The firm may also receive, in certain circumstances, further payments up to a maximum of US$10mln taking the total to US$71.5mln.
Following the deal, Empyrean is left with a 58.084% working interest (WI) in the Eagle Oil Pool Development project in southern California; a 7.5% WI in two producing wells and lesser interests in another six producing wells located in the Sugarloaf Block A operated by ConocoPhillips.
It also has a 10% WI in the Riverbend Project, located in the Tyler and Jasper counties, onshore Texas.
Empyrean plans to use the money for various things, including closing out its existing debt facility with Macquarie Bank and settle payments due to Marathon Oil.
Chief executive Tom Kelly said: "It is with great pleasure that Empyrean is able to announce the completion of what has been a very challenging transaction in perhaps the most difficult market conditions for the global oil and gas sector for decades.
"The transaction represents an excellent outcome for our Company and its shareholders given the backdrop of continued low oil prices and particularly limited development capital availability. We move forward debt free and with the potential to unlock further significant value for shareholders."
Empyrean shares eased 2.08% to stand at 5.875p.