Liberia's state run oil company NOCAL negotiated the latest deal, announcing on Thursday that it would give the state a 10 percent free equity stake and royalties of between 5 and 10 percent once production began.

It will also require Exxon and COPL to jointly pay a $50 million up-front signing bonus to Liberia's government.

COPL shares gained more than 20 percent on Thursday after the announcement, reflecting the importance of the Liberian project on its relatively small portfolio of projects, while Exxon shares slipped 1 percent.

The terms are slightly better for Liberia than those of some previous oil deals, some of which waive royalty payments or offer reduced tax rates and state equity share.

But the latest deal still falls short of the country's current laws, which call for royalties of no less than 10 percent and a free stake for the state of 20 percent.