The £47 billion agreed bid for BG Group by Royal Dutch Shell, which would be the world's second biggest oil and gas takeover ever, comes with short-term risks for Shell, according to shareholders in both companies. But most think it will turn out well in the long run.

The combined cash-and-shares transaction, announced by Shell on Wednesday morning, would hand it BG's prized oilfields off the coast of Brazil, as well as adding substantial capabilities in liquefied natural gas. Shell also estimates it could generate $2.5 billion a year in cost synergies by eliminating overlaps between the companies.