said to remain the largest undeveloped oil discovery on the UK Continental shelf (Coney et al, 1993). During late 2001, BP and its partners decided to go ahead with development (Harts E&P, Dec. 2001).

The literature concerning hydrocarbons in the UK contains occasional references to trace hydrocarbons found in the Cornubian granites of southwest England. These Hercynian granites are intruded into rocks of Devonian, Carboniferous and Permian age and minor traces of a 'mineral' pitch are found on some fracture surfaces within the granite. No significant accumulations of hydrocarbon have been found in any of the extensive mining activities (as deep as 1000 m) in the region nor in boreholes as deep as 2800 m near the centre of the Carnmenellis granite. All the mines in the region are now closed (the last one, South Crofty, was shut down in March 1998) and the few samples that are accessible are at the Camborne School of Mines.

Massonat et al (1993) have described the geological and reservoir approaches to optimising the oil recovery from a field in the United Kingdom (the authors have not included the name of the field, except that it is owned by Elf). The reservoir consists of several hundred metres of sandstones, conglomerates, siltstones, and shales with poor reservoir characteristics. The well productivity is generally low. However, a vertical well has shown higher productivity coming from both the sandstones and the underlying fractured granite basement. The authors expect that a horizontal well within the basement and intersecting the major fracture orientations could possibly produce hydrocarbons from the overlying and adjacent accumulation with good productivity (Massonat et al, 1993).

There is another unnamed reservoir in the UK sector of the North Sea that has found oil in a crystalline basement during testing but no details have been released yet.

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