Speaking at a conference organized by Global Pacific and Partners in Cape Town, Jeff Schellebarger, Chevron Texaco's general manager, said new oilfields must be developed to replace older ones in which reserves would run out.

Ebbie Haan, Shell International's regional vice-president for African exploration and new business development, said it was drilling an average of 100 wells for exploration every year.

"We spend about half our costs on drilling. There are large parts of Africa which do not yet have a Shell presence, and we are looking for opportunities," he said.

Growth

Experts at the conference said that demand for oil was expected to grow at a rate of 2 to 3% a year from its current demand of 120 million barrels a day.

Production from established oilfields would decline, creating a gap of 80 million barrels a day.

Half of this gap would be filled by fields not yet in production.

Africa, which has about 9% of the world's oil and gas reserves, has the potential to provide a large part of this.

Duncan Clarke, the chief executive of the London-based Global Pacific and Partners, said Africa had become a "critical" source of oil and gas and there were huge opportunities for exploration and production.