Brazil's Petrobras terminated its contract for Diamond Offshore's semi-submersible drilling rig Ocean Valor. Diamond Offshore said in an SEC filing that it could take legal action. The drilling contract was extended in 2014 and had been scheduled to terminate in October 2018.
"The company does not believe that Petrobras had a valid or lawful basis for terminating the contract, and the company intends to defend the rights of its subsidiary under the contract," Diamond Offshore said in a SEC filing Thursday.
According to the filing, the rig had been earning an estimated $455,000 per day during the extension period.
In a note to investors, Capital One analyst Luke Lemoine said "this could be a drawn-out process, and Petrobras has received the more favorable outcome in the past."
Lemoine rates Diamond Offshore stock as underweight with a $20 price target, based on the fact that offshore drilling fundamentals are still rough, despite improved oil prices.
The stock was on track for its worst daily performance since February, 2016. Thursday was Diamond Offshore's seventh negative session in a row.
Shares are off by more than 21 percent year to date, which would be its fourth consecutive year of double digit losses.
-CNBC's Chris Hayes contributed to this report