Premiums for Nigerian benchmark grades Qua Iboe and Bonny Light jumped more than $1.50 per barrel to Brent plus $4.00 when the Libyan uprising began and traders had suggested that values might ease back after the initial rise.

But premiums have risen even further this week as trading in the new May cargoes has started and a cargo of Qua Iboe was reported sold as high as dated Brent plus $4.30/$4.30, up to 20 cents above the highest previous deal, traders said. Angolan grades have also been trading at higher levels for May than April cargoes, again reflecting the higher premiums for low sulphur grades.

Qua Iboe: Exxon was reported to have sold its cargo loading May 1-2 to Sun at between dated Brent plus $4.30 to plus $4.35. The previous highest deal for Qua Iboe was reported at around dated plus $4.15 for April lifting. Bonny Light: assessed at close to dated Brent plus $4.00 to $4.25, a slight discount to Qua Iboe; while Forcados: reported sold for May on an OSP-related basis at a "hefty premium", one trader said. The official selling price for Forcados for April lifting was set at dated Brent plus $3.80 per barrel.

West African crude oil differentials strengthened on Thursday as the first wave of cargoes sold for May reflected increased demand for low sulphur crudes due to the loss of Libyan exports.

Libya normally exports around 1.3 million barrels per day of relatively low sulphur crude, the equivalent of more than 40 cargoes per month of West African size.

This void has been filled partly by extra sales of higher sulphur crudes from the Middle East but much of the demand is for lower sulphur oil such and West African grades are perfectly suited to meet these specifications.