The buyer is Carrier Energy (CEP II), a Houston-based privately-owned specialist in US oil fields that has the backing of private equity group Riverstone.
Empyrean has a 3% working interest in Sugarloaf, which is operated by Marathon and has about 290 wells in production currently.
Average daily production net to Empyrean was 1,250 barrels in the three months to September.
The initial consideration will be US$61.5mln, with up to US$10mln on an oil price contingency basis.
Tom Kelly, chief executive, said given the weak oil price and financial resources required to maintain its interest as Marathon develops the field, now was the time to sell.
Marathon’s indicative drill schedule for 2016 is for around 80-100 new wells.
“We believe that we have reached the right time to realise the value of the Sugarloaf asset and de-risk our position against continued depressed oil prices.”
Kelly added the money will repay a near US$20mln facility with Macquarie.
Thereafter the plan is to retain its existing interests and use the cash it receives to look at possible deals for assets owned by companies in distress due to the current oil market turmoil.
Empyrean has three other exploration assets: an interest in another Sugarloaf block operated by Conoco Phillips; Riverbend in Texas; and Eagle Oil in California.
Shares rose 45% to 7.6p, valuing Empyrean at £10mln.