Sullivan & Cromwell and Davis Polk & Wardwell have taken lead roles on BG Group's inaugural debt offering into the US, with a $1bn (£640m) notes offer.

Sullivan advised BG Energy Capital on the deal, which marks the New York firm's first mandate for the company. The firm advised on both English and US law, fielding a team led by London co-managing partner Vanessa Blackmore (pictured) and corporate partner Bill Plapinger.

Davis Polk advised the underwriters – Merrill Lynch, Goldman Sachs, JP Morgan and Fenner & Smith. The firm's London-based team was led by corporate partner John Banes, who was assisted by corporate associates Jeremy Entwisle and Andrea Buti, while tax partner John Paton also advised.

The deal comprised an aggregate of $1bn (£640m) of BG Energy Capital's 2.5% guaranteed notes due in 2015 and 4% guaranteed notes due in 2020.

Plapinger said: "This was BG Group's debut entrance into the US debt market, and it was our job to make the transaction as smooth as possible from a US and English law point of view. BG Group is a large and complex company with diverse operations around the world, and it was a fascinating deal for Sullivan to work on with the very fine BG legal and finance teams."

The news comes two months after BG Group announced its first formal legal panel, with four firms appointed to the energy company's roster. The company named Allen & Overy, Freshfields Bruckhaus Deringer, Herbert Smith and CMS Cameron McKenna as its four panel advisers.

BG Group was created following the demerger of British Gas in 1997, which also saw Centrica take over the gas sales arm of the business under the British Gas brand.

The existing gas exploration section of the business was spun off under the BG brand, which also develops and connects gas to markets. BG's headquarters are in the UK but 60% of its staff are based internationally.