THE SEARCH for oil and gas in ever deeper water and harsher marine environments has spurred a $3.2 billion merger of two of the world's leading offshore drillers, which believe the move will herald a structural shift across the sector. The combination of Transocean Offshore with Schlumberger unit Sedco Forex Offshore will create the world's fourth largest oilfield service company behind Halliburton, Schlumberger and Baker Hughes with a market capitalisation of more than $6 billion -- 45% larger than its closest competitor -- and a strong presence in each of the world's most important offshore drilling regions.