THE discovery that has done more than any other in recent years to draw attention to the latent potential of West of Shetland is surely Rosebank/Lochnagar, over which there has been much speculation as to size – mostly in the 400-600mmb range, but could be larger, with the prospect of a future separate gas find in the same locale.
What matters is that Chevron has embarked on development concept studies, evaluating the options that basically distil down to a floating production unit and a suite of subsea wells. A production vessel (FPSO) remains the most likely option as ship specialist BW Offshore has carried out work on the project.
It was in late-2004 that Chevron, with Statoil, OMV and DONG as partners, made the Rosebank/Lochnagar discovery on UK block 213/27. Well 213/27-1z drilled the crest of a large anticlinal structure named Rosebank. Two oil and gas accumulations were encountered with that well, since when a patient appraisal programme has been ongoing.
During a production test of the second of the initial three appraisal wells in July, 2007, the well flowed at choked rates of 6,000bpd.
The 205/1-1 well is located at the southern end of the Rosebank structure and was drilled to a vertical depth of just over 9,000ft. The oil was found to be very light, with a gravity of 38deg API. North Sea oil is normally of 20-30 API quality and the Clair, Foinaven and Schiehallion fields are around 25 API.
The very lightness of Rosebank/Lochnagar oil explodes another myth that has grown up around the UK’s Atlantic Frontier, namely that any oil found will be heavy.
However, not satisfied with the amount of data thus far accumulated, Chevron has embarked on a further drilling programme, with up to five exploration/appraisal wells targeting Rosebank, Rosebank North/Lochnagar North, Aberlour, Lochside and Milburn reservoir/ prospects. These prospects collectively make up the Greater Rosebank Area, which straddles blocks 213/17, 213/23, 213/27 and 213/28. It is located 10-15km from the UK-Faroe median line, about 113km west of Shetland and 160km south-east of the Faroe Islands. Water depths at Greater Rosebank are about 1,150m.
The campaign is being conducted using Stena Drilling’s new drillship, Stena Carron, which paused briefly at Invergordon prior to going on station and spudding the first well. Drilling is expected to last for 340 days, ending tentatively in February, 2010.
Chevron said at the Aberdeen Share Fair last month that it expected FEED (front-end engineering and design) to start in 2011.
But not everyone is as talkative. Take Shell and Hess’s 2008 probes. In Shell’s case, the rig, Leiv Eiriksson, drilled the South Uist Prospect on block 214/21a. For Unst, geo-technical data had predicted that there was an 18% chance of hydrocarbon discovery being made and that the target most likely contained dry gas/gas condensate.
This prospect is located on block 214/21a and lies north of the Torridon gas discovery and east of Chevron’s Rosebank/Lochnagar oil find. But, in characteristic style, having drilled the well and sent the Leiv Eiriksson on its way to Dooish, Shell then classified South Unst as a tight hole.
Hess said nothing about the outcome of its 2008 appraisal sidetrack, 204/25a-8Y, on the Schiehallion attic oil prospect, Amos. In fact, the US group did locate oil and, according to OMV, a stakeholder in the licence, the find may be commercial. And being just four kilometres from BP-operated Schiehallion, a subsea satellite tie-in development would appear to be the logical approach. It is understood that Hess will in 2009 be revisting its Cambo discovery about which nothing has been disclosed.