Mayer Brown and Freshfields Bruckhaus Deringer have won roles on the planned $4.2bn (£2.7bn) merger of former BP chief executive Tony Hayward's investment vehicle Vallares with Turkish oil company Genel Energy International.

Mayer Brown's London office is advising Genel, the largest oil producer in the Kurdistan region of Iraq, fielding a team led by global energy co-head Robert Hamill, working with partners including Kate Ball-Dodd 
and Richard Smith (corporate), James Hill 
(tax) and Mark Compton (financial services).

Mayer Brown worked alongside Turkish law firm Yazici, where senior partner and Genel GC Murat Yazici took the lead role.

Freshfields is advising Vallares on the transaction, the first for the company since it was formed this summer by Hayward and financier Nathaniel Rothschild and listed on the London Stock Exchange. London corporate partners Julian Makin and Steve Hewes are leading the Freshfields' team, with corporate partner Ben Spiers also advising.

US law firm Gibson Dunn & Crutcher is acting for Vallares' founders, fielding a team led by London corporate partner James Barabas and tax partner Nicholas Aleksander.

The deal will see Vallares issue new shares worth $2.1bn (£1.3bn) to acquire a 100% stake in Genel, which will retain a 50% stake in the renamed Genel Energy.

Hamill said: "This is one of the most exciting energy deals of the year and signals a move towards consolidation of oil and gas companies in the region."

Hayward left BP last year after the oil giant's Gulf of Mexico spill. He will become chief executive of the 
new entity, with Rothschild taking a role as a non-executive director.