Royal Dutch Shell (RDSa.L) said on Wednesday it had agreed to buy BG Group (BG.L) for $70 billion in a bid to close the gap on the world’s biggest oil company, U.S. ExxonMobil (XOM.N). In a joint statement, the two firms said that as part of the recommended deal Shell would pay a mix of cash and shares that would value each BG share at around 1,350 pence($20). Anglo-Dutch Shell also expects to start a share buyback program in 2017 of at least $25 billion for the period 2017 to 2020.