In 2007, shale gas fields included the #2 (Barnett/Newark East) and #13 (Antrim) sources of natural gas in the United States in terms of gas volumes produced.[2] The number of unconventional natural gas wells in the U.S. rose from 18,485 in 2004 to 25,145 in 2007 and is expected to continue increasing[3] until about 2040.