(Adds details on the tender rounds, allocations, joint ventures)
By David Alire Garcia and Tomás Sarmiento
MEXICO CITY, Aug 13 (Reuters) - Mexico expects to attract $50.5 billion in new private and foreign investment by 2018 as part of a historic opening of its oil sector that begins next year with a first round of contracts, the country's top oil officials said on Wednesday.
The so-called Round One tender will offer up 169 exploration and extraction blocks, including a mix of both onshore and offshore areas, and cover a total of 28,500 square km.
The tender will be organized by Mexico's national hydrocarbons commission and will happen as early as May 2015 and no later than September, said commission president Juan Carlos Zepeda.
The landmark tender will prioritize areas that boost output quickly and leave trickier deep water projects for later, he added.
Separately, the energy ministry on Wednesday assigned 83 percent of Mexico's probable and possible reserves to Pemex under a so-called Round Zero allocation.
The allocation provides the Mexican oil company with a new, slimmed-down portfolio of assets to develop on its own or enter into joint ventures with international oil majors such Chevron Corp and BP Plc.
The rounds are among the first steps of an energy overhaul championed by President Enrique Pena Nieto to break Pemex's 75-year-old monopoly and reverse a decade-long slide in crude output.
The energy ministry said it had assigned 21 percent of Mexico's prospective resources to Pemex, versus the 31 percent the company had asked for.
The total area assigned to Pemex under Round Zero is equal to 20.6 billion barrels of proven and probable oil reserves. But the company was also given prospective resources totaling 22.1 billion barrels of oil equivalent covering 90,000 square km.
"Pemex will continue to be the big business of Mexico," said Energy Minister Pedro Joaquin Coldwell.
Mexico is the world's 10th largest crude producer, but since hitting peak production of 3.38 million bpd in 2004, output slipped to 2.52 million bpd last year. Last month, Pemex revised its output forecast for this year down to 2.44 million bpd.
Pemex Chief Executive Emilio Lozoya said the company wanted to set up joint ventures with private companies on 10 different projects.
"Pemex faces the biggest challenge in its history, the challenge of competing all along the value chain," Lozoya said.
Pemex's top exploration and production executive, Gustavo Hernandez, said the company still planned to compete in Round One for blocks it was not awarded in Round Zero.
The energy ministry said the Round Zero allocation provides Pemex with a production "floor" of about 2.5 million bpd over the next two decades.
Pemex will by February seek new contractual arrangements for 11 fields it was assigned to take advantage of a more favorable tax structure under the reform.
The company will also seek tie-ups for projects at particularly complex, costly fields, including heavy oil offshore areas, large deep water gas developments and some acreage in the deep water Perdido Fold Belt. (With reporting by Adriana Barrera and Anahi Rama; Writing by Simon Gardner and David Alire Garcia; Editing by Kieran Murray and Andre Grenon)