Spain's Repsol energy firm is threatening legal action against companies or investors that help Argentina exploit petroleum Repsol planned to tap before its Argentine unit YPF was nationalized.

Repsol discovered vast shale oil deposits under the "Vaca Muerta" ("Dead Cow") basin of Neuquen province that could increase Argentina's oil reserves by at least 750 million barrels. Experts say Argentina will need strategic partners to tap it.

Repsol YPF spokesman Kristian Rix says the company "is reserving the rights to take legal action against any investor in YPF or its assets following the unlawful expropriation of that company."

Rix spoke Monday as Repsol shares continued to fall following last week's nationalization announcement.