The company said that it intends to promptly start discussions on the counter-offer with CNOOC Limited though it reiterated that there was no assurance that any agreement would be reached.
It added that its board of directors recommendation of the Chevron bid to its shareholders remained in effect.
CNOOC Limited's offer is $1.5 billion higher than Chevron's and while Unocal has already reached an acquisition agreement with Chevron in April it had said that it would consider the CNOOC Limited's offer.
