Oil and gas tops off this week's list of billion dollar deals, with Royal Dutch Shell plc's $70 billion acquisition of BG Group plc.

The deal marks the largest in the space since Exxon's $82 billion buy of Mobil in 1999, and may serve as a forebearer to additional oil and gas megadeals given relatively low oil prices, according to reports. It also further solidifies Shell's position as the global leader in liquefied natural gas.

Magic Circle and Am Law firms landed roles in the transaction, with Slaughter and May, Cravath, Swaine & Moore and De Brauw Blackstone Westbroek advising Shell, and Freshfields Bruckhaus Deringer working on behalf of BG Group.