Though the volume of energy-related M&A transactions has dipped to a nine-year low, several law firms are closing out 2013 by grabbing lead roles on a spate of big deals tied to the industry.

In the United States, shale plays continued to be the main driver of energy-related transactional work in 2013, says Joseph Dilg, an oil and gas industry veteran at Vinson & Elkins who headed the firm from 2002 through 2011.