Tullow Oil expanded its west African exploration interests by acquiring a 15 per cent stake in an oilfield off the coast of Angola.
Under the terms of the "farm-in" agreement, Tullow will agree to share the exploration costs with Devon Energy the current operators of the field.
A farm-in agreement is an arrangement under which a holder of an exploration or mining right agrees to assign a portion of its interest to another party in exchange for investment or the performance of specific activities.
Two prospective wells have already been identified within the licence area and drilling is under way in the first well.
Aidan Heavey, Tullow's chief executive, said: 'We are delighted to have the opportunity to expand our West African business into Angola, one of the major petroleum provinces in Africa.
"This is the first step in our strategy to gain a significant position in the country and we look forward to doing further business in the region."