(Adds details on the tender rounds, allocations, jointventures)
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 aspart of a historic opening of its oil sector that begins nextyear with a first round of contracts, the country's top oilofficials said on Wednesday.
The so-called Round One tender will offer up 169 explorationand extraction blocks, including a mix of both onshore andoffshore areas, and cover a total of 28,500 square km.
The tender will be organized by Mexico's nationalhydrocarbons commission and will happen as early as May 2015 andno later than September, said commission president Juan CarlosZepeda.
The landmark tender will prioritize areas that boost outputquickly and leave trickier deep water projects for later, headded.
Separately, the energy ministry on Wednesday assigned 83percent of Mexico's probable and possible reserves to Pemexunder 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 enterinto joint ventures with international oil majors such ChevronCorp and BP Plc.
The rounds are among the first steps of an energy overhaulchampioned by President Enrique Pena Nieto to break Pemex's75-year-old monopoly and reverse a decade-long slide in crudeoutput.
The energy ministry said it had assigned 21 percent ofMexico's prospective resources to Pemex, versus the 31 percentthe company had asked for.
The total area assigned to Pemex under Round Zero is equalto 20.6 billion barrels of proven and probable oil reserves. Butthe company was also given prospective resources totaling 22.1billion barrels of oil equivalent covering 90,000 square km.
"Pemex will continue to be the big business of Mexico," saidEnergy Minister Pedro Joaquin Coldwell.
Mexico is the world's 10th largest crude producer, but sincehitting peak production of 3.38 million bpd in 2004, output slipped to 2.52 million bpd last year. Last month, Pemex revisedits 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 differentprojects.
"Pemex faces the biggest challenge in its history, thechallenge of competing all along the value chain," Lozoya said.
Pemex's top exploration and production executive, GustavoHernandez, said the company still planned to compete in RoundOne for blocks it was not awarded in Round Zero.
The energy ministry said the Round Zero allocation providesPemex with a production "floor" of about 2.5 million bpd overthe next two decades.
Pemex will by February seek new contractual arrangements for11 fields it was assigned to take advantage of a more favorabletax structure under the reform.
The company will also seek tie-ups for projects atparticularly complex, costly fields, including heavy oiloffshore areas, large deep water gas developments and someacreage in the deep water Perdido Fold Belt. (With reporting by Adriana Barrera and Anahi Rama; Writing bySimon Gardner and David Alire Garcia; Editing by Kieran Murrayand Andre Grenon)
BP (BP.)