Mexico's government plans to auction off one oil field per year beginning in 2015 once the process of assigning areas requested by state-owned Petroleos Mexicanos has concluded, a senior official said.

"There will likely be resources for at least 10 rounds, but we're thinking that after this year's Zero Round we would hold a new round in 2015 and do one per year after that," Deputy Energy Secretary Lourdes Melgar Palacios told the media.

President Enrique Peña Nieto's administration will likely look to hold five rounds through 2018, "while aware that there are resources for more rounds," she said Wednesday.

The plan is to auction off fields ranging in area from 20,000 sq. kilometers (7,720 sq. miles) to 25,000 sq. kilometers, "although they may be larger," she added.

Under the energy overhaul that was approved in December and opened the door to private investment in the sector, Pemex has first dibs on oil fields in the so-called Zero Round before private and foreign companies will be invited to participate in subsequent bidding processes.

Pemex is trying to hold on to 83 percent of its proved and probable reserves in the Zero Round, while it has requested the right to develop a much smaller number of technically challenging deep-water fields.

In its wish list, submitted last week, the state-owned giant asked the Energy Secretariat to allow it to retain 100 percent of fields where proven reserves are available and the company has already starting investing.

Melgar Palacios said the decision on which oil fields to directly assign to Pemex will be announced on Sept. 17, after a thorough assessment of the state-owned company's technical, operational and financial ability to develop those areas.

The information will be made public and include a map showing which zones will be assigned to Pemex and which ones the government will retain, she said.

In the Zero Round, Pemex requested it be assigned fields covering a total of 160,000 sq. kilometers (61,800 sq. miles), or around 30 percent of the 407,000 sq. kilometers currently under its control.

The move to end Pemex's monopoly, in place since the company was created in 1938 as part of then-President Lazaro Cardenas nationalization of the oil industry, is a thorny issue in Mexico because the state-owned firm has long been a symbol of national sovereignty. EFE