ExxonMobil has placed a big bet on US shale oil, agreeing a deal worth up to $6.6bn to buy companies with drilling rights on about 250,000 acres in the Permian Basin region of Texas and New Mexico. The deal, which is worth $5.6bn in Exxon shares plus up to $1bn more in cash, is the company’s largest since the $41bn acquisition of XTO Energy, agreed in 2009. It is also the first big decision for Darren Woods, the new chief executive who took over from Rex Tillerson at the end of last year. Exxon is buying a package of companies from the Bass family of Fort Worth, Texas, which was identified by Forbes magazine as one of the 30 richest families in the US in 2015, controlling an “intensely private fortune”. The deal is the clearest indication yet of how Exxon and other large US oil groups have shifted their attention from large multibillion-dollar projects to more flexible developments where individual wells are much less expensive. The acquisition will double Exxon’s resource base in the Permian region to about 6bn barrels of oil equivalent. The deal is also the latest example of the intense interest from oil producers in the Permian region, which holds a number of different shale layers including the Wolfcamp, Bone Spring and Spraberry formations.