Abu Dhabi has not invited current partner BP to bid for its biggest oil concession, Abu Dhabi Company for Onshore Oil Operations (Adco), after its current deal with the international oil company (IOC) ends in 2014.

Abu Dhabi National Oil Company (Adnoc) sent letters to three other Adco partners ExxonMobil, Shell and Total inviting them to prequalify for the new concession, according to a report by Petroleum Intelligence Weekly.

The four IOCs each hold a 9.5 per cent stake in Adco, which operates Abu Dhabi’s major onshore oil fields and accounts for almost 60 per cent of the UAE’s total crude output. Adnoc owns 60 per cent of the concession, with Portugal-based Partex holding the remaining two per cent.

It remains unclear why BP was not invited to renew its partnership. Norway’s Statoil, which has no interest in the UAE, has reportedly been invited to bid.

BP owns a 14.66 per cent stake in the UAE’s largest offshore concession, Abu Dhabi Marine Operating Company (Adma-Opco), which expires in 2018.

The full list of companies prequalifying to bid for Adco has not been disclosed by Adnoc. The state-owned firm said last week that it was becoming more open to new partners, including South Korean and Chinese companies.