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Second Sierra Leone discovery lifts new oil province

Anadarko has made a second oil discovery off Sierra Leone, validating its interpretation of the area's geology and raising prospects for a new province spanning four countries

IN November, the company's Mercury-1 well – only the second deep-water well drilled off Sierra Leone – hit more than 41 metres of oil zones. The well was drilled 65 km southeast of the firm's first find, Venus, of September 2009, which gave hopes for a new oil province stretching nearly 1,500 km across Sierra Leone, Liberia, Ivory Coast and Ghana.

"The Mercury well demonstrates that the stratigraphic trapping systems we have identified are working and that the petroleum system is generating high-quality oil," Anadarko said. The company worked closely with Kosmos, the small US firm that drilled a stratigraphic trap off Ghana to discover the large Jubilee field, due on stream in December. Stratigraphic structures, in which oil is held in place by changes in the porosity and permeability of overlying rocks, are often overlooked because structural traps show up better on seismic surveys.

The Mercury-1 well, drilled by Transocean's Deepwater Millennium drillship, found 34.7 metres of zones holding light 34°-42°API crude and 6.4 metres holding heavier 24°API crude. Anadarko said it was suspending the well for possible testing or sidetracking. The discovery lies in 1,600 metres of water in the SL-07B-10 block, where interests are Anadarko, 65%, Repsol, 25%, and Tullow Oil, 10%. Venus is in the neighbouring SL-06 block, held by the same group.

Anadarko said 3-D seismic surveys over its five contiguous licences off eastern Sierra Leone and western Liberia has shown 17 prospects or leads, with its Ivory Coast and Ghana acreage raising the total to over 30. It plans to accelerate exploration and appraisal of the area in 2011 – although, when Mercury operations have been completed, the Deepwater Millennium is committed to move to Ghana.

In Ghana, the drilling unit will be used on appraisal work at the Owo and Tweneboa fields, to the west of Jubilee, which Anadarko says are likely to be sanctioned for development in 2011. The fields lie in the Tullow-operated Deep water Tano licence, in which Anadarko participates.

In late-November, Tullow said some crude had been produced from the Jubilee field and was being used to commission the processing equipment on the floating production, storage and offloading (FPSO) vessel. Jubilee is due to start flowing on 15 December, the firm said – only three and a half years after its discovery, by Kosmos, in June 2007. Initial production of 55,000 barrels a day (b/d) is expected, rising over three to six months to the plateau rate of 120,000 b/d for the first-phase development.

Interests in the unitised Jubilee field are Tullow (unit operator), 34.7047%, Kosmos (technical operator of the integrated project team), 23.4913%, Anadarko, 23.4913%, Ghana National Petroleum Corporation, 13.75% (of which 10.0% is carried), Sabre, 2.8127%, and EO, 1.75%.


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