President Cristina Fernandez defended the deal her administration signed earlier this week with U.S.-based supermajor Chevron Corp. for the development of the Vaca Muerta shale oil and gas field in southwestern Argentina amid criticism from opponents of the agreement.

"I know the really important agreement we signed with one of the world's most important oil companies, Chevron, under the framework of an extensive development decree covering all petroleum companies that invest more than $1 billion in Argentina, bothered some people," Fernandez said during an appearance Wednesday in Chaco province.

The president questioned the criticism of the $1.24 billion deal signed Tuesday by YPF, the oil company controlled by the Argentine state following the expropriation of a 51 percent stake from Spain's Repsol, and San Ramon, California-based Chevron.

Adolfo Perez Esquivel, winner of the 1980 Nobel Peace Prize, former Energy Secretary and YPF chief Daniel Montamat, and several opposition members of Congress called on the administration to clarify the deal.

"They are setting an extremely serious legal precedent. We have to make an official request to YPF so they will give us the studies on the environmental impact and so we can see what the agreement with Chevron is based on," Perez Esquivel said, adding that Texaco, now a unit of the U.S. supermajor, has been sued for harming the environment between 1964 and 1990.

The deal will be governed by U.S. law and could run for up to 35 years, the La Nacion newspaper reported.

Leftist groups protested the agreement in Buenos Aires, while Mapuche Indians staged demonstrations in Neuquen province, where the oil production will take place.

YPF announced the discovery of non-conventional oil and natural gas reserves in Vaca Muerta in 2011 after successful results in the exploration phase.

Development of that region, which has already begun on a small scale, is seen as a solution to Argentina's current energy deficit. EFE