Royal Dutch Shell (RDSa.L) agreed to buy smaller rival BG Group (BG.L) for $70 billion in the first oil super-merger in more than a decade to close the gap with the world's largest oil firm U.S. ExxonMobil (XOM.N).

In a joint statement, the two firms said Shell would pay a mix of cash and shares that would value each BG share at around 1,350 pence ($20). It said this represented a premium of around 52 percent to the 90-day trading average.

Britain's BG had a market capitalisation of $46 billion as of Tuesday close, Shell was worth $202 billion while Exxon, the world's largest oil company by market value, was worth $360 billion.

"We have two very strong portfolios combining globally in deep water and integrated gas," Shell's CEO Ben van Beurden told a news conference.

The deal would give Anglo-Dutch Shell access to BG's multi-billion dollar projects in Brazil, East Africa, Australia, Kazakhstan and Egypt, including some of the world's most ambitious liquefied natural gas (LNG) projects.

Van Beurden said the presence of two large firms in Australia, Brazil and China and the European Union might require a detailed conversation with anti-trust authorities.

The collapse in global oil prices has sparked much speculation about mergers in the industry and BG has often been cited as a potential target.

The merger is the biggest announced deal globally this year.

BG shares have tumbled nearly 28 percent since mid-June, when the slump in global oil prices started. That compares with a 17.2 percent decline in the FTSE All Share Oil and Gas Index .FTASX0530 and a 3.3 percent fall for Shell's Amsterdam-listed shares.

In early trade in London, BG's shares had risen 42 percent, while Shell's were down 1.8 percent.

MEGA MERGER

The halving in crude prices on the back of a shale oil boom in the United States and a decision by Saudi Arabia not to cut production has created an environment similar to the early 2000s, when many large mergers took place.

Back then, oil major BP (BP.L) acquired rival Amoco and Arco, Exxon bought Mobil and Chevron (CVX.N) merged with Texaco.

The deal, which should generate pretax synergies of around 2.5 billion pounds per year, will result in BG shareholders owning around 19 percent of the combined group.

The two groups said Shell would pay a dividend of $1.88 per ordinary share in 2015 and at least the same amount in 2016.

Shell also expects to start a share buyback programme in 2017 of at least $25 billion from 2017 to 2020.

Shell said the deal would boost its proven oil and gas reserves by 25 percent. The firm also plans to increase asset sales to $30 billion between 2016-2018 on the back of the deal.