MEXICO CITY (AP) — Mexican officials said Monday they expect to open bidding on the first round of private oil contracts in the first half of 2015.

A constitutional reform passed last year opened the state-owned energy sector to private investment. But enabling legislation is still before congress.

Assistant Energy Secretary Lourdes Melgar said the first round of bidding is expected to offer a range of onshore, offshore and deep water fields.

Melgar said Mexico hopes to have private companies producing a half million barrels of oil a day by 2018, while state-owned oil company Petroleos Mexicanos should maintain production at about 2.5 million barrels a day.

Mexican tax authorities said they expect government revenue, on average, to be about 50 percent of production value from private projects operated under contract, license or concession.

Petroleos Mexicanos, known as Pemex, will have first choice of which fields to develop. Those announcements are expected by Sept. 17.

Copyright 2014 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.